Re: [Haskell-cafe] a little parsec enhancement

[Antoine Latter]

The exported `mkPT` is equivalent to the old 'ParsecT' data constructor
from parsec 3.0.x.

I wouldn't mind exporting a similar alias for the new 'ParsecT' constructor
from 3.1.x.


On Fri, Sep 6, 2013 at 2:21 PM, Petr Pudlák  wrote:

> Dne 09/05/2013 01:38 PM, Roman Cheplyaka napsal(a):
>
>  * Petr Pudlák  [2013-09-05 11:18:25+0200]
>>
>>> Unfortunately |ParsecT| constructor isn't exported so I'm not able to
>>> implement it outside /parsec/.
>>>
>> No, but there's an 'mkPT' function which is equivalent to the ParsecT
>> constructor.
>>
>> (Although I, too, wish the ParsecT constructor were exposed.)
>>
>> Roman
>>
> Yes, I tried to use `mkPT`, but the result looked very complicated and I
> wasn't quite sure if it'll be working correctly in all cases. Implementing
> the same thing with the `ParsecT` constructor is simple and comprehensible.
>
>   Best regards,
>   Petr
>
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Re: [Haskell-cafe] Generating Haskell Code out of Haskell AST (GHC API)

[Antoine Latter]

The package haskell-src-exts is a lot less intimidating if all you are
trying to do is programmatically generate Haskell source:

http://hackage.haskell.org/package/haskell-src-exts

The base types are here:
http://hackage.haskell.org/packages/archive/haskell-src-exts/1.13.5/doc/html/Language-Haskell-Exts-Syntax.html#t:Module

This module has some helper function for generating parts of the AST:
http://hackage.haskell.org/packages/archive/haskell-src-exts/1.13.5/doc/html/Language-Haskell-Exts-Build.html


On Thu, Jul 18, 2013 at 1:11 PM, John Blackbox
 wrote:
> Hi!
> I dont know GHC API very well, but I want to generate AST of a program using
> GHC API.
> Is there any standard method to generate Haskell code out of it? (something
> like "print_this_for_me_please" function? :D
>
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[Haskell-cafe] Fwd: Basic Parsec float & integer parsing question

[Antoine Latter]

Forwarding to the list.


-- Forwarded message --
From: Fredrik Karlsson 
Date: Fri, Jul 5, 2013 at 11:42 AM
Subject: [Haskell-cafe] Basic Parsec float & integer parsing question
To: haskell-cafe@haskell.org


Dear list,

Sorry for asking a simple parsec question, but both Parsec and Haskell
is new to me, so please be gentle :-)

I have this code:


import Text.ParserCombinators.Parsec
import Text.Parsec.Token
import Text.ParserCombinators.Parsec.Char


data VariableLine = VariableLine String String deriving Show
data TierType = IntervalTier | PointTier deriving Show

data Tier = Tier TierType String Float Float Integer
data Label = Interval Float Float String
data LabelFile = LabelFile Float Float

symbol :: Parser Char
symbol = oneOf "!#$%&|*+-/:<=>?@^_~"

testString = "intervals [1]:\nxmin = 0 \nxmax =
0.028192 \ntext = \"\""
headTS1 = "File type = \"ooTextFile\"\nObject class = \"TextGrid\"\n\nxmin ="

header :: Parser LabelFile
header = do
headTS1
start <- float
string "\nxmax = "
end <- float
string "\ntiers? \nsize = "
integer
char '\n'
return $ LabelFile start end



Loading it into ghci I get :

Prelude> :l parsectest.hs
[1 of 1] Compiling Main ( parsectest.hs, interpreted )

parsectest.hs:21:9:
Couldn't match type `[]'
  with `Text.Parsec.Prim.ParsecT
  String () Data.Functor.Identity.Identity'
Expected type: Text.Parsec.Prim.ParsecT
 String () Data.Functor.Identity.Identity Char
  Actual type: [Char]
In a stmt of a 'do' block: headTS1
In the expression:
  do { headTS1;
   start <- float;
   string
 "\
 \xmax = ";
   end <- float;
    }
In an equation for `header':
header
  = do { headTS1;
 start <- float;
 string
   "\
   \xmax = ";
  }

parsectest.hs:22:18:
Couldn't match expected type `Text.Parsec.Prim.ParsecT
String ()
Data.Functor.Identity.Identity Float'
with actual type `GenTokenParser s0 u0 m0
  -> Text.Parsec.Prim.ParsecT s0 u0 m0 Double'
In a stmt of a 'do' block: start <- float
In the expression:
  do { headTS1;
   start <- float;
   string
 "\
 \xmax = ";
   end <- float;
    }
In an equation for `header':
header
  = do { headTS1;
 start <- float;
 string
   "\
   \xmax = ";
  }

parsectest.hs:24:16:
Couldn't match expected type `Text.Parsec.Prim.ParsecT
String ()
Data.Functor.Identity.Identity Float'
with actual type `GenTokenParser s1 u1 m1
  -> Text.Parsec.Prim.ParsecT s1 u1 m1 Double'
In a stmt of a 'do' block: end <- float
In the expression:
  do { headTS1;
   start <- float;
   string
 "\
 \xmax = ";
   end <- float;
    }
In an equation for `header':
header
  = do { headTS1;
 start <- float;
 string
   "\
   \xmax = ";
  }

parsectest.hs:26:9:
Couldn't match expected type `Text.Parsec.Prim.ParsecT
String () Data.Functor.Identity.Identity a0'
with actual type `GenTokenParser s2 u2 m2
  -> Text.Parsec.Prim.ParsecT s2 u2 m2 Integer'
In a stmt of a 'do' block: integer
In the expression:
  do { headTS1;
   start <- float;
   string
 "\
 \xmax = ";
   end <- float;
    }
In an equation for `header':
header
  = do { headTS1;
 start <- float;
 string
   "\
   \xmax = ";
  }
Failed, modules loaded: none.

I'm sure I'm doing something really stupid here, but I need help to
get through this problem. I've used the predefined "letter" parser at
other places in the code, so I can't understand why "float" and
"integer" does not work.

/Fredrik

--
"Life is like a trumpet - if you don't put anything into it, you don't
get anything out of it."

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Re: [Haskell-cafe] Layer on a layer of record syntax in the type synonym?

[Antoine Latter]

You could look into the "Generic Monoid" solution proposed in your
other thread, then you wouldn't need your "Socket" types  - you would
use the "Generic Monoid" machinery to make a Monoid instance for
whatever type needed it.

This approach loses some type-safety, as you might pass on version of
a Scoket3 to a function that was meant to take a different type of
Socket3.

On Fri, Dec 21, 2012 at 4:50 PM, Christopher Howard
 wrote:
> On 12/21/2012 04:52 AM, Daniel Trstenjak wrote:
>>
>> Why having a Socket3 in the first place, what's the point of it?
>>
>
> The idea was to have some generic structures (Sockets) which were
> already instanced into the Monoids-within-Monoids abstraction, yet could
> still be made concrete into anything more specific.
>
> So, I have...
>
> code:
> 
> data Socket3 a b c = Socket3 a b c
>   deriving (Show)
>
> instance (Monoid a, Monoid b, Monoid c) => Monoid (Socket3 a b c) where
> mempty = Socket3 mempty mempty mempty
> Socket3 a b c `mappend` Socket3 w x y =
> Socket3 (a <> w) (b <> x) (c <> y)
>
> nullSocket3 :: (Monoid a, Monoid b, Monoid c) => Socket3 a b c
> nullSocket3 = Socket3 mempty mempty mempty
> 
>
> ...which allows me to have...
>
> code:
> 
> type ShipSys = Socket3 (Last Engine) (Last RotThruster) [LinThruster]
>
> nullShipSys :: ShipSys
> nullShipSys = nullSocket3
>
> setEngineSocket (Socket3 a b c) x = Socket3 x b c
>
> engineSys :: Engine -> ShipSys
> engineSys a = setEngineSocket nullShipSys (Last (Just a))
>
> mk1Engine = engineSys (Engine 100 1 "Mark I")
>
> -- etc.
> 
>
> And so, with each individual component being wrapped as a generic
> ShipSys (ship system), I can make a complete system simply by composition:
>
> code:
> 
> h> :t mk1Engine
> mk1Engine :: ShipSys
> h> :t stdRearThruster
> stdRearThruster :: ShipSys
> h> :t stdFrontThruster
> stdFrontThruster :: ShipSys
> h> :t stdRotThruster
> stdRotThruster :: Power -> ShipSys
> h> mk1Engine <> stdRearThruster <> stdFrontThruster <> stdRotThruster 10
> Socket3 (Last {getLast = Just (Engine 100.0 1.0 "Mark I")}) (Last
> {getLast = Just (RotThruster 10.0)}) [LinThruster 3.1415927
> 1.0,LinThruster 0.0 0.5]
> 
>
> This seems to work well enough so far. But the issue I was concerned
> about is: if I can't layer record syntax onto the type synonym, then I
> have to rewrite a whole bunch of getters / setters each time I want to
> add an attribute (e.g., requiring a switch from a Socket3 to a Socket4.)
> If this is the case, then perhaps it would be better just to define the
> ShipSys type directly, and directly instance it into the monoid abstraction.
>
> --
> frigidcode.com
>
>
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Re: [Haskell-cafe] SOX - play simple

[Antoine Latter]

The example is assuming you have an import statement like this:

import qualified Sound.Sox.Option.Format as Option

On Wed, Nov 28, 2012 at 8:48 AM, Gary Klindt  wrote:
> Dear Cafe,
>
> after installing the Sox library
> (cabal install sox)
> I wanted to let run a minimal example from
> http://hackage.haskell.org/packages/archive/sox/0.2.2.2/doc/html/Sound-Sox-Play.html
>
>
> module Main where
>
> import Sound.Sox.Play
> import Sound.Sox.Signal.List
> --import Sound.Sox.Option.Format
>
> import Data.Int
>
> main = do
> simple Sound.Sox.Signal.List.put
>Option.none
>11025
>(iterate (1000+) (0::Data.Int.Int16))
>
> in this version, I get the error:
>  Not in scope: `Option.none'
>
> So, I imported Sound.Sox.Option.Format, because there is a none with the
> right type. I also changed Option.none to none.
>
> Then the program compiles and I get the runtime error:
> fd:4: hClose: resource vanished (Broken pipe)
>
>
> What is wrong here?
>
> I appreciate your help!
>
> Best regards, Gary
>
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Re: [Haskell-cafe] How do I specify language literals in hsparql?

[Antoine Latter]

On Sun, Oct 21, 2012 at 10:41 AM, Andrew Pennebaker
 wrote:
> If the raw SPARQL is rdfs:label "D (programming language)"@en, what would
> the hsparql syntax be?
>
> The docs don't include any language literal examples.
>

Have you emailed the maintainer of the package? Not all package
authors subscribe to high-traffic lists like haskell-cafe.

Antoine

> --
> Cheers,
>
> Andrew Pennebaker
> www.yellosoft.us
>
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Re: [Haskell-cafe] How to correctly benchmark code with Criterion?

[Antoine Latter]

On Thu, Oct 18, 2012 at 9:06 AM, Janek S.  wrote:
>> So the evaluation will be included in the benchmark, but if "bench" is
>> doing enough trials it will be statistical noise.
>
> When I intentionally delayed my dataBuild function (using delayThread 
> 100) the estimated time
> of benchmark was incorrect, but when I got the final results all runs were 
> below 50ms, which
> means that initial run that took 1 second was discarded. So it seems to me 
> that the first
> evaluation is discarded. Would be good if someone could definitely confirm 
> that.
>

You could email Bryan, the author of criterion.

> Janek

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Re: [Haskell-cafe] How to correctly benchmark code with Criterion?

[Antoine Latter]

On Thu, Oct 18, 2012 at 4:23 AM, Janek S.  wrote:
> Dear list,
>
> during past few days I spent a lot of time trying to figure out how to write 
> Criterion benchmarks,
> so that results don't get skewed by lazy evaluation. I want to benchmark 
> different versions of an
> algorithm doing numerical computations on a vector. For that I need to create 
> an input vector
> containing a few thousand elements. I decided to create random data, but that 
> really doesn't
> matter - I could have as well use infinite lists instead of random ones.
>
> My problem is that I am not certain if I am creating my benchmark correctly. 
> I wrote a function
> that creates data like this:
>
> dataBuild :: RandomGen g => g -> ([Double], [Double])
> dataBuild gen = (take 6 $ randoms gen, take 2048 $ randoms gen)
>
> And I create benchmark like this:
>
> bench "Lists" $ nf L.benchThisFunction (L.dataBuild gen)
>

The argument value will be evaluated by the first run of the
bench-mark, and then laziness will keep the value around for the next
few hundred runs that the "bench" function performs.

So the evaluation will be included in the benchmark, but if "bench" is
doing enough trials it will be statistical noise.

Antoine

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Re: [Haskell-cafe] ANNOUNCE: uuid-1.2.6

[Antoine Latter]

On Fri, Sep 21, 2012 at 12:30 AM, David Fox  wrote:
> I was wondering about this:
>
>   -- My goal with this instance was to make it work just enough to do what
>   -- I want when used with the HStringTemplate library.
>   instance Data UUID where
>   toConstr uu  = mkConstr uuidType (show uu) [] (error "fixity")
>   gunfold _ _  = error "gunfold"
>   dataTypeOf _ = uuidType
>
> Is there any reason not to just say "deriving Data" in the type declaration?
>

I didn't want my 'Data' instance to leak the details of the
constructor. Also I don't know much about how 'Data' works.

I think a similar conversation is going on about the Data declarations
in the 'containers' library.

Antoine

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Re: [Haskell-cafe] What is the good way to work with list comprehension and UTCTime?

[Antoine Latter]

On Sun, Sep 16, 2012 at 5:04 PM, Richard O'Keefe  wrote:
>
> On 15/09/2012, at 5:14 AM, Chris Heller wrote:
>
>> You might want to have a look at the time-recurrence package: 
>> http://hackage.haskell.org/package/time-recurrence
>>
>> For your simple cases you would do something like:
>>
>> Each second:
>>
>> starting (UTCTime ...) $ recur secondly
>>
>> Each minute:
>>
>> starting (UTCTime ...) $ recur minutely
>
> Ouch.  Look up "minutely" (my-NEWT-ly) in an English
> dictionary.  Look up "secondly" while you're there.
>
>

You can blame RFC 5545 for that one. In section 3.3.10. our frequencies are:

freq= "SECONDLY" / "MINUTELY" / "HOURLY" / "DAILY"
   / "WEEKLY" / "MONTHLY" / "YEARLY"

Antoine

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Re: [Haskell-cafe] Data structure containing elements which are instances of the same type class

[Antoine Latter]

It should be pretty easy to write an adapter function of type "String ->
(Show a => a)".
On Aug 11, 2012 12:34 PM, "Patrick Palka"  wrote:

> On Sat, Aug 11, 2012 at 4:14 AM,  wrote:
>
>>
>> I'd like to point out that the only operation we can do on the first
>> argument of MkFoo is to show to it. This is all we can ever do:
>> we have no idea of its type but we know we can show it and get a
>> String.
>>
>
> That's not all you can do: you can also pass the first argument of MkFoo
> to a function that expects a Show a => a argument, like the function
> 'print'. Can you do that with just a String (that represents show x for
> some x)?
>
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Re: [Haskell-cafe] parsec: parserFail & multiple error messages

[Antoine Latter]

On Wed, Aug 8, 2012 at 8:26 PM, silly  wrote:
> Inserting a character into the stream can be expensive if for example
> the stream is a ByteString.
> I tried the following crazy solution and it seems that it works:
>
> succeed :: Parser ()
> succeed = mkPT $ \st ->
> return $ Consumed $ return $ Ok () st $ unknownError st
>
> succeed is a parser that always succeeds without really consuming any
> input but it also resets the error state.
>

Because you're using the 'Consumed' constructor, you're also telling
parsec not the back-track if there any errors following this parsers.

This means that 'succeed >> failingParser' won't backtrack, even if
'failingParser' doesn't consume input.

Are you using your original parser within a larger parser? Are the
error messages also not great?

Antoine

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Re: [Haskell-cafe] Capturing the parent element as I parse XML using parsec

[Antoine Latter]

On Sun, Jul 29, 2012 at 1:21 AM, C K Kashyap  wrote:
> Hi,
>
> With the help of the cafe I've been able to write up the xml parser using
> parsec -
> https://github.com/ckkashyap/really-simple-xml-parser/blob/master/RSXP.hs
>
> I am struggling with an idea though - How can I capture the parent element
> of each element as I parse? Is it possible or would I have to do a second
> pass to do the fixup?
>

What are you trying to do? Maybe you could give an example of what
you'd like to produce?

Generally speaking, having tree elements in a Haskell datatype point
to their parent and their children is asking for trouble - it means
you can't change any part of the tree without re-building the entire
tree (otherwise your parent pointers point to the parent in the old
version of the tree).

If you're interested in complex traversals and transformation of XML
trees, I like the cursor API here:
http://hackage.haskell.org/packages/archive/xml/1.3.12/doc/html/Text-XML-Light-Cursor.html

HaXML is also popular for whole-tree queries and transformations.

Antoine

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Re: [Haskell-cafe] Need help with learning Parsec

[Antoine Latter]

On Sun, Jul 22, 2012 at 11:00 AM, C K Kashyap  wrote:
> What's the function to access it?
>

The function 'runParser' returns either a result or a ParseError. You
can extract the error position with the 'errorPos' function, and then
you can extract the name of the file from the position with
'sourceName'.

The the 'Show' instance of ParseError does this.

Antoine

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Re: [Haskell-cafe] Fwd: hackage compile failure with QuickCheck 2.5

[Antoine Latter]

Cabal doesn't play well with version constraints on the template-haskell
package - it doesn't know it can't reinstall template-haskell.

The workaround is to figure out why QuickCheck has version constraints on
template-haskell and solve that problem in the QuickCheck package a
different way - perhaps with CPP conditonal compilation macros - and then
remove the problematic version constraints.
On Jul 17, 2012 11:44 AM, "Alexander Foremny" 
wrote:

> Dear Levent,
>
> unfortunately I am at a loss here. As far as I understand it this
> should be fixed in QuickCheck's .cabal file or on Hackage. But I am
> not experienced enough to decide.
>
> You best wait for someone else to comment on this. Depending on
> template-haskell in your .cabal file is not the way to go as far as I
> understand it. But maybe it's a possible work-around in case you
> depend on the package being available on Hackage timely.
>
> Regards,
> Alexander Foremny
>
> 2012/7/17 Levent Erkok :
> > It builds fine locally on my box; but not on hackage. Here's the page:
> > http://hackage.haskell.org/package/sbv-2.2
> >
> > Thanks for looking into this Alexander, I appreciate your help.
> >
> > -Levent.
> >
> >
> > On Tue, Jul 17, 2012 at 9:09 AM, Alexander Foremny
> >  wrote:
> >>
> >> Which package are you trying to build? Is it a local package that
> >> fails to build or something on Hackage? Its .cabal file or at least
> >> full dependencies would be of interest.
> >>
> >> Regards,
> >> Alexander Foremny
> >>
> >> 2012/7/17 Levent Erkok :
> >> > Thanks Alexander. However, I'm not sure how to use the workaround
> >> > described
> >> > so I can get hackage to properly compile my package. It sounds like I
> >> > have
> >> > to add a "template-haskell >= 2.7.0.0"  dependency to my own cabal
> file,
> >> > which sounds like the wrong thing to do in the long-run.
> >> >
> >> > Is there something that can be done on the hackage/ghc side to avoid
> >> > this
> >> > issue? Or something less drastic than adding a template-haskell
> >> > dependency
> >> > on my own package's cabal file?
> >> >
> >> > Thanks,
> >> >
> >> > -Levent.
> >> >
> >> >
> >> > On Tue, Jul 17, 2012 at 7:31 AM, Alexander Foremny
> >> >  wrote:
> >> >>
> >> >> Dear Levent,
> >> >>
> >> >> I think this [1] could be related.
> >> >>
> >> >> Regards,
> >> >> Alexander Foremny
> >> >>
> >> >> PS. Sent this to Levent directly. Here's a copy for the mailing list.
> >> >> Sorry for the noise.
> >> >>
> >> >> [1]
> >> >>
> >> >>
> http://haskell.1045720.n5.nabble.com/Bad-interface-problem-td5714184.html
> >> >>
> >> >> -- Forwarded message --
> >> >> From: Alexander Foremny 
> >> >> Date: 2012/7/17
> >> >> Subject: Re: [Haskell-cafe] hackage compile failure with QuickCheck
> 2.5
> >> >> To: Levent Erkok 
> >> >>
> >> >>
> >> >> Dear Levent,
> >> >>
> >> >> I think this [1] could be related.
> >> >>
> >> >> Regards,
> >> >> Alexander Foremny
> >> >>
> >> >> [1]
> >> >>
> >> >>
> http://haskell.1045720.n5.nabble.com/Bad-interface-problem-td5714184.html
> >> >>
> >> >> 2012/7/17 Levent Erkok :
> >> >> > [This message is more appropriate for a hackage mailing list I
> >> >> > presume,
> >> >> > but
> >> >> > that doesn't seem to exist. Let me know if there's a better place
> to
> >> >> > send
> >> >> > it.]
> >> >> >
> >> >> > I'm having a hackage compile failure for a newly uplodaded package
> >> >> > that
> >> >> > has
> >> >> > a QuickCheck 2.5 dependence. The error message is:
> >> >> >
> >> >> > [13 of 13] Compiling Test.QuickCheck.All ( Test/QuickCheck/All.hs,
> >> >> > dist/build/Test/QuickCheck/All.o )
> >> >> >
> >> >> > Test/QuickCheck/All.hs:15:1:
> >> >> > Bad interface file:
> >> >> >
> >> >> >
> >> >> >
> /usr/local/tmp/archive/install/lib/template-haskell-2.6.0.0/ghc-7.4.1/Language/Haskell/TH.hi
> >> >> > Something is amiss; requested module
> >> >> > template-haskell-2.6.0.0:Language.Haskell.TH differs from name
> found
> >> >> > in
> >> >> > the
> >> >> > interface file template-haskell:Language.Haskell.TH
> >> >> >
> >> >> >
> >> >> > The full log file is at (search for "Something is a miss" in it):
> >> >> >
> >> >> >
> http://hackage.haskell.org/packages/archive/sbv/2.2/logs/failure/ghc-7.4
> >> >> >
> >> >> > Needless to say, I don't see this problem when I compile this
> package
> >> >> > at
> >> >> > home with the same compiler (ghc 7.4.1) as hackage is using; also
> >> >> > Hackage
> >> >> > has a successfully compiled QuickCheck 2.5 package.
> >> >> >
> >> >> > Could it be something related to the particular cabal/ghc
> >> >> > installation
> >> >> > on
> >> >> > the hackage server? In particular, I don't understand why it picks
> >> >> > template-haskell 2.6.0.0 when there's a newer version (2.7.0.0). As
> >> >> > far
> >> >> > as I
> >> >> > can see, QuickCheck doesn't put an upper limit on its template
> >> >> > haskell
> >> >> > version dependency.
> >> >> >
> >> >> > I'd appreciate any pointers with this. (Googling and questions on
> th

Re: [Haskell-cafe] strange hangs with -threaded runtime (now with test case)

[Antoine Latter]

If the MVar was set up prior to the fork, I can imagine things going crazy
trying to use it on the ther side of the fork - especially if their were
waiting readers or writers while the fork was executing.
On Jul 14, 2012 3:01 PM, "Joey Hess"  wrote:

> I've isolated the hang further to hPipeFrom's use of System.Log.Logger.
> Commenting out calls to logging functions makes it reliably run ok. See
> the "LINE OF DEATH" in the attached updated test case.
>
> ... And it turns out that System.Log.Logger uses a MVar to hold the
> logger information. So I think this points to a MissingH bug, although I
> don't yet understand it.
>
> --
> see shy jo
>
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Re: [Haskell-cafe] strange hangs with -threaded runtime (now with test case)

[Antoine Latter]

Well, hPipeFrom does indeed call forkProcess internally. I don't fully
understand when it is and is not safe to use 'forkProcess' with the
threaded runtime of GHC.

Which version of GHC are you using?

Antoine

On Sat, Jul 14, 2012 at 1:24 PM, Joey Hess  wrote:
> I've found a minimal test case that seems to demonstrate a bug in either
> MissingH or ghc's threaded runtime. Or I'm doing something stupid in
> fewer lines of code than usual. ;)
>
> When built with the threaded runtime, after a short while it hangs in
> hGetContents.
>
> import System.Cmd
> import System.IO
> import System.Cmd.Utils
> import System.Posix.Process
> import Control.Monad
>
> main :: IO ()
> main = forever $ pipeRead ["hello", "world"]
>
> pipeRead :: [String] -> IO ()
> pipeRead params = do
> -- removing this next line avoids the hang somehow
> print $ "pipeRead in " ++ show params
> (p, h) <- hPipeFrom "echo" params
> print "pipeRead getcontents"
> c <- hGetContents h
> print $ "got: " ++ c
> _ <- getProcessStatus True False $ processID p
> -- removing this last line avoids the hang somehow
> print "pipeRead out"
>
> joey@wren:~>ghc --make -threaded test
> [1 of 1] Compiling Main ( test.hs, test.o )
> Linking test ...
> joey@wren:~>./test
> "pipeRead in [\"hello\",\"world\"]"
> "pipeRead getcontents"
> "got: hello world\n"
> "pipeRead out"
> "pipeRead in [\"hello\",\"world\"]"
> "pipeRead getcontents"
> "got: hello world\n"
> "pipeRead out"
> 
> "pipeRead in [\"hello\",\"world\"]"
> "pipeRead getcontents"
> 
>
> Ghc 7.4.2, Debian Linux
>
> --
> see shy jo
>
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Re: [Haskell-cafe] Question on proper use of Data.IORef

[Antoine Latter]

On Fri, Jun 22, 2012 at 11:18 AM, Captain Freako  wrote:
> Okay, I get it now. Thanks to all of you for your quick replies!
>
> So, here's what I need to do:
>
> My Haskell code gets called by a higher level C function and asked to
> initialize itself.
> As part of that initialization, I'm expected to return a pointer to an
> instance of a particular data structure, which gets created/initialized in
> Haskell.
> I create a stable pointer to a data structure instance, using newStablePtr,
> and return that pointer to the calling C function.
> The same C function then, at a later time, calls a different function in my
> Haskell code, sending it that very same pointer, and expects that code to
> modify the data structure pointed to, in place.
>
> So, I wrote some code that looks like this:
>
> secondFunction :: StablePtr DataStructure -> IO ()
> secondFunction dsPtr = do
>     ds <- deRefStablePtr dsPtr
>     theRef <- newIORef ds
>     writeIORef theRef newDs
>
> which, of course, didn't work, because I didn't understand the true nature
> of IORef.
>
> So, my question is: How do I do this? That is, how do I modify, in place, a
> data structure, which I originally created and made stable, using a pointer
> to that structure, which is being passed back to me, by the higher level C
> function that is calling me?
>

You could use a StablePtr (IORef DataStructure).

> Thanks, all!
> -db
>
>
> On Fri, Jun 22, 2012 at 8:43 AM, Max Rabkin  wrote:
>
>>
>> On Fri, Jun 22, 2012 at 5:30 PM, Captain Freako 
>> wrote:
>> >  12 main = do
>> >  13 let theValue = 1
>> >  14 print theValue
>> >  15 theValueRef <- newIORef theValue
>> >  16 bump theValueRef
>> >  17 return theValue
>>
>>
>> theValue is a plain old immutable Haskell variable. "newIORef" creates
>> an IORef whose initial value is equal to the argument; it doesn't
>> create a pointer to the value or something like that. Change "return
>> theValue" to "readIORef theValueRef" to extract the changed value in
>> the IORef.
>>
>> --Max
>>
>
>
>
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[Haskell-cafe] ANNOUNCE: uuid-1.2.6

[Antoine Latter]

Hi folks,

I'm happy to announce a new point release of the uuid library, 1.2.6:

http://hackage.haskell.org/package/uuid-1.2.6

The 'uuid' package implements most of RFC 4122[1] including random
generation and generation based on hardware MAC addresses.

I haven't announced a point-release in a while. The changes since 1.2.1 include:

* When generating UUIDs from the hardware MAC address, if the MAC
address is not available we now use a random seed for our (hidden,
global) state machine

* The 'Read' instance now drops leading spaces

* Added the functions 'toWords' and 'fromWords', primary to support
the package uuid-quasi[2].

Take care,
Antoine

1: http://tools.ietf.org/html/rfc4122
2: http://hackage.haskell.org/package/uuid-quasi

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Re: [Haskell-cafe] Troubles understanding Parsec Error Handling

[Antoine Latter]

On Wed, May 30, 2012 at 5:47 PM, Roman Cheplyaka  wrote:
>
> With this patch your code prints:
>
>    parse error at (line 1, column 7):
>    unexpected "Hallofb", expecting one of ["Hello","Hallo","Foo","HallofFame"]
>

Hi folks,

Roman's patch has been included in the newly-released parsec 3.1.3:

http://hackage.haskell.org/package/parsec-3.1.3

Enjoy,

Antoine

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Re: [Haskell-cafe] GHC 7.4 and TypeSynonymInstances

[Antoine Latter]

On Fri, Apr 20, 2012 at 12:25 AM, Ivan Lazar Miljenovic
 wrote:
> Has there been a change in the behaviour/requirement of
> TypeSynonymInstances as of GHC-7.4.1? (Not sure if this behaviour
> occurs with 7.2.1 as I don't have it installed)
>
> I had an instance for String for a class which ghc accepted whilst
> using FlexibleInstances; however, when trying to load it in 7.0.* it
> stated that TypeSynonymInstances was needed (which I forgot to add
> since it didn't seem to need it).
>

Previously FlexibleInstances implied TypeSynonymInstances, I think?

I can't find any documentation for the change or a bug report, so I
could be mis-remembering.

Antoine

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Re: [Haskell-cafe] Re-exports of resourcet in conduit

[Antoine Latter]

Is it possible that you are puuling in two different versions of the
package that defines the MonadThrow class?

That is, package a was built against version 1, but package b was built
against version 2? This would make GHC think the type-class were
incompatable.

This is just a guess - I have not tried what you are trying.
On Jun 2, 2012 6:35 PM, "Myles C. Maxfield" 
wrote:

> To: Michael Snoyman
> CC: haskell-cafe
>
> Hello,
> I'm having a problem working with the conduit library, and was hoping
> you could help me out.
>
> Data.Conduit re-exports ResourceT, MonadResource, and MonadThrow (but
> not ExceptionT) from Control.Monad.Trans.Resource. I have a conduit
> which operates on a monad in the MonadThrow class. I am trying to
> figure out which MonadThrow class this should be (the
> Data.Conduit.MonadThrow class, or the
> Montrol.Monad.Trans.Resource.MonadThrow class, since apparently GHC
> doesn't recognize them as the same, even though one is just a
> re-export of the other).
>
> If a user of this conduit wants to chain this conduit up with
> something like sourceFile, the underlying monad has to be a member of
> Data.Conduit.MonadResource and whatever MonadThrow class I chose to
> use. I would like to be able to use an existing instance to lift the
> class of the inner monad to the class of the entire monad stack (so I
> don't have to tell the user of my conduit that they have to define
> their own instances), and the only rule that I can find that does that
> is the following from Data.Conduit:
>
> Data.Conduit.MonadThrow m => Data.Conduit.MonadThrow
> (Data.Conduit.ResourceT m)
>
> However, GHC doesn't seem to think that
> Control.Monad.Trans.Resource.ExceptionT is an instance of
> Data.Conduit.MonadThrow:
>
>No instance for (Data.Conduit.MonadThrow (ExceptionT IO))
>  arising from a use of `.'
>
> Control.Monad.Trans.Resource has a similar instance:
>
> Control.Monad.Trans.Resource.MonadThrow m =>
> Control.Monad.Trans.Resource.MonadThrow
> (Control.Monad.Trans.Resource.ResourceT m)
>
> but because sourceFile operates in the Data.Conduit.MonadResource
> class, and Control.Monad.Trans.Resource.ResourceT isn't a member of
> that class (it's only a member of
> Control.Monad.Trans.Resource.MonadResource), that doesn't help:
>
>No instance for (Data.Conduit.MonadResource
>   (Control.Monad.Trans.Resource.ResourceT (ExceptionT
> IO)))
>  arising from a use of `.'
>
> It should be noted that neither module defines anything like the following:
>
> MonadResource m => MonadResource (ExceptionT m)
>
> It seems like the underlying problem here is that:
> 1) I am required to use the Control.Monad.Trans.Resource.ExceptionT
> class, because Data.Conduit doesn't re-export it
> 2) I am required to use the Data.Conduit.MonadResource class, because
> sourceFile and others require it
> 3) There doesn't seem to be an existing instance that bridges between the
> two.
>
> This seems like a fundamental flaw with re-exporting; it can only work
> if you re-export every single last thing from the original module.
> This doesn't seem tenable because the orignal module might not be
> under your control, so its author can add new symbols whenever he/she
> wants to.
>
> I see two solutions to this problem:
> 1) Re-export Control.Monad.Trans.Resource.ExceptionT in Data.Conduit.
> This will work until someone adds something to the resourcet package
> and someone wants to use the new addition and Data.Conduit.ResourceT
> in the same stack
> 2) Don't re-export anything in Data.Conduit; make sourceFile and
> others explicitly depend on types in another module, but this might
> break compatibility with existing programs if they use fully-qualified
> symbol names.
> 3) Make anyone who wants to use a monad stack in MonadThrow and
> MonadResource define their own instances. This is probably no good
> because it means that many different modules will implement the same
> instance in potentially many different ways.
>
> I feel like option 2) is probably the best solution here. I'm
> perfectly happy to issue a pull request for whichever option you think
> is best, but I don't know which solution you think is best for your
> project. What do you think?
>
> --Myles
>
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Re: [Haskell-cafe] Troubles understanding Parsec Error Handling

[Antoine Latter]

On Wed, May 30, 2012 at 4:18 PM, Kevin Charter  wrote:
> On Wed, May 30, 2012 at 3:11 PM, Kevin Charter  wrote:
>>
>> What version of parsec 3 are you using? In version 3.1.1, I get (using
>> Text.Parsec.String instead of Text.Parsec.Text):
>
>
> Ah, answered my own question. I gather you're using 3.1.2, since it's the
> first and so far only version with the Text.Parsec.Text module.
>

We changed how 'try' handled errors in some cases in between 3.1.1 and
3.1.2. I'll take a look at this.

Antoine

> Kevin
> --
> Kevin Charter
> kevin.char...@acm.org
>
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Re: [Haskell-cafe] Problem with packet documentation generated by cabal on windows 7

[Antoine Latter]

On Fri, May 25, 2012 at 6:28 PM, Nicu Ionita  wrote:
>
>
> I have Haskell Platform 2011.2.0.1 and I assumed that haddock comes with it.
> Now I checked the version - it is 2.9.2 and cabal info tells me that the
> last version is 2.10.0 and that I don't have the package installed (?).
>
> Ok, now I see, haddock is that from the patform and I never install it with
> cabal...
>

Also 'cabal' doesn't track executables, only libraries.

Antoine

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Re: [Haskell-cafe] Template Haskell antiquotation in user-defined quasiquoters

[Antoine Latter]

On Fri, May 25, 2012 at 2:51 PM, Sam Lindley  wrote:
> Template Haskell supports antiquotation for built-in quasiquotes, e.g.:
>
>  [| \x -> x + $([|3 * 4|]) |]
>
> However, as far as I can tell, there is no way of supporting antiquotation
> in user-defined quasiquoters, because the only way to specify a new
> quasiquoter is through a quoteExp function of type String -> Q Exp. Of
> course, it is perfectly possible to write a parser for some fragment of
> Haskell inside your quoteExp function, but that seems crazy given that
> Template Haskell or rather GHC already implements a parser for the whole
> language.
>
> I know about Language.Haskell.Exts.Parser in haskell-src-exts, which
> provides parseExp :: String -> ParseResult Exp, but that Exp is a different
> type to the one provided by Template
> Haskell.
> I'm also aware of Dominic Orchard's syntax-trees package, which supports
> converting between the two representations using a cunning hack that
> pretty-prints the haskell-src-exts representation to a string and uses
> Template Haskell to parse it back.
>
> Is there a saner way of simulating antiquotation in user-defined
> quasiquoters?
>

Have you looked at:

http://hackage.haskell.org/package/haskell-src-exts-qq
http://hackage.haskell.org/package/haskell-src-meta

The might help you pull something together.


Antoine

> Sam
>
>
> --
> The University of Edinburgh is a charitable body, registered in
> Scotland, with registration number SC005336.
>
>
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Re: [Haskell-cafe] Safe Haskell at the export symbol granularity?

[Antoine Latter]

On Thu, May 17, 2012 at 8:50 AM, Ryan Newton  wrote:
> Thanks David.
>
> I'm glad to see it was discussed in the wiki.  (Btw, my 2 cents is that I
> like the comment pragmas more than new keywords.)
>
> The issue that I think doesn't make it into the wiki is of splitting, not
> modules, but type-classes. That's where I think it becomes a more serious
> issue.
>
> Do you think a symbol-level Safe Haskell would be able to distinguish one
> method of a type class as unsafe, while the others are safe?
>

You can still do this at the module level, with the down-side of
potentially not being able to implement a class with the safe version:

> module Unsafe where
>
> class MyClass a where
>   safeOp :: a -> Int -> IO ()
>   unsafeOp :: a -> Int -> IO ()
>
> instance MyClass A where ...


> module Safe
>   (MyClass(safeOp))
>   where
>
> import Unsafe

I think this works.

Antoine

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Re: [Haskell-cafe] Datapacker?

[Antoine Latter]

On Wed, May 9, 2012 at 10:09 PM, sharon kimble  wrote:
> On Thursday 10 May 2012 04:01:56 you wrote:
>> What is datapacker? Can you summarize the sort of problem you are having?
>
> https://github.com/jgoerzen/datapacker/wiki  datapacker is a tool
> to group files by size. It is perhaps most often used to fit a set of files 
> onto
> the minimum number of CDs or DVDs.
>
> I just want to check the syntax of the command before i commit it to stone for
> backing up my mp3s?
>

You should probably ask the author - that link is a part of the
author's personal github page. Here is some contact information:

http://www.complete.org/JohnGoerzen

Antoine

> Thanks
> Sharon.
>>
>> On May 9, 2012 9:34 PM, "sharon kimble"  wrote:
>> > Is this the right place to ask questions about datapacker please? If not,
>> > can
>> > anyone tell me where is please?
>> >
>> > Thanks
>> > Sharon.
>> > --
>
> --
> A taste of linux = http://www.sharons.org.uk/taste/index.html
> efever = http://www.efever.blogspot.com/
> efever = http://sharon04.livejournal.com/
> Debian 6,0.4, Gnome 1:2.30+7, LibreOffice 3.4.6
> Registered Linux user 334501

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Re: [Haskell-cafe] Datapacker?

[Antoine Latter]

What is datapacker? Can you summarize the sort of problem you are having?
On May 9, 2012 9:34 PM, "sharon kimble"  wrote:

> Is this the right place to ask questions about datapacker please? If not,
> can
> anyone tell me where is please?
>
> Thanks
> Sharon.
> --
> A taste of linux = http://www.sharons.org.uk/taste/index.html
> efever = http://www.efever.blogspot.com/
> efever = http://sharon04.livejournal.com/
> Debian 6,0.4, Gnome 1:2.30+7, LibreOffice 3.4.6
> Registered Linux user 334501
>
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Re: [Haskell-cafe] Uploading a new hsc2hs

[Antoine Latter]

On Wed, Apr 25, 2012 at 4:59 PM, Thomas DuBuisson
 wrote:
> Warning:
>
> I, not the maintainer of hsc2hs, will be uploading a trivial fix for
> hsc2hs to hackage (new build deps).  Even after public attempts to
> contact anyone in charge of hsc2hs (last January) there still has been
> no word.  Speak now or forever hold your peace.
>

I don't think I've ever installed hsc2hs from Hackage as it ships with GHC.

It looks like the version on Hackage was last updated in 2006 ... I'm
pretty sure there have been changes to hsc2hs since then.

Antoine

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Re: [Haskell-cafe] prettyprint with IO

[Antoine Latter]

Hi Warren,

On Thu, Apr 12, 2012 at 6:31 PM, Warren Harris  wrote:
> I wrote a parsec parser that does symbols lookups during the parsing process 
> (ParsecT String Store IO a). Now I'd like to write a pretty printer that does 
> the reverse. Unfortunately there doesn't appear to be a transformer version 
> of Text.PrettyPrint.HughesPJ. Can anyone suggest a way to do this? Thanks,

It seems like the opposite would be a function of type 'a -> Store -> IO Doc'.

Maybe a function of type 'a -> ReaderT Store IO Doc' could be easier
to work with.

If you go this route you could write a lifted versions of (<>), (<+>), hcat etc.

An example:

(<>) :: Applicative m => m Doc -> m Doc -> m Doc

I haven't tried any of this, so I'm not sure if you would get any big
win over just using the first suggestion (a function of type 'a ->
Store -> IO Doc') and using the stock combinators and threading the
store around by hand.

But do let me know if something works out.

Antoine

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Re: [Haskell-cafe] using FlexibleInstances and OverlappingInstances

[Antoine Latter]

On Sat, Apr 7, 2012 at 12:08 PM, TP  wrote:
> Hello,
>
> In a module I am writing, I would like to use FlexibleInstances and
> OverlappingInstances.
> But I get errors, so I am trying to reproduce the problems on a smaller
> program:
>

Is your actual issue with Showing a list? If so, you might be better
off using the 'showList' member of the 'Show' typeclass:

instance Show Foo where
   show x = ...
   showList xs = ...

Then your 'showList' method will be called when 'show' is called on a
list of 'Foo' values.

The first error is because 'map show l' is the wrong type - mapping
show over a list will give you a list of strings, but 'show' must
return a string. I think you could use 'concatMap' here.

Other than that the only advice I can give is that I try my hardest to
avoid OverlappingInstances.

Antoine

Antoine

> 
> {-# LANGUAGE FlexibleInstances, OverlappingInstances #-}
>
> data Foo = Foo Int
>            deriving ( Show )
>
> instance Show [Foo] where
>    show [] = "[0]"
>    show l  = map show l
>
> main = do
>    let l = [ Foo 1, Foo 2 ]
>    print l
> 
>
> The first error I obtain is:
> 
> test_overlappinginstances.hs:7:19:
>    Couldn't match expected type `Char' with actual type `[Char]'
>    Expected type: a0 -> Char
>      Actual type: a0 -> String
>    In the first argument of `map', namely `show'
>    In the expression: map show l
> 
>
> Where does this "Char" come from? How to solve this problem?
>
> The second error is:
> 
> test_overlappinginstances.hs:11:5:
>    Overlapping instances for Show [Foo]
>      arising from a use of `print'
>    Matching instances:
>      instance Show a => Show [a] -- Defined in GHC.Show
>      instance [overlap ok] Show [Foo]
>        -- Defined at test_overlappinginstances.hs:5:10-19
> 
>
> The overlap is ok ("overlap ok" does not appear if not using the pragma
> OverlappingInstances), so it should work?
>
> Thanks in advance,
>
> TP
>
>
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Re: [Haskell-cafe] Is there a generic way to detect "mzero"?

[Antoine Latter]

On Mon, Mar 26, 2012 at 4:25 PM, Ting Lei  wrote:
> Hi Antoine and Tobias (and everyone else),
>
> Thanks a lot for your answers. They are really helpful
>
> Can you please show me how to use the (Eq m) constraint to do this?
>
> Also, my general question (probably novice-level) is that in monadic
> programming, you can convert not necessarily monadic codes into monadic
> ones.
> I know for many cases, it is impossible to do the reverse conversion, e.g.
> you can't make a function involving real IO operations into a pure code.
> In other cases, for example, I may need to using things like Nothing as the
> "null" value as in other programming languages, just to represent a special
> "missing" value outside the regular type.
> Is mzero a reasonable replacement for this or is there any reasonable
> (abstract) approximation in Haskell for doing this? (Like "null", I need the
> ability to detect it.)

I think using 'Maybe' (with Nothing) is perfect for this - this
function should come in handy:

http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Maybe.html#v:isNothing

Antoine

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Re: [Haskell-cafe] Is there a generic way to detect "mzero"?

[Antoine Latter]

On Mon, Mar 26, 2012 at 1:33 PM, Ting Lei  wrote:
> Hi,
>
> I was writing a code trying to use MonadPlus to detect some error cases
> (representing missing values etc. in pure code). With the Maybe monad, I can
> do this:
>
> can0 :: (a -> Maybe b) -> a -> Bool
> can0 f x = case f x of
>   Nothing -> False
>   Just  x -> True
>
> And I got the expected result:
>
> *Main> can0 (\x -> Just x) 1
> True
>
> But, when I try to generalize this using MonadPlus, as follows:
>
> can :: (MonadPlus m) => (a -> m b) -> a -> Bool
> can f x = case f x of
>     mzero -> False
>     _ -> True
>
>
> I got a warning:
>
> __testError.hs:31:11:
>     Warning: Pattern match(es) are overlapped
>  In a case alternative: _ -> ...
> Ok, modules loaded: Main.
>

Well, you can sort of do it with only MonadPlus - but it really
depends on your choice of Monad whether or not it does anything like
what you want:

can :: (MonadPlus m) => (a -> m ()) -> a -> m Bool
can f x = (f x >> return True) <|> return false

For 'Maybe' this works great, but for something like 'List' I couldn't
even tell you what it would do without reasoning through it.

So you might be better off with the suggestion from Tobias using Eq

Antoine

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Re: [Haskell-cafe] Quickest way to pass Text to C code

[Antoine Latter]

On Wed, Mar 21, 2012 at 3:35 AM, Yves Parès  wrote:
> Hello,
>
> I have to interact with a C++ library that accepts as string types (putting
> c++ strings aside) pointers of wchar_t (CWString in Haskell) or unsigned
> 32-bit int (Ptr Word32 for UTF-32 codepoints).
>
> I have read what text, bytestring and base provide, but Text can only be
> directly converted to (Ptr Word16), and if I use encodeUTF32 to get a
> ByteString, then I only get useAsCString, no direct conversion to CWString
> or Ptr WordXX is possible.

A CString is a (Ptr CChar). You can then use castPtr to get whichever
pointer type you need, if you believe the underlying buffer has the
representation you want (in this case, UTF-32).

It still won't be null-terminated, however.

Antoine

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Re: [Haskell-cafe] cabal test-suite library recompilation

[Antoine Latter]

On Tue, Mar 13, 2012 at 11:16 AM, Ozgur Akgun  wrote:
>
> While waiting for a build to finish [ :) ], I just wanted to write an email
> and check if this is intentional or only an oversight. Or maybe a technical
> limitation for the time being?
>

If your library code and test code are in separate sub-directories and
you reference your library as a package dependency for your test then
Cabal won't re-build your library.

That is, the 'Hs-source-disr' for the library and the test should not
overlap. Otherwise GHC will compile the file-on-disk instead of
grabbing the module from a package.

Antoine

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Re: [Haskell-cafe] Impact of "try" on Parsec performance

[Antoine Latter]

On Fri, Mar 2, 2012 at 10:30 PM, Omari Norman  wrote:
> The Parsec documentation says that Parsec performs best on predictive
> grammars, and Parsec does not backtrack by default to improve performance
> (though this also improves error messages).
>

A big thing that parsec strives to do is to "forget" consumed portions
of the input as quickly as possible, allowing it to be collected.

Using 'try' forces the program to hold on to input for longer than
would be required if the grammar were factored to avoid 'try'.

> The Parsec documentation says there is a penalty for using try and suggests
> left-factoring grammars when possible. Real World Haskell says that
> excessive use of try can degrade Parsec performance quite significantly. On
> the other hand, some of the combinators provided with Parsec, such as
> notFollowedBy, use try.
>

The assumption is that 'notFollowedBy' is primarily used with 'short'
parsers, which means the amount of input we need to preserve is less
(also, I think using 'try' is the only way to implement
'notFollowedBy').

> So my question is: what is the practical impact of using try? I ask because
> as a novice I have a simple grammar that could, perhaps, be factored to
> avoid use of try, but it's quite a brain puzzle for me and I wonder if it
> would ever be worth it. Does it matter how many characters "try" would have
> to store, or how often it would run?
>

I guess you could test running a parser which always fails without
consuming input both underneath a 'try' and not, to see what
non-storage overhead 'try' has.

I'm guessing the overhead would be minimal, but I have not measured it.

Antoine

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Re: [Haskell-cafe] Impact of "try" on Parsec performance

[Antoine Latter]

On Fri, Mar 2, 2012 at 10:35 PM, Erik de Castro Lopo
 wrote:
> Omari Norman wrote:
>
>> So my question is: what is the practical impact of using try?
>
> My experience is based mostly on trying to improve the error location
> reporting in Ben Lippmeier's DDC compiler. What I found was that
> removing Parsec.try's almost always resulted in the parser generating
> better location information.
>

Roman Cheplyaka made some tweaks to 'try' error reporting in parsec
3.1.2, so this might be different now.

Antoine

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Re: [Haskell-cafe] Compressed Data.Map for more efficient RAM usage?

[Antoine Latter]

On Thu, Feb 16, 2012 at 4:29 PM, Johan Tibell  wrote:
> On Thu, Feb 16, 2012 at 2:03 PM, Antoine Latter  wrote:
>> You could have a re-implemented HashMap which would un-pack the
>> payload's ByteString constructor into the leaves of the HashMap type
>> itself.
>>
>> Then you would save on both the keys and the values.

Hah hah - forget what I said. HashMap may be smaller in terms of the
structure of the tree it builds, but it can't be smaller in the size
of its keys since the hash is lossy :-/

>
> Note that ByteString has a high per-value overhead due to the internal
> use of ForeignPtrs and indicies that track offset/size:
> http://blog.johantibell.com/2011/06/memory-footprints-of-some-common-data.html
>

You could store just the raw GHC ByteArray, which drops the indices.
You could then promote to a ByteString to call into standard
deserialization libraries. I don't know at what point we descend too
far into voodoo-magic.

> -- Johan

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Re: [Haskell-cafe] Compressed Data.Map for more efficient RAM usage?

[Antoine Latter]

On Thu, Feb 16, 2012 at 3:51 PM, Jeremy Shaw  wrote:
> Sometimes we  want to store very large collection types in RAM -- such as a
> Data.Map or Data.IxSet.
>
> It seems like we could trade-off some speed for space savings by compressing
> the values in RAM.
>
> Lemmih has previously created compact-map:
>
>  http://hackage.haskell.org/package/compact-map
>
> which mightybyte used to create:
>
> https://github.com/mightybyte/compact-ixset
>
> compact-map converts the Haskell values to a more compact binary
> representation. But could we do even better by compressing the bytestrings?
>
> Here is a useless implementation:
>
> http://hpaste.org/63836
>

You could have a re-implemented HashMap which would un-pack the
payload's ByteString constructor into the leaves of the HashMap type
itself.

Then you would save on both the keys and the values.

This assumes you're not using the ordering provided by Data.Map.

Antoine

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Re: [Haskell-cafe] Haskell-Cafe Tag (was: Optimizations and parallel execution in the IO for a small spellchecker)

[Antoine Latter]

On Wed, Feb 15, 2012 at 1:49 PM, JP Moresmau  wrote:
> I was not aware that it was something a poster was supposed to do, but
> now I notice that while the messages I get from the list have
> [Haskell-Cafe] on their subject, the ones I post don't. The
> Haskell-Cafe info page
> (http://www.haskell.org/mailman/listinfo/haskell-cafe) does not
> mention that you should have that tag in the subject. I'm probably a
> uncouth Frenchman knowing nothing about netiquette, but surely the
> list software could add such a tag if it was required?
>

The tag appears to be added automatically by the mailing-list software
that haskell-cafe runs on. I see it on threads that you have started,
so there is no problem with your emails.

If you add it manually, it looks like the software won't double-add it.

Antoine

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Re: [Haskell-cafe] Building a Haskell project which depends on an old version of base

[Antoine Latter]

On Tue, Feb 14, 2012 at 3:53 PM, Rogan Creswick  wrote:
> On Tue, Feb 14, 2012 at 12:14 PM, Steven J. Murdoch
>  wrote:
>> $ ghc-pkg list base
>> /Users/ghc6/homebrew/Cellar/ghc/6.12.3/lib/ghc/package.conf.d
>>   base-3.0.3.2
>>   base-4.2.0.2
>
> I'm a bit fuzzy on the details when it comes to the core packages that
> are distributed with ghc, but my understanding is that the compiler is
> /very/ needy about which base it can use.  As far as I know, the only
> way to change versions of Base is by changing the compiler to the
> version that uses the desired base.
>
> With that in mind, having two base libraries installed in the same
> package database seems like a recipe for disaster.
>

GHC 6.12.1 shipped with two versions of base, so this is okay in this case.

But I really don't know if we export two different versions of GHC.*
from the different versions of base.

Antoine

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Re: [Haskell-cafe] [haskell-cafe] Some reflections on Haskell

[Antoine Latter]

On Tue, Feb 14, 2012 at 9:13 AM, Doug McIlroy  wrote:
>
> Nevertheless, I share Jardine's concern about the central problem.
> It is hard to find one's way in this ecosystem.  It needn't be,
> as Java illustrates.  To my mind Java's great contribution to the
> world is its library index--light years ahead of typical
> "documentation" one finds at haskell.org, which lacks the guiding
> hand of a flesh-and-blood librarian.  In this matter, it
> seems, industrial curation can achieve clarity more easily than
> open source.
>

Hi Doug,

I'm not really familiar with the Java ecosystem - most of my
experience has involved a couple Apache projects, which had decent
wikis next to really terrible raw API documentation (with links from
the wikis into the raw API documentation as if it were a useful
thing).

But I assume my experience isn't typical - I'm not a Java dev.

Do you have any links to examples that we should imitate? Or a summary
of how it works "over there"?

Thanks,
Antoine

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Re: [Haskell-cafe] Hackage 2 maintainership

[Antoine Latter]

On Mon, Feb 13, 2012 at 10:17 PM, Antoine Latter  wrote:
> On Mon, Feb 13, 2012 at 5:44 PM, Ben Gamari  wrote:
>> Hey all,
>>
>> Those of you who follow the Haskell subreddit no doubt saw today's post
>> regarding the status of Hackage 2. As has been said many times in the
>> past, the primary blocker at this point to the adoption of Hackage 2
>> appears to be the lack of an administrator.
>>
>> It seems to me this is a poor reason for this effort to be held
>> up. Having taken a bit of time to consider, I would be willing to put in
>> some effort to get things moving and would be willing to maintain the
>> haskell.org Hackage 2.0 instance going forward if necessary.
>>
>> I currently have a running installation on my personal machine and
>> things seem to be working as they should. On the whole, installation was
>> quite trivial, so it seems likely that the project is indeed at a point
>> where it can take real use (although a "logout" option in the web
>> interface would make testing a bit easier).
>>
>
> One thing that made testing hard for me last time was that existing
> 'cabal install' client treats the server "hackage.haskell.org"
> different from any other server you put in your config files, so
> simulating the effect of a cutover on existing users can be tricky.
>

Oh, and it isn't absolutely terrible - it only affects the 'upload'
command, I think (and the upload + check).

> If you don't know what I'm talking about I can look it up. I had a
> hacked 'cabal' client at one point, and I could probably re-create the
> patches.
>
> Antoine

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Re: [Haskell-cafe] Hackage 2 maintainership

[Antoine Latter]

On Mon, Feb 13, 2012 at 5:44 PM, Ben Gamari  wrote:
> Hey all,
>
> Those of you who follow the Haskell subreddit no doubt saw today's post
> regarding the status of Hackage 2. As has been said many times in the
> past, the primary blocker at this point to the adoption of Hackage 2
> appears to be the lack of an administrator.
>
> It seems to me this is a poor reason for this effort to be held
> up. Having taken a bit of time to consider, I would be willing to put in
> some effort to get things moving and would be willing to maintain the
> haskell.org Hackage 2.0 instance going forward if necessary.
>
> I currently have a running installation on my personal machine and
> things seem to be working as they should. On the whole, installation was
> quite trivial, so it seems likely that the project is indeed at a point
> where it can take real use (although a "logout" option in the web
> interface would make testing a bit easier).
>

One thing that made testing hard for me last time was that existing
'cabal install' client treats the server "hackage.haskell.org"
different from any other server you put in your config files, so
simulating the effect of a cutover on existing users can be tricky.

If you don't know what I'm talking about I can look it up. I had a
hacked 'cabal' client at one point, and I could probably re-create the
patches.

Antoine

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Re: [Haskell-cafe] Stable pointers: use of cast to/from Ptr

[Antoine Latter]

On Sun, Feb 12, 2012 at 3:09 PM, Yves Parès  wrote:
> But then,
> In use case 1), how can a Haskell function modify the data addressed?
>

http://hackage.haskell.org/packages/archive/base/latest/doc/html/Foreign-StablePtr.html#v:deRefStablePtr

Antoine

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Re: [Haskell-cafe] Stable pointers: use of cast to/from Ptr

[Antoine Latter]

On Sun, Feb 12, 2012 at 8:18 AM, Yves Parès  wrote:
> Hello,
>
> According to the documentation
> (http://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html/Foreign-StablePtr.html),
> StablePtrs aims at being opaque on C-side.
> But they provide functions to be casted to/from regular void*'s.
> Does that mean if for instance you have a StablePtr CInt you can cast it to
> Ptr () and alter it on C-side?
>
> void alter(void* data)
> {
>     int* x = (int*)data;
>     *x = 42;
> }
>
> --
>
> -- using 'unsafe' doesn't change anything.
> foreign import ccall safe "alter"
>     alter :: Ptr () -> IO ()
>
> main = do
>     sptr <- newStablePtr (0 :: CInt)
>     deRefStablePtr sptr >>= print
>     alter (castStablePtrToPtr sptr)  -- SEGFAULTS!
>     deRefStablePtr sptr >>= print
>     freeStablePtr sptr
>
>
> But I tried it, and it doesn't work: I got a segfault when 'alter' is
> called.
>

I think that 'castStablePtrToPtr' exists because many C APIs use
'void*' to mean 'opaque lump of data', and these exist to conform to
that sort of API.

Antoine

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Re: [Haskell-cafe] Happstack-state in non-happstack production apps

[Antoine Latter]

On Fri, Feb 10, 2012 at 3:56 PM, Tom Murphy  wrote:
> Hi,
>     Is it common to use happstack-state without happstack for
> real-world code (web or otherwise)?
>

The package 'acid-state' is considered the successor to
happstack-state, and it doesn't include 'happstack' in it's name to
advertise that it is quite usable outside of happstack.

I don't have a list of who does, though.

http://hackage.haskell.org/package/acid-state

Antoine

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Re: [Haskell-cafe] Contributing to http-conduit

[Antoine Latter]

On Tue, Feb 7, 2012 at 10:28 PM, Myles C. Maxfield
 wrote:
> I have been looking around at possibly making a Browser module for
> Network.HTTP.Conduit on top of Control.Monad.State. I came across this
> roadbump:
>
> In order to implement redirection following, client code must call 'http'
> with a redirection count of 0. If there is a redirect, 'http' will throw an
> exception. In order to catch this exception and continue on with the
> redirection chain, the 'catch' function must be called. The problem is that
> the 'catch' function has a type of (catch :: Exception e => IO a -> (e -> IO
> a) -> IO a) which means that it can only be used in the IO monad. A call to
> 'http' inside the first argument of 'catch' must be wrapped in a
> 'runResourceT'
>

Does this help?

http://hackage.haskell.org/packages/archive/lifted-base/0.1.0.3/doc/html/Control-Exception-Lifted.html

It should handle part of your problem, at least.

Antoine

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Re: [Haskell-cafe] The State of Testing?

[Antoine Latter]

On Thu, Feb 2, 2012 at 4:30 PM, Michael Craig  wrote:
> I've been picking up Haskell as a side project for the last few months, and
> I'm now considering publishing some useful code I've written and reused in
> several small projects. So far I've done relatively little with testing
> (these have been non-mission-critical applications) but I feel I should get
> my act together before pushing my work into the common ecosystem.
>
> I'm comfortable writing tests in QuickCheck and HUnit and bundling them as
> optional executables with cabal, but I understand there's a better
> way. Specifically, I'm looking at the test-framework package and cabal's
> (newish) test-suite sections. Are these two used together or each to the
> exclusion of the other? Is there something else I should be using?
>

I use Cabal's test-suite support to make building and running a test
executable easy, and I use test-framework to write my 'main' method
and provide nice output to the user.

They have worked well together for me.

Antoine

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Re: [Haskell-cafe] help with safecopy + acid-state

[Antoine Latter]

On Tue, Jan 31, 2012 at 8:27 AM, Johannes Waldmann
 wrote:
>
>> > Can I really rename  old.T => new.T_orig ?
>> > It looks as if then tries to load the wrong acid-state snapshot.
>>
>> The name of your data type doesn't matter as acid-state doesn't store
>> that on the disk.
>
> I think it does - because file names are  state/T/*.log   and so on?
>

The function 'openLocalState' in AcidState uses the name of the passed
in state type to locate the log files on disk.

So as long as you always call 'openLocalState' with types of the same
name to represent the same state you'll be fine - this is why it is
safe to rename your old type, because you call 'openLocalState' with
the new type.

Alternatively, you can call 'openLocalStateFrom', which doesn't base
anything on names of types (you can tell because there is no
'Typeable' constraint on its arguments).

Antoine

> J.W.
>
>
>
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Re: [Haskell-cafe] [Haskell] ANNOUNCE multiarg - parser combinators for command line parsing

[Antoine Latter]

On Mon, Jan 30, 2012 at 8:19 AM, Henning Thielemann
 wrote:
>
> On Sun, 29 Jan 2012, Simon Meier wrote:
>
>> I'm currently using Neil Mitchell's cmdargs package [1]. How does your
>> package compare to that?
>
>
> Last time I checked cmdargs it was not referential transparent. Is multiarg
> better in this respect?
>

It has since been re-architectured as an impure library around a pure
core library:

http://hackage.haskell.org/packages/archive/cmdargs/0.9.2/doc/html/System-Console-CmdArgs-Explicit.html

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Re: [Haskell-cafe] C++ Parser?

[Antoine Latter]

On Tue, Jan 24, 2012 at 4:06 AM, Christopher Brown
 wrote:
> Hi,
>
> I have stumbled across language-c on hackage and I was wondering if anyone is 
> aware if there exists a full C++ parser written in Haskell?
>

I'm not aware of one.

When it comes to parsing C++, I've always been a fan of this essay:
http://www.nobugs.org/developer/parsingcpp/

It's a hobbyist's tale of looking into parsing C++ and then an
explanation of why he gave up. It's older, so perhaps the
state-of-the-art has advanced since then.

Antoine

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Re: [Haskell-cafe] In-memory Handle for testing

[Antoine Latter]

On Fri, Jan 20, 2012 at 5:53 AM, Simon Hengel  wrote:
> For testing I want to stub handles, performing all reads and writes in
> memory (and in process, so no mmap).  From looking at the documentation
> of mkFileHandle[1], I think this should be possible.  But it requires
> some work.  Is there already something out there?
>

John Miliken has one on Hackage:

http://hackage.haskell.org/package/knob

I had a read-only version that I used for testing things a ways back:

http://hackage.haskell.org/trac/ghc/attachment/ticket/4144/ByteStringHandle.hs

There might still be some things in GHC.IO.Handle that assume FD
handles - I haven't tried it in a while.

Antoine

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Re: [Haskell-cafe] Not an isomorphism, but what to call it?

[Antoine Latter]

On Thu, Jan 19, 2012 at 3:24 PM, Sean Leather  wrote:
> I have two types A and B, and I want to express that the composition of two
> functions f :: B -> A and g :: A -> B gives me the identity idA = f . g :: A
> -> A. I don't need g . f :: B -> B to be the identity on B, so I want a
> weaker statement than isomorphism.
>
> I understand that:
> (1) If I look at it from the perspective of f, then g is the right inverse
> or section (or split monomorphism).
> (2) If I look at from g, then f is the left inverse or retraction (or split
> epimorphism).
>
> But I just want two functions that give me an identity on one of the two
> types and I don't care which function's perspective I'm looking at it from.
> Is there a word for that?
>

I don't think it makes sense to say you want one label for the
situation when looking from either end - the relation you're labeling
is non-symmetric.

> Regards,
> Sean
>
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[Haskell-cafe] ANNOUNCE: quickcheck-instances

[Antoine Latter]

Hello!

I'd like to announce the first release of the quickcheck-instances
package, which aims to consolidate commonly needed class instances for
use with QuickCheck.

These instances are appropriate when your tests don't have strong
requirements on the nature of the input data - for example, if you are
testing a network-protocol parser, the ByteString instance provided in
this package might not be what you want to use.

Other limitations:

* I do not have full coverage of the Haskell Platform - I have been
adding instances as I need them. Patches are warmly welcomed, however.
* I only have instances of the 'Arbitrary' class, however I would be
happy to accept patches for instances of CoArbitrary and other classes
that would be useful in quickcheck-based testing.

On Hackage: http://hackage.haskell.org/package/quickcheck-instances

Ordinarily I frown on orphaned instances in library packages, but the
fact that quickcheck properties should never appear in a library Cabal
package, most of the downsides of orphans are mitigated.

Thanks,
Antoine

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Re: [Haskell-cafe] STM: "nested atomically" error

[Antoine Latter]

On Thu, Jan 12, 2012 at 6:48 AM, Johan Brinch  wrote:
> Hi all,
>
> I'm seeing the "Control.Concurrent.STM.atomically was nested" error,
> but I just can't figure out what's happening. I'm not using any unsafe
> IO (only for debug printing), and the test program is only running one
> thread. Where is a transaction being nested?
>
> What are the scenarios where this error is reported?
>

Where is 'evalCacheSTM' defined?

> The behaviour is consistent on GHC 7.4.0 RC and GHC 7.2.2 (stable).
>
> That function that's being run can be found here:
> https://gist.github.com/30b94760abc27b05ec7c
>
> And here is the last output from the runtime system:
> https://gist.github.com/4356ae541895becb4169
>
>
> Any ideas to track this down will be greatly appreciated!
>
> --
> Johan Brinch,
> Dept. of Computer Science,
> University of Copenhagen
>
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Re: [Haskell-cafe] When will constraint kinds roll out?

[Antoine Latter]

On Tue, Jan 10, 2012 at 1:04 PM, Nicholas Tung  wrote:
> Just a quick, logistical question: I see constraint kinds didn't make it to
> GHC 7.2.2; does anyone have guesses when the extension will roll out in an
> official release? Our research team is interested in using Haskell for an
> EDSL, and having both constraint kinds and access to all of the libraries on
> Hackage would be optimal. (I've been writing an EDSL with monad transformers
> and the rmonad package myself, but think it is too difficult and cumbersome
> if we are to use Haskell EDSL approaches more broadly).
>

Constraint kinds are in the release branch for the upcoming 7.4
release, and were in the build of 7.4.1 RC 1 that went out a bit ago:

http://www.haskell.org/pipermail/glasgow-haskell-users/2011-December/021310.html

I'm not on the GHC team, but from what I've seen new features like
this generally don't get added in point releases (something like from
7.1.1 to 7.2.2).

Antoine

> Thanks in advance,
> Nicholas — https://ntung.com — 4432-nstung
>
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Re: [Haskell-cafe] "too many open files" using snap

[Antoine Latter]

2012/1/8 Eric Wong :
> I run both of these commands and found out that there're sockets leaking.
> Most of them are TCP connections in CLOSE_WAIT state. There're also some
> socket with "can't identify protocol".
>
> So, What's the problem? Is it related to the timeout in the server config?
> I'm using the default value.
>
>

Since they're sockets, it could be either the database connections or
the HTTP connections, right? Is there some way you can insert a fake
database layer to see if that makes your problem go away, to try and
isolate the problem?

Antoine

> 在 2012-1-8，下午10:56， Brandon Allbery 写道：
>
> On Sun, Jan 8, 2012 at 09:50, Felipe Almeida Lessa 
> wrote:
>>
>> If you're on Linux, then you may
>>
>>  $ ls -lah /proc/PID/fd
>>
>> to see where these open files are pointing to.
>
>
> And on other systems lsof can determine this information.
>
>     $ lsof -p PID
>
> --
> brandon s allbery                                      allber...@gmail.com
> wandering unix systems administrator (available)     (412) 475-9364 vm/sms
>
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Re: [Haskell-cafe] [web-devel] [ANNOUNCE] First release of crypto-conduit

[Antoine Latter]

On Sun, Jan 8, 2012 at 12:39 AM, Thomas DuBuisson
 wrote:
>> Why? I don't actually need the hash object for anything, usually. All
>> I need is the ByteString, and then I need to learn how to use the
>> cereal package to get it...
>
> What would you think if Crypto.Classes exported Data.Serialize.encode?

This suggestion is probably a win even if you do nothing else.

>  Or how about if it exported Antoine's hash suggestion (under a
> different name)?
>
> Cheers,
> Thomas
>
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Re: [Haskell-cafe] [web-devel] [ANNOUNCE] First release of crypto-conduit

[Antoine Latter]

On Sat, Jan 7, 2012 at 10:12 PM, Aristid Breitkreuz
 wrote:
> 2012/1/8 Vincent Hanquez :
>> What would you prefer ?
>>
>> At the moment, i'm inclined to someday move cryptohash apis to be similar to
>> crypto-api. i.e. from a result type being a bytestring to an opaque type
>> with serialize/show instance.
>>
>
> Why? I don't actually need the hash object for anything, usually. All
> I need is the ByteString, and then I need to learn how to use the
> cereal package to get it...
>
> So talking purely about convenience, I would prefer hash :: HashType
> -> InputData -> ByteString. That would probably not be extensible
> enough, so I'm not sure how to optimally do it.
>

You could use 'hash :: Hash h => Proxy h -> ByteString -> ByteString'.

where 'data Proxy a = Proxy'. Defined for you in the 'tagged' package.

Antoine

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[Haskell-cafe] Package for QuickCheck instances

[Antoine Latter]

Hi Haskell,

I was writing some tests that involved a large number of quickcheck
properties which don't ship with the library itself, so I thought I
would package them all together and put the orphan instances on
Hackage. Here's what I have so far:

https://github.com/aslatter/qc-instances/tree/93a87fa06b5575ddcc12c2f9eb0f4672c6f9a317

The policy would be to allow anything into to package which is a part
of the Haskell Platform or is a GHC build library.

Has anyone already done this? I didn't find anything from grepping the
Hackage package list.

Has anyone already talked about doing this sort of thing? Are their
any non-obvious pitfalls (aside from the package eventually becoming a
cesspool of CPP conditions)?

Thanks,
Antoine

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Re: [Haskell-cafe] Sudden monomorphism with -XTypeFamilies

[Antoine Latter]

On Thu, Jan 5, 2012 at 3:32 PM, Rian Hunter  wrote:
> hello
>
> i'm getting inconsistent monomorphism behavior with the same code only
> depending on whether or not -XTypeFamilies is enabled:
>

Which version of GHC are you using?

Starting with GHC 7.0, the TypeFamilies extension implies the
MonoLocalBinds language feature, which looks like what you're running
into. Here's a blog post from the GHC folks about it:

http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7

Antoine

> 
> {-# LANGUAGE TypeFamilies #-}
>
> snda :: a -> b -> b
> snda ba = id
>
> main = do
>  let ma = (return () :: IO ())
>  mBox bb = snda ma bb
>
>  mBox $ return (4 :: Int)
>  mBox $ return "G"
>
>  return ()
> 
>
> in the preceding example, if -XTypeFamilies is enabled then "mBox" is
> monomorphic and the program will terminate early, otherwise,
> it's polymorphic and the program will complete successfully.
>
> i think i understand why this is the case but i couldn't find documentation
> on this inconsistency anywhere. is this expected behavior or is this a bug
> in GHC? thanks!
>
> rian
>
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Re: [Haskell-cafe] (no subject)

[Antoine Latter]

On Thu, Jan 5, 2012 at 10:54 AM, Christoph Breitkopf
 wrote:
> Hello,
>
> I'm trying to figure out how to handle versioning of my IntervalMap
> package. I've just read the package versioning
> policy: http://www.haskell.org/haskellwiki/Package_versioning_policy
>
> I don't quite understand all the recommendations in the above document,
> though:
>
> a) You are not allowed to remove or change the types of existing stuff. Ok.
>
> b) You are allowed to add new functions. But that can break compilation
> because of name conflicts. Seems to be allowed on the grounds that this is
> easy to fix in the client code.

This will never break clients who are using qualified imports, or only
importing the symbols they use, which is strongly recommended
behavior.

>
> c) You are not allowed to add new instances. I don't get this - how is this
> any worse than b)?

Unlike adding functions, there is no way for a client of your library
to control which instances they import.

Antoine

>
> I do understand that it is not generally possible to prevent breaking code
> - for example if the client code depends on buggy behavior that gets fixed
> in a minor version update. That seems unavoidable - after all, bugfixes are
> _the_ reason for minor updates.
>
> Regards,
>
> Chris
>
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Re: [Haskell-cafe] [Haskell-beginners] WinRT and Haskell

[Antoine Latter]

On Thu, Jan 5, 2012 at 9:55 AM, AbdulSattar Mohammed
 wrote:
> WinRT has a concept of projections that expose its API. Microsoft has
> implemented projections for Native (C and C++), HTML/Javascript and .NET
> (from Miguel de Caza's WinRT demystified
> post: http://tirania.org/blog/archive/2011/Sep-15.html). There's no mention
> of creating your own projections (or I have seen none). I don't see any
> reason why we can't create our own projections. If it's possible a Haskell
> projection could really help.
>

Yep, that's what I was thinking of.

I'm pretty sure the C/C++ projections rely on compiler extensions for
the MS C/C++ compiler, so I don't think we could use those directly
from Haskell.

Antoine

>
> On Thu, Jan 5, 2012 at 8:11 PM, Antoine Latter  wrote:
>>
>> On Thu, Jan 5, 2012 at 5:12 AM, AbdulSattar Mohammed
>>  wrote:
>> > I suppose this should go into the GUI mailing list, but it is filled
>> > with
>> > spam. So, WinRT does not depend on the .NET Framework. C++ applications
>> > can
>> > directly compile to x86 and be able to use WinRT. Do we have a room for
>> > Haskell development there?
>> >
>>
>> From what I understand, WinRT is a set of COM libraries - C++ can
>> directly compile to it because the MS C++ compiler has special
>> extensions to handle the COM resources in the library.
>>
>> There is a COM library for Haskell:
>>
>> http://hackage.haskell.org/package/com
>>
>> But I've never used it.
>>
>> The API is encoded in a special meta-data format, which is then used
>> by C++, .NET and Javascript to create the language-specific APIs - it
>> might be possible to generate Haskell bindings to the COM components
>> from this metadata.
>>
>> Antoine
>
>
>
>
> --
> Warm Regards,
>
> AbdulSattar Mohammed

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Re: [Haskell-cafe] haxr standalone server?

[Antoine Latter]

On Thu, Jan 5, 2012 at 9:40 AM, Johannes Waldmann
 wrote:
> How could I use haxr (http://www.haskell.org/haskellwiki/HaXR)
> to build a stateful server?
>
> It should listen on some port,
> and fork threads (inside Haskell land) to handle incoming calls.
> Any of the Haskell web frameworks can do this?
>
> I guess this is the same question as:
> http://www.haskell.org/pipermail/haskell-cafe/2009-December/071185.html
>

Pretty much any of the Haskell web frameworks listen on a port, accept
HTTP requests and fork a new GHC thread into a handler. I'm more
familiar with the Happstack/Snap approach to writing handlers
(although the Snap approach is evolving away from Happstack), and a
lot of people have good luck with Yesod.

Happstack crash course: http://happstack.com/docs/crashcourse/index.html
Snap quickstart: http://snapframework.com/docs/quickstart
Installing and starting yesod in five minutes:
http://www.yesodweb.com/page/five-minutes

In particular, I know that the folks working on yesod have spent a lot
of time on documentation.

Antoine

> Thanks - J.W.
>
>
>
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Re: [Haskell-cafe] Solved but strange error in type inference

[Antoine Latter]

On Wed, Jan 4, 2012 at 9:08 AM, Thiago Negri  wrote:
> Do not compile:
>
> f :: a -> a
> f x = x :: a
>
>    Couldn't match type `a' with `a1'
>      `a' is a rigid type variable bound by
>          the type signature for f :: a -> a at C:\teste.hs:4:1
>      `a1' is a rigid type variable bound by
>           an expression type signature: a1 at C:\teste.hs:4:7
>    In the expression: x :: a
>    In an equation for `f': f x = x :: a
>
>
> Any of these compiles:
>
> f :: a -> a
> f x = undefined :: a

Re-written:

f :: forall a . a -> a
f x = undefined :: forall a . a

Renaming variables to avoid shadowing:

f :: forall a . a -> a
f x = undefined :: forall b . b

Which is allowed.

The rest of your examples are similar - a new value is introduced with
a new type that can unify with the required type.

This is different from the failing case:

g :: a -> a
g x = x :: a

Let's go through the same process.

Insert foralls:

g :: forall a . a -> a
g x = x :: forall a . a

Rename shadowed variables:

g :: forall a . a -> a
g x = x :: forall b . b

In the function body we have declared that the value 'x' may take on
any value. But that's not true! The value 'x' comes from the input to
the function, which is a fixed 'a' decided by the caller.

So it does not type-check.

Antoine

>
> f :: Num a => a -> a
> f x = undefined :: a
>
> f :: Int -> Int
> f x = undefined :: a
>
> f :: Int -> Int
> f x = 3 :: (Num a => a)
>
>
> Can someone explain case by case?
>
> Thanks,
> Thiago.
>
> 2012/1/4 Yves Parès :
>>> I don't see the point in universally quantifying over types which are
>> already present in the environment
>>
>> I think it reduces the indeterminacy you come across when you read your
>> program ("where does this type variable come from, btw?")
>>
>>
>>> So is there anyway to "force" the scoping of variables, so that
>>> f :: a -> a
>>> f x = x :: a
>>> becomes valid?
>>
>> You mean either than compiling with ScopedTypeVariables and adding the
>> explicit forall a. on f? I don't think.
>>
>> 2012/1/4 Brandon Allbery 
>>
>> On Wed, Jan 4, 2012 at 08:41, Yves Parès  wrote:
>>>
>>> Would you try:
>>>
>>> f :: a -> a
>>>
>>> f x = undefined :: a
>>>
>>> And tell me if it works? IMO it doesn't.
>>
>>>  It won't
>>
>> Apparently, Yucheng says it does.
>>
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Re: [Haskell-cafe] Windows: openFile gives permission deniedwhenfilein use

[Antoine Latter]

On Thu, Dec 29, 2011 at 12:28 PM, Donn Cave  wrote:
> Quoth Antoine Latter ,
> ...
>> Would this program then loop:
>>
>> append fromFilePath toFilePath = do
>>   str <- readFile fromFile
>>   writeFile toFile str
>>
>> if 'from' and 'to' where the same file?
>>
>> Currently the locking prevents this.
>
> Do you mean, locking makes that work, or just makes it fail in a
> different way?  Anyway, I think that's an example that would have
> the same issues without hGetContents - that is, any way you set
> out to overwrite a file by modifying its contents, you have the
> same problem.
>

Locking makes this fail with an exception, I think. Not locking would
make this mis-behave.

And lets pretend I wrote the function that was in my head:

append fromFilePath toFilePath = do
   str <- readFile fromFile
   appendFile toFile str

Here, without locking, I would expect to fill my disk up, whereas
previously my program would have crashed before writing out any data.

Maybe these are pathological examples, but they are (I think!) the
justification for having the locking in the first place.

Antoine

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Re: [Haskell-cafe] Windows: openFile gives permission denied whenfilein use

[Antoine Latter]

On Thu, Dec 29, 2011 at 11:49 AM, Donn Cave  wrote:
> Quoth Antoine Latter ,
> ...
>>> http://www.haskell.org/pipermail/libraries/2011-October/016978.html
>>>
>>> ... wherein Ian Lynagh proposed to remove this feature and let the
>>> programmer enforce locking or not, as in other programming languages'
>>> base I/O libraries.  This met with enthusiastic universal support,
>>> so whatever the Report may say, it looks to me like the GHC libraries
>>> will eventually not do this.
>>>
>>
>> Wouldn't this lead to 'getContents' and friends being much less safe
>> than they already are? Or would we have to do something at the GHC
>> runtime level of things to add locking?
>
> Interesting question.  I tend to steer clear of that function, rather
> than try to keep track of all the things that can go wrong with it!
> but I would guess, the effect is the same on any I/O strategy that
> executes the same way: e.g. if you were to read pieces of the file
> iteratively you would be similarly exposed to the possibility of
> concurrent modifications.
>
> That risk may be less obvious with getContents if you take the naive
> view that it returns the contents of the file in the same way that normal
> I/O operations return data, but then you will eventually be punished for
> that naivety anyway!
>
> Are you aware of some possible special issue with getContents?
>

Well, the issue would more be with 'hGetContents'.

Would this program then loop:

append fromFilePath toFilePath = do
  str <- readFile fromFile
  writeFile toFile str

if 'from' and 'to' where the same file?

Currently the locking prevents this.

Antoine

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Re: [Haskell-cafe] On the purity of Haskell

[Antoine Latter]

On Thu, Dec 29, 2011 at 11:14 AM, Gregg Reynolds  wrote:
>
> On Dec 29, 2011, at 11:01 AM, Iustin Pop wrote:
>
>> And to clarify better my original email: yes, (bar x) always gives you
>> back the same IO action;
>
> More precisely: the same *type*.
>

I'm confused - what do you mean by "type"? I don't think that Iustin's
statement needs any sort of qualifier - (bar x) always returns the
same IO action when called with the same value for x, no matter how
many times you call it.

Antoine

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Re: [Haskell-cafe] Windows: openFile gives permission denied whenfile in use

[Antoine Latter]

On Thu, Dec 29, 2011 at 10:53 AM, Donn Cave  wrote:
> Quoth Andrew Coppin ,
>> On 29/12/2011 04:29 AM, Antoine Latter wrote:
> ...
>>> This bug and its discussion is similar, but not identical:
>>> http://hackage.haskell.org/trac/ghc/ticket/4363
>>
>> This one has been rumbling on for ages. As others have said, the Report
>> demands that locking occur, which is probably a mistake.
>
> The rationale that followed may have been a little sketchy, but
> apparently everyone agrees with with the point.  I see the ticket
> led to a discussion on the libraries list -
> http://www.haskell.org/pipermail/libraries/2011-October/016978.html
>
> ... wherein Ian Lynagh proposed to remove this feature and let the
> programmer enforce locking or not, as in other programming languages'
> base I/O libraries.  This met with enthusiastic universal support,
> so whatever the Report may say, it looks to me like the GHC libraries
> will eventually not do this.
>

Wouldn't this lead to 'getContents' and friends being much less safe
than they already are? Or would we have to do something at the GHC
runtime level of things to add locking?

Antoine

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Re: [Haskell-cafe] Windows: openFile gives permission denied when file in use

[Antoine Latter]

On Wed, Dec 28, 2011 at 3:52 PM, Michael Snoyman  wrote:
> Hi all,
>
> I just received a bug report from a client that, when an input file is
> open in FrameMaker, my program gives a "permission denied error". This
> bug is reproducible with a simple Haskell program:
>

This bug and its discussion is similar, but not identical:
http://hackage.haskell.org/trac/ghc/ticket/4363

> import System.IO
>
> main = do
>    putStrLn "here1"
>    h <- openFile "filename.txt" ReadMode
>    putStrLn "here2"
>
> I tried writing a simple C program using fopen, and it ran just fine.
> Does anyone have experience with this issue, and know of a workaround?
>
> Thanks,
> Michael
>
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Re: [Haskell-cafe] On the purity of Haskell

[Antoine Latter]

On Wed, Dec 28, 2011 at 3:45 PM, Steve Horne
 wrote:
> On 28/12/2011 20:44, Heinrich Apfelmus wrote:
>>
>> Steve Horne wrote:
>>>
>>> This is just my view on whether Haskell is pure, being offered up for
>>> criticism. I haven't seen this view explicitly articulated anywhere before,
>>> but it does seem to be implicit in a lot of explanations - in particular the
>>> description of Monads in SBCs "Tackling the Awkward Squad". I'm entirely
>>> focused on the IO monad here, but aware that it's just one concrete case of
>>> an abstraction.
>>>
>>> Warning - it may look like trolling at various points. Please keep going
>>> to the end before making a judgement.
>>>
>>> To make the context explicit, there are two apparently conflicting
>>> viewpoints on Haskell...
>>>
>>> 1. The whole point of the IO monad is to support programming with
>>>   side-effecting actions - ie impurity.
>>> 2. The IO monad is just a monad - a generic type (IO actions), a couple
>>>   of operators (primarily return and bind) and some rules - within a
>>>   pure functional language. You can't create impurity by taking a
>>>   subset of a pure language.
>>>
>>> My view is that both of these are correct, each from a particular point
>>> of view. Furthermore, by essentially the same arguments, C is also both an
>>> impure language and a pure one. [...]
>>
>>
>> Purity has nothing to do with the question of whether you can express IO
>> in Haskell or not.
>>
> ...
>
>
>> The beauty of the IO monad is that it doesn't change anything about
>> purity. Applying the function
>>
>>   bar :: Int -> IO Int
>>
>> to the value 2 will always give the same result:
>>
> Yes - AT COMPILE TIME by the principle of referential transparency it always
> returns the same action. However, the whole point of that action is that it
> might potentially be executed (with potentially side-effecting results) at
> run-time. Pure at compile-time, impure at run-time. What is only modeled at
> compile-time is realized at run-time, side-effects included.
>

I don't think I would put it that way - the value 'bar 2' is a regular
Haskell value. I can put it in a list, return it from a function and
all other things:

myIOActions :: [IO Int]
myIOActions = [bar 2, bar (1+1), bar (5-3)]

And I can pick any of the elements of the list to execute in my main
function, and I get the same main function either way.

> Consider the following...
>
> #include 
>
> int main (int argc, char*argv)
> {
>  char c;
>  c = getchar ();
>  putchar (c);
>  return 0;
> }
>
> The identifier c is immutable. We call it a variable, but the compile-time
> value of c is really just some means to find the actual value in the "big
> implicit IORef" at runtime - an offset based on the stack pointer or
> whatever. Nothing mutates until compile-time, and when that happens, the
> thing that mutates (within that "big implicit IORef") is separate from that
> compile-time value of c.
>
> In C and in Haskell - the side-effects are real, and occur at run-time.
>
> That doesn't mean Haskell is as bad as C - I get to the advantages of
> Haskell at the end of my earlier post. Mostly unoriginal, but I think the
> bit about explicit vs. implicit IORefs WRT an alternate view of transparency
> is worthwhile.
>
> I hope If convinced you I'm not making one of the standard newbie mistakes.
> I've done all that elsewhere before, but not today, honest.
>
>
>
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Re: [Haskell-cafe] Windows: openFile gives permission denied when file in use

[Antoine Latter]

On Wed, Dec 28, 2011 at 3:52 PM, Michael Snoyman  wrote:
> Hi all,
>
> I just received a bug report from a client that, when an input file is
> open in FrameMaker, my program gives a "permission denied error". This
> bug is reproducible with a simple Haskell program:
>
> import System.IO
>
> main = do
>    putStrLn "here1"
>    h <- openFile "filename.txt" ReadMode
>    putStrLn "here2"
>
> I tried writing a simple C program using fopen, and it ran just fine.
> Does anyone have experience with this issue, and know of a workaround?
>

When GHC opens files for reading, it asks windows to disallow write
access to the file. I'm guessing that Framemaker has the file open for
writing, so GHC can't get that permission.

I imagine that the Haskell runtime does this to make lazy-io less crazy.

Here's the call GHC makes:
https://github.com/ghc/packages-base/blob/0e1a02b96cfd03b8488e3ff4ce232466d6d5ca77/include/HsBase.h#L580

To open a file for reading in your C demo in a similar way you could
do something like:

fd = _sopen("file_name", _O_RDONLY | _O_NOINHERIT,_SH_DENYWR, 0);

Here "_SH_DENYWR" is telling windows to deny others from writing to this file.

Here's the msdn link for _sopen and _wsopen:
http://msdn.microsoft.com/en-us/library/w7sa2b22%28VS.80%29.aspx

I haven't tested any of that, but that should help you in reproducing
how GHC opens files for read on windows.

You should be able to use System.Win32.File.createFile with a share
mode of (fILE_SHARE_READ .|. fILE_SHARE_WRITE) and then wrangling a
Haskell Handle out of that, but I haven't tried it.

Or you could modify the call to _wsopen and FFI call that - it takes
fewer parameters and might be less confusing.

Antoine

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Re: [Haskell-cafe] Non-exhaustive pattern match warning (incorrect?)

[Antoine Latter]

On Mon, Dec 26, 2011 at 2:19 PM, Michael Orlitzky  wrote:
> On 12/26/11 13:42, Antoine Latter wrote:
>>>
>>> Am I overlooking something, or did I already match Octet.None?
>>>
>>
>> What is your definition of the 'Octet' type?
>>
>
> -- An Octet consists of eight bits. For our purposes, the most
> -- significant bit will come "first." That is, b1 is in the 2^7
> -- place while b8 is in the 2^0 place.
> data Octet = None | Octet { b1 :: Bit,
>                            b2 :: Bit,
>                            b3 :: Bit,
>                            b4 :: Bit,
>                            b5 :: Bit,
>                            b6 :: Bit,
>                            b7 :: Bit,
>                            b8 :: Bit }
>           deriving (Eq)
>

The error is warning you that the record update 'oct { b8 = bit }' can
fail at run-time if 'oct' is None.

Since it looks like you've checked for that you shouldn't have a
problem, but the compiler doesn't know that.

If you decompose your type into 'Octet' without the 'None' case, and
'Maybe Octet' for the times when 'None' is appropriate, the compiler
will have enough information to not give warnings like this.

I can't be the one to tell you if that is worth it or not.

Antoine

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Re: [Haskell-cafe] Non-exhaustive pattern match warning (incorrect?)

[Antoine Latter]

On Mon, Dec 26, 2011 at 1:21 PM, Michael Orlitzky  wrote:
> I'm cleaning up some old projects, and hit this:
>
>  src/Octet.hs:47:27:
>    Warning: Pattern match(es) are non-exhaustive
>    In a record-update construct: Patterns not matched: Octet.None
>
> But in the source, I've checked for that case:
>
>  class Maskable a where
>    apply_mask :: a -> Maskbits -> Bit -> a
>
>  instance Maskable Octet where
>    apply_mask _ Maskbits.None _ = Octet.None
>    apply_mask Octet.None _ _    = Octet.None
>    apply_mask oct mask bit
>        | mask == Eight = oct
>        | mask == Seven = oct { b8 = bit } -- Line 47
>        ...
>        | otherwise = Octet.None
>
>
> Am I overlooking something, or did I already match Octet.None?
>

What is your definition of the 'Octet' type?

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Re: [Haskell-cafe] Images in Haddock documentation: best practices?

[Antoine Latter]

On Sun, Dec 25, 2011 at 12:04 AM, Brent Yorgey  wrote:
> Hi all,
>
> Although it doesn't seem to be documented in the user manual (!),
> Haddock supports inline images, using a <> syntax.  I'd like to
> include some images in the documentation for a package I'm writing,
> but not sure of the best way.
>

In case nothing else works out:

http://en.wikipedia.org/wiki/Data_URI_scheme

They do not work in IE 7, and in IE 9 they are limited to 32k.

They also are not cached separately from the containing page.

Antoine

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Re: [Haskell-cafe] Mitigating state-threading through an application loop

[Antoine Latter]

On Tue, Dec 20, 2011 at 11:08 AM, Michael Serra  wrote:
> Hello Haskellers,
>   I'm implementing a simple tree-manipulating (for sports tournaments)
> application prototype, with SDL for graphics and simple user interaction.
> For reference, I've posted the code on hpaste.  My question is about code
> organization: everything was simple and elegant until I started writing the
> program's display/event loop.  Every function in this section has to be
> passed the same parameters - the application window to draw on, the font to
> display text with, the tree representing the current application state,
> etc.  The font is an especially egregious example of the problem, because
> it's only used by one function but to get there it must be threaded through
> all of them (looking at the hpaste, you will see I don't want to call
> openFont on every invocation of drawTexts; what's needed is to call it once
> in main and have the resulting value available to drawTxt.  So my question:
> how can I mitigate the ugliness of this state-threading?  I understand this
> is one purpose for monads; am I supposed to implement a monad transformer
> for this?
>
> I think it would be great if I could define a type AppState as a tuple of
> the various things I need to thread, and specify some kind of automatic
> as-pattern, so that every function taking this AppState parameter would
> implicitly have its components bound to certain preset names.  I've never
> seen anyone do this however.  What is the right solution?
>

I've done that before in web-apps. It is conventional enough.

To ease the threading-through of paramters you can use a Reader monad
(perhaps ReaderT AppState IO). I don't know how well this fits in with
the SDL libraries, though.

Antoine

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Re: [Haskell-cafe] [Alternative] change some/many semantics

[Antoine Latter]

On Thu, Dec 15, 2011 at 2:20 AM, Gregory Crosswhite
 wrote:
>
> On Dec 15, 2011, at 6:19 PM, Gregory Crosswhite wrote:
>
> After all, we already have the Monad typeclass which gives them essentially
> the same functionality.
>
>
> Make that the *Monoid* typeclass.  :-)

And this is an interesting discussion all of its own!

Should the monoid instance of a Functor do what List does - which is
analogious to its append or choice operation (where applicable), or
should it do what Maybe does, which is lift the operation into its
contained type? (That is, (Just x) `mappend` (Just y) ==> Just (x
`mappend` y)).

Since the Monoid instance for Maybe doesn't offer choice between
Nothing and Some, it would be nice to have a standard choice operation
that we could use for Maybe.

Which is sort of what Alternative is - offering choice over a functor
which supports it. Except that the notion of what choice means is
richer in a parser than in Maybe (parsers may backtrack (like List)
and parsing has some sort of stateful effect, which affects the
success of future parses).

It is an interesting dilemma.

I am also fond of using Alternative (disguised as MonadPlus) in the
Happstack sense, for building a web-site routing table. In the truest
sense I am composing alternative responses to an input request, but
using 'many', 'some', or 'sepEndBy` in this context would be odd.

Antoine

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Re: [Haskell-cafe] [Alternative] change some/many semantics

[Antoine Latter]

On Thu, Dec 15, 2011 at 1:40 AM, Antoine Latter  wrote:
> On Thu, Dec 15, 2011 at 1:23 AM, Gregory Crosswhite
>  wrote:
>>
>> On Dec 15, 2011, at 3:36 PM, Antoine Latter wrote:
>>
>>> There are a lot of combinators you can build from (<|>) and empty that
>>> go terribly wrong for Maybe and List but are still quite useful.
>>>
>>
>> Yes, you *could* do that, but the whole point is that you shouldn't.
>>  Typeclasses generally come with informal laws that must be obeyed.  If your
>> instance does not obey those laws, then it should not be an instance.
>>
>

To clarify - I dropped Greg's sentence "Incidentally, exactly what use
cases do you have in mind?", which is most of what I was addressing in
my previous email.

> I said 'combinators', not 'instances'. A lot of popular parsers
> combinators can be written exclusively from (<|>) and empty, but make
> little sense for List and Maybe, and may not even function properly.
> The 'trifecta' package includes a nice reference:
>
> http://hackage.haskell.org/packages/archive/trifecta/0.49.1/doc/html/Text-Trifecta-Parser-Combinators.html
>
> See 'skipSome' through 'chainr1' - I wouldn't be surprised if most of
> these lead to the same infinite loop behavior for Maybe as the stock
> 'many' and 'some' in base.
>
> These sorts of functions are what Alternative is for.
>
> Maybe I'm missing something fundamental here.
>
> Antoine

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Re: [Haskell-cafe] [Alternative] change some/many semantics

[Antoine Latter]

On Thu, Dec 15, 2011 at 1:23 AM, Gregory Crosswhite
 wrote:
>
> On Dec 15, 2011, at 3:36 PM, Antoine Latter wrote:
>
>> There are a lot of combinators you can build from (<|>) and empty that
>> go terribly wrong for Maybe and List but are still quite useful.
>>
>
> Yes, you *could* do that, but the whole point is that you shouldn't.
>  Typeclasses generally come with informal laws that must be obeyed.  If your
> instance does not obey those laws, then it should not be an instance.
>

I said 'combinators', not 'instances'. A lot of popular parsers
combinators can be written exclusively from (<|>) and empty, but make
little sense for List and Maybe, and may not even function properly.
The 'trifecta' package includes a nice reference:

http://hackage.haskell.org/packages/archive/trifecta/0.49.1/doc/html/Text-Trifecta-Parser-Combinators.html

See 'skipSome' through 'chainr1' - I wouldn't be surprised if most of
these lead to the same infinite loop behavior for Maybe as the stock
'many' and 'some' in base.

These sorts of functions are what Alternative is for.

Maybe I'm missing something fundamental here.

Antoine

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Re: [Haskell-cafe] [Alternative] change some/many semantics

[Antoine Latter]

On Wed, Dec 14, 2011 at 10:57 PM, Brandon Allbery  wrote:
> On Wed, Dec 14, 2011 at 23:49, Antoine Latter  wrote:
>>
>> Or we could not use 'some' and 'many' with list and maybe :-)
>
>
> Yes, yes, we get the message, a wink and a nod is all that's needed to
> discard the nonsensical notion that types and typeclasses *mean* something.
>

That's the interesting thing about type-classes like Alternative and
Functor - they mean very little, and are used in widely varying
contexts. Heck, Control.Monad.void has the type signature "Functor f a
=> f a -> f ()" - how many functors is that operator sensible for?

There are a lot of combinators you can build from (<|>) and empty that
go terribly wrong for Maybe and List but are still quite useful. Even
the operators at hand ('many' and 'some') are partial in parsing, but
I'm not prepared to throw them out.

Antoine

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Re: [Haskell-cafe] [Alternative] change some/many semantics

[Antoine Latter]

On Wed, Dec 14, 2011 at 10:33 PM, Gregory Crosswhite
 wrote:
>
> On Dec 15, 2011, at 2:13 PM, Antoine Latter wrote:
>
> Isn't this what Ross previously suggested? I think his suggested
> instance methods for Maybe return the elements of the lists
> incrementally.
>
>
> Yes and no.  Yes, his excellent suggestion is one of my favorite ideas for
> what we should do with Alternative that I have seen so far and was the
> inspiration for my proposal, but no it is not the same idea at all.  Whereas
> his suggestion keeps the types and generic definitions of some and many the
> way that they are but overrides them manually to work for types such as
> Maybe, my proposal is that we instead change the types and generic
> definitions of some and many themselves so that they automatically do the
> right thing for the Maybe and List types.
>

Unless the Alternative and Applicative type classes offer class
methods to guarantee laziness, we'll have a hard time writing
functions with those guarantees. Such is the cost of parametric
polymorphism! But also the beauty - in the absence of any laziness
guaranteeing functions on the class, we are promised that a
polymorphic function can't be using magic fairy dust behind our back.

We could add some sort of laziness guaranteeing combinators to the
class interface, but that would restrict its membership even further.

Or we could not use 'some' and 'many' with list and maybe :-)

Antoine

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Re: [Haskell-cafe] [Alternative] change some/many semantics

[Antoine Latter]

On Wed, Dec 14, 2011 at 9:58 PM, Gregory Crosswhite
 wrote:
> Hey everyone,
>
> This is even more out there than my previous posts, but the following just
> occurred to me:  is it absolutely necessary that some/many have produced the
> entire list of results before returning?  Couldn't we change their semantics
> so that the list of results is computed and/or extracted lazily?  For
> example, if this were the case, then we would have that (some (Just 1))
> returns an infinite list rather than running in an infinite loop (even
> without defining a special case "some" implementation), which is exactly
> what my intuition would expect.
>

Isn't this what Ross previously suggested? I think his suggested
instance methods for Maybe return the elements of the lists
incrementally.

> Of course, this is not a simple change at all because it would have to be
> done in such a way as to respect the ordering of actions --- that is, we
> can't have each action executed only when the corresponding element of the
> list demanded is forced, or else actions would undesirably interleave.  For
> example, in a parser when we use "many v" we expect everything matching v to
> be consumed by the time "many v" returns, but if instead "many v" only
> consumed as much of the input as we demanded from its result list then we
> might see a chunk of input matching v in another part of our parser despite
> having assumed we'd gotten rid of it, which would cause our parser to be
> broken.
>
> Nonetheless, if there were some way that we could use magic fairy dust to
> have the results returned by some and many be lazily generated lists, then
> this might solve many of the problems that come up with them performing
> infinite loops in cases where it seems like they shouldn't.  :-)
>
> Cheers,
> Greg
>
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Re: [Haskell-cafe] Splitting off many/some from Alternative

[Antoine Latter]

On Wed, Dec 14, 2011 at 8:03 PM, Ross Paterson  wrote:
> The current definition says that some and many should be the least
> solutions of the equations
>
>    some v = (:) <$> v <*> many v
>    many v = some v <|> pure []
>
> We could relax that to just requiring that they satisfy these equations
> (which I think is what John wants).  In that case there would be another
> possible definition for Maybe:
>
>    some Nothing = Nothing
>    some (Just x) = Just (repeat x)
>
>    many Nothing = Just []
>    many (Just x) = Just (repeat x)
>

This seems to generalize to list:

some [] = []
some xs = [cycle xs]

many [] = [[]]
many xs = [cycle xs]

Although I'm still not sure why I would be using these operations in
maybe or list.

Antoine

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Re: [Haskell-cafe] Splitting off many/some from Alternative

[Antoine Latter]

On Mon, Dec 12, 2011 at 4:30 PM, John Meacham  wrote:
> Yes, they are major pains for frisby, which is a parser but needs to
> be cleverer about recursion, the many and some that come with
> applicative actually cause infinite loops.
>

That's why 'many' and 'some' were promoted up to being class methods,
so that folks that needed a more specific implementation could provide
one.

But now they look as if they are of equal importance with the other
class methods, which is not really true.

Antoine

>    John
>
> On Sun, Dec 11, 2011 at 9:18 PM, Gregory Crosswhite
>  wrote:
>> Hey everyone,
>>
>> I am sure that it is too late to do more than idly speculate about this, but
>> could we split the some/many methods out from Alternative?  They simply
>> don't make sense except in a subset of possible Alternatives --- in most
>> cases they just result in an infinite loop.  That is to say, it is not just
>> that they are extraneous functionality, but most of the time they are
>> *impossible* functionality to even implement!  In a way, it is a lie to be
>> including them in Alternative since people making use of the class might
>> incorrectly (but quite reasonably!) assume that any type that is an instance
>> of Alternative *has* a well-defined some and many method, when this is
>> actually the exception rather than the rule.
>>
>> It is only recently that I have been able to grok what some and many are
>> even about (I think), and they seem to only make sense in cases where
>> executing the Alternative action results in a portion of some input being
>> consumed or not consumed.  "some v" means "consume at least one v and return
>> the list of items consumed or fail", and "many v" means "consume zero or
>> more v and return the list of items consumed or the empty list of none are
>> consume".  It thus makes sense for there to be some subclass of Alternative
>> called something like "Consumptive" that contains these methods.  The
>> majority of "Alternative" instances would no longer have these methods, and
>> again that would actually be an improvement since in such cases some/many
>> were unworkable and did nothing but cause infinite loops anyway.
>>
>> Normally it would be unthinkable to even consider such a change to the base
>> libraries, but I suspect that there are not more than a couple of packages
>> out there that make active use of the some/many instances of Alternative;
>>  it is really only parser packages that need some/many, and users most
>> likely use the versions included with the packages themselves rather than
>> the Alternative version.  Could we verify that this is the case, and if so
>> split them away?  I thought I heard a trick whereby people were able to grep
>> all the source on Hackage, but I can't remember what it was.  :-)
>>
>> Just a thought.  :-)
>>
>> Thanks,
>> Greg
>>
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Re: [Haskell-cafe] [ANNOUNCEMENT] xmobar 0.14

[Antoine Latter]

On Sun, Dec 11, 2011 at 10:23 AM, Antoine Latter  wrote:
>
> All of the imports in the ./src/Parsers.hs are from the
> Text.ParserCombinators.Parsec.* module space, which was intended to be
> a compatibility layer, and all of the parsers and parser-combinators
> in ./ser/Parsers.hs appear to be written using the 'Parser' type
> synonym, also from the compatibility layer, which has the same token
> type in both parsec-2.x and parsec-3.x.
>
> So after digging deeper I'm even more curious :-)
>

Okay, I just wasn't reading closely enough.

We generalized the type of 'notFollowedBy' in parsec-3 - the
generalization isn't parsec-3 specific at all, it just hadn't been
done yet.

The intent of the compatibility module was to supply backwards
compatibility - I never thought about the problems of forward
compatibility.

Maybe some warning or deprecation pragmas are in order.

Antoine

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Re: [Haskell-cafe] [ANNOUNCEMENT] xmobar 0.14

[Antoine Latter]

On Sun, Dec 11, 2011 at 10:10 AM, Brandon Allbery  wrote:
> On Sun, Dec 11, 2011 at 10:44, Antoine Latter  wrote:
>>
>> On Sat, Dec 10, 2011 at 5:31 PM, Brandon Allbery 
>> wrote:
>> > On Sat, Dec 10, 2011 at 18:25, Peter Simons  wrote:
>> >>    src/Parsers.hs:163:52:
>> >>        Couldn't match expected type `Char' against inferred type
>> >> `[Char]'
>> >
>> > xmobar currently requires parsec 3.x; the above is the symptom of
>> > building
>> > it against 2.x.
>>
>> It's not clear to me from reading the sources and type signatures of
>> 'notFollowedBy' why this is different in parsec 2 vs. parsec 3.
>
>
> It's not necessarily going to be visible in individual combinators, as I
> understand it; the issue is that Parsec2 can really only handle fundamental
> types such as Char, whereas Parsec3 can handle more complex types.  xmobar
> appears to be making use of this so its higher level parsing is at the
> string instead of the character level, factoring out lower level issues to a
> lower parsing "layer".
>

All of the imports in the ./src/Parsers.hs are from the
Text.ParserCombinators.Parsec.* module space, which was intended to be
a compatibility layer, and all of the parsers and parser-combinators
in ./ser/Parsers.hs appear to be written using the 'Parser' type
synonym, also from the compatibility layer, which has the same token
type in both parsec-2.x and parsec-3.x.

So after digging deeper I'm even more curious :-)

> It's not going to be something easily retrofitted into Parsec2, in any case;
> Parsec3 needed a fair amount of work to do it to begin with, and even more
> to make it do so efficiently, which is why Parsec2 remained the default in
> the 6.12/6.14 days.
>
> --
> brandon s allbery                                      allber...@gmail.com
> wandering unix systems administrator (available)     (412) 475-9364 vm/sms
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Re: [Haskell-cafe] [ANNOUNCEMENT] xmobar 0.14

[Antoine Latter]

On Sat, Dec 10, 2011 at 5:31 PM, Brandon Allbery  wrote:
> On Sat, Dec 10, 2011 at 18:25, Peter Simons  wrote:
>>
>> previous versions of xmobar used to compile fine with GHC 6.10.4, but
>> the new version no longer does:
>>
>>    src/Parsers.hs:163:52:
>>        Couldn't match expected type `Char' against inferred type `[Char]'
>>          Expected type: GenParser Char st Char
>>          Inferred type: GenParser Char st String
>>        In the second argument of `($)', namely `wrapSkip $ string "Run"'
>>        In a stmt of a 'do' expression:
>>              notFollowedBy $ wrapSkip $ string "Run"
>
>
> xmobar currently requires parsec 3.x; the above is the symptom of building
> it against 2.x.
>

It's not clear to me from reading the sources and type signatures of
'notFollowedBy' why this is different in parsec 2 vs. parsec 3.

Am I missing something easy? Because if it isn't obvious perhaps it
wasn't on purpose.

Antoine

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Re: [Haskell-cafe] [ANNOUNCEMENT] xmobar 0.14

[Antoine Latter]

On Sun, Dec 11, 2011 at 8:48 AM, Peter Simons  wrote:
> Hi Jose,
>
>  > Peter, would using parsec 3.x be an acceptable solution to you?
>
> well, we can link xmobar with parsec 3.x on NixOS. The situation
> is tricky, though, because the latest version of parsec that we
> have, 3.1.2, doesn't compile with GHC 6.10.4 anymore, so we'd
> have to use some older version to work around that problem. That
> kind of setup somewhat complicated to maintain, which is why I
> would prefer to compile xmobar with parsec 2 2 if at all
> possible.
>

Hi Peter,

What errors are you getting compiling with GHC 6.10.4? If its a small
thing I certainly don't mind patching things.

> Generally speaking, though, GHC 6.10.4 support is not a high
> priority. I just thought it might be worth pointing out that
> backwards compatibility has been lost in the 0.14 release,
> because earlier versions worked just fine.
>
> Take care,
> Peter
>
>
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Re: [Haskell-cafe] HaXml 1.13 -> 1.22 upgrade

[Antoine Latter]

On Sun, Dec 11, 2011 at 12:08 AM, Michael Orlitzky  wrote:
> I'm trying to migrate one my programs from the old HaXml API to the new.
> Please, someone save me.
>
> I'm currently stuck with this, which works in 1.13. All of the filters work
> on Content, so I make some from the root element with the "root_elem = CElem
> root" line.
>
>  -- |Takes an XML String as an argument, and returns the
>  -- status that was parsed from it. Should only be used
>  -- on XML string where a  is a top-level element.
>  parse_status :: String -> [Status]
>  parse_status xml_data =
>    catMaybes maybe_status
>    where
>      (Document _ _ root _) = xmlParse xml_file_name xml_data
>      root_elem = CElem root
>      status_element = (single_status root_elem)
>      maybe_status = map status_from_content status_element
>
> In the new API, xmlParse returns the root element with type (Element i)
> rather than just Element. And the Content constructor I have to use is
> (CElem (Element i) i), but I have no way to pass the correct 'i' to it.
>

It looks like the function 'xmlParse' returns a value of type
'Document Posn', according to the API docs. I'm guessing the 'Posn'
value is used to annotate the position in the source document a
particular piece of XML came from, so you can report errors better.

Since the pretty-printing functions ignore it, you can replace it with
whatever you want, even with a value of a different type if you have a
need to annotate the tree.

> I just want to parse a few elements from an XML file.
>
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Re: [Haskell-cafe] Generating Code

[Antoine Latter]

On Fri, Dec 9, 2011 at 6:32 PM, Brandon Allbery  wrote:
> On Fri, Dec 9, 2011 at 17:27, Antoine Latter  wrote:
>>
>> On Fri, Dec 9, 2011 at 2:17 PM, Erik Hesselink 
>> wrote:
>> > Since you ask how other packages solve this problem, and since most
>> > packages use template haskell, I have to ask: why can't you use
>> > template haskell for this?
>>
>> For my case, template haskell can't create modules, and template
>> haskell solves a different problem - I've not interested in creating
>> Haskell declarations from Haskell declarations - I'm interested in
>> creating Haskell modules from an external, formal,  specification. In
>> a sense I'm compiling to Haskell.
>
>
> It occurs to me that c2hs (or more appropriately the gtk2hsc2hs fork) is
> intended to solve this problem; have you looked into it?
>

That may be good for the opengl-raw, I was working on generating
Haskell modules from an XML spec - I crashed the thread with my own
(similar) problems :-)

> --
> brandon s allbery                                      allber...@gmail.com
> wandering unix systems administrator (available)     (412) 475-9364 vm/sms
>

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Re: [Haskell-cafe] Generating Code

[Antoine Latter]

On Fri, Dec 9, 2011 at 2:17 PM, Erik Hesselink  wrote:
> On Fri, Dec 9, 2011 at 20:45, L Corbijn  wrote:
>> So I'm interested if there are other libraries that are more suitable
>> to the task of generating haskell code for library use, and thus
>> generate 'human readable' exported code (so no TH). I'm also
>> interested in how other projects generate code for their packages.
>
> Since you ask how other packages solve this problem, and since most
> packages use template haskell, I have to ask: why can't you use
> template haskell for this?
>

For my case, template haskell can't create modules, and template
haskell solves a different problem - I've not interested in creating
Haskell declarations from Haskell declarations - I'm interested in
creating Haskell modules from an external, formal,  specification. In
a sense I'm compiling to Haskell.

> Another option (also not code generation, but very useful in reducing
> boilerplate) is generic programming, for example using the 'regular'
> package, or the new generics in GHC 7.2.
>
> Erik
>
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Re: [Haskell-cafe] Generating Code

[Antoine Latter]

On Fri, Dec 9, 2011 at 1:45 PM, L Corbijn  wrote:
> Hello,
>
> In an attempt to reduce the amount of boring repetitive work to update
> the OpenGLRaw package I've created a generator to do it partially for
> me. It currently uses haskell-src-exts for representing the haskell
> source of the modules. Though haskell-src-exts does an excellent job
> for representing haskell source, it seems to be more aimed at parsing
> haskell source and doing something with it, rather than generating it.
> For cpp macros, say for the use of different calling conventions used,
> can't be used directly, nor is there a really good way to use comments
> (at least so it seems to me).
>
> So I'm interested if there are other libraries that are more suitable
> to the task of generating haskell code for library use, and thus
> generate 'human readable' exported code (so no TH). I'm also
> interested in how other projects generate code for their packages.
>

I've used 'haskell-src' to generate code, but thankfully I didn't need
to include any CPP.

I've wanted to experiment with using haskell-src-exts to generate code
with Haddocks, but it looks like it will be painful, since it doesn't
include comments in the syntax tree proper. This is appropriate for
arbitrary comments, but Haddocks can only appear in specific locations
(I think?).

Here is a utility module I use as a wrapper around haskell-src to make
it a bit friendlier for generating modules:

http://code.haskell.org/~aslatter/code/xhb/build-utils/src/HaskellCombinators.hs

Maybe you can add an 'OtherCall String' calling convention to haskell-src-exts?

I'm not sure what to do about comments.

Antoine

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Re: [Haskell-cafe] Cabal issue

[Antoine Latter]

On Fri, Dec 9, 2011 at 5:20 AM, Kevin Jardine  wrote:
> I understand that this may have been addressed before on this list in some
> form, so I'll be brief:
>
> Had problem with deprecated package, was told my only option was to wipe my
> Haskell install and start over. Is this true and if so, doesn't this mean
> that Cabal (or the package management system that it is a part of) is
> broken?
>

It's hard to say for sure what the fix is without knowing what the error is.

Wiping your installed packages db is easy, so it is often recommended.

> Details here:
>
> https://plus.google.com/u/0/111705054912446689620/posts/V1186HGWEap
>
> Kevin
>
>
>
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Re: [Haskell-cafe] Converting string to System.Time.ClockTime

[Antoine Latter]

On Thu, Dec 8, 2011 at 9:30 AM, dokondr  wrote:
> Ok, maybe you could advise what packages to use for this simple scenario:
>
> I have two text strings with dates:
>
> s1 = "Wed, 07 Dec 2011 10:09:21 +"
> s2 = "Wed, 07 Dec 2011 10:11:00 +"
>
> I need:
> 1) Find how many seconds are between  these dates
> 2) Calculate the date in the middle between these dates

It looks like you already have 1) and 2) finished, using the 'time' package.

> 3) Print out all three dates in the different format, like these:
>  2011,  7 Dec, Wed, 10:11:00

If you need to convert into a specific time-zone you can use the
'utcToLocalTime' function in the 'time' package, which takes a UTCTime
and a TimeZone to create a 'LocalTime'. I'm just guessing that you
might want this, as your output format doesn't include time-zone
information.

Then for formatting, the 'Data.Time.Format' module in the 'time'
package has the function 'formatTime', which uses the same sort of
format string used by 'parseTime'.

I hope that helps! It took me a while to find my way around the 'time'
package properly.

Antoine

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Re: [Haskell-cafe] Converting string to System.Time.ClockTime

[Antoine Latter]

On Thu, Dec 8, 2011 at 9:13 AM, Antoine Latter  wrote:
> On Thu, Dec 8, 2011 at 9:01 AM, dokondr  wrote:
>> Now, when I have managed to convert UTCTime to seconds (see code below) I
>> got stuck trying to convert from UTCTime to CalendarTime, how to do this?
>>
>>
>
> It might be easier to use 'diffUTCTime' and 'addUTCTime' instead of
> converting to and from POSIX seconds.
>

For those reading along at home, 'addUTCTime' and 'diffUTCTime' are
implemented in terms of 'posixSecondsToUTCTime' and
'utcTimeToPOSIXSeconds'. So it's pretty similar in the end.

Antoine

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Re: [Haskell-cafe] Converting string to System.Time.ClockTime

[Antoine Latter]

On Thu, Dec 8, 2011 at 9:01 AM, dokondr  wrote:
> Now, when I have managed to convert UTCTime to seconds (see code below) I
> got stuck trying to convert from UTCTime to CalendarTime, how to do this?
>
>

It might be easier to use 'diffUTCTime' and 'addUTCTime' instead of
converting to and from POSIX seconds.

What do you need the 'CalendarTime' for? I recommend not mixing the
'time' and 'old-time' packages if you can avoid it.

If you really need to for inter-operating with some other library, it
looks like you can use the 'datetime' package to convert from a
UTCTime to a ClockTime, and then you can use the 'old-time' package to
convert from a 'ClockTime' to a 'CalendarTime'.

Antoine

> import Data.Time.Format
> import Data.Time.Clock
> import Locale
> import Data.Maybe
> import Data.Time.Clock.POSIX
>
>
> s1 = "Wed, 07 Dec 2011 10:09:21 +"
> s2 = "Wed, 07 Dec 2011 10:11:00 +"
> t1 = fromJust $ tryParseTime s1
> t2 = fromJust $ tryParseTime s2
> pt1 = utcTimeToPOSIXSeconds t1  -- :: UTCTime -> POSIXTime
> pt2 = utcTimeToPOSIXSeconds t2
> pt3 = pt1 + (pt2 - pt1) / 2
> t3 = posixSecondsToUTCTime pt3
>
>
> t = compare t1 t2
>
> tryParseTime :: String -> Maybe UTCTime
> tryParseTime timeStr = tryFormat (parseTime defaultTimeLocale timeFormat1
> timeStr :: Maybe UTCTime)
>    where
>  tryFormat time
>     | time == Nothing = parseTime defaultTimeLocale timeFormat2 timeStr
> :: Maybe UTCTime
>     | otherwise = time
>
>  timeFormat1 = "%a, %d %b %Y %T %z"
>  timeFormat2 = "%m/%e/%Y %l:%M:%S %p"
> -- timeFormat1 = "%m/%d/%Y %l:%M:%S %p"
>
>
>
> On Thu, Dec 8, 2011 at 6:30 PM, dokondr  wrote:
>>
>> I need to parse time strings like "Wed, 07 Dec 2011 10:09:21 +" to a
>> type that:
>> 1) implements Eq, Ord
>> 2) is numerical, so I could subtract one value from another to find the
>> difference or interval length
>>
>> To answer 1) requirement I wrote the following snippet. Yet I could not
>> subtract  UTCTime values. How can I convert them to milliseconds?
>>
>> import Data.Time.Format
>> import Data.Time.Clock
>> import Locale
>> import Data.Maybe
>>
>> s1 = "Wed, 07 Dec 2011 10:09:21 +"
>> s2 = "Wed, 07 Dec 2011 10:11:00 +"
>> t1 = fromJust $ tryParseTime s1
>> t2 = fromJust $ tryParseTime s2
>>
>> t = compare t1 t2
>>
>> tryParseTime :: String -> Maybe UTCTime
>> tryParseTime timeStr = tryFormat (parseTime defaultTimeLocale timeFormat1
>> timeStr :: Maybe UTCTime)
>>    where
>>  tryFormat time
>>     | time == Nothing = parseTime defaultTimeLocale timeFormat2
>> timeStr :: Maybe UTCTime
>>     | otherwise = time
>>
>>  timeFormat1 = "%a, %d %b %Y %T %z"
>>  timeFormat2 = "%m/%e/%Y %l:%M:%S %p"
>>
>>
>>
>>
>> On Thu, Dec 8, 2011 at 6:12 PM, Erik Hesselink 
>> wrote:
>>>
>>> I'm not sure if you really need ClockTime (from old-time), but if you
>>> don't, the types from the 'time' package are all parseable with
>>> `parseTime` [1].
>>>
>>> Erik
>>>
>>> [1]
>>> http://hackage.haskell.org/packages/archive/time/latest/doc/html/Data-Time-Format.html#v:parseTime
>>>
>>> On Thu, Dec 8, 2011 at 14:16, dokondr  wrote:
>>> > Hi,
>>> > What would be the simplest way to convert strings like "Wed, 07 Dec
>>> > 2011
>>> > 10:09:21 +" to System.Time.ClockTime ?
>>> >
>>> > Thanks!
>>> >
>>> >
>>> >
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>>> >
>>
>>
>>
>>
>>
>
>
>
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Re: [Haskell-cafe] ismzero operator possible without equal constraint

[Antoine Latter]

On Sat, Dec 3, 2011 at 10:55 AM, edgar klerks  wrote:
> Hi list,
>
> I am using MonadSplit
> (from http://www.haskell.org/haskellwiki/New_monads/MonadSplit )  for a
> project and now I want to make a library out of it. This seems to be
> straightforward, but I got stuck when I tried to move miszero out of the
> class:
>
> miszero :: m a -> Bool
>
> It tests if the provided monad instance is empty. My naive attempt was:
>

You can write:

miszero :: MonadPlus m => m a -> m Bool
miszero m = (m >> return False) <|> return True

but that will invoke any monadic effects as well as determining the
nature of the value, which may not be what you want.

Antoine

> miszero :: (Eq (m a), MonadPlus m) => m a -> Bool
> miszero =  ( == mzero )
>
> This works, but not correctly. It adds an Eq constraint that is unneeded. I
> would prefer to have something like:
>
> miszero :: MonadPlus m => m a -> Bool
>
>
> Because I am not comparing the contents of the monad. I don't even touch it.
>  Is this possible to write?
>
> with kind regards,
>
> Edgar
>
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Re: [Haskell-cafe] (no subject)

[Antoine Latter]

On Mon, Nov 28, 2011 at 4:12 PM, Willem Obbens  wrote:
> Hello,
> I get this error when I try to derive an instance of the Show typeclass:
> Abc.hs:21:60:
>     Couldn't match expected type `Vector' with actual type `[Point]'
>     In the first argument of `show'', namely `xs'
>     In the second argument of `(++)', namely `show' xs'
>     In the second argument of `(++)', namely `", " ++ show' xs'
> Failed, modules loaded: none.
> Here's the faulty code:
> newtype Point = Point Int
> instance Show Point where
>    show (Point a) = [chr $ a + 48]
>
> data Vector = Vector [Point]
> instance Show Vector where
>    show (Vector ys) =
>       let show' (Vector [z])     = show z
>           show' (Vector (x:xs))  = show x ++ ", " ++ show' xs
>           show' (Vector [])      = []
>       in  "(" ++ show' ys ++ ")"

Here you're treating the value 'ys' as if its type was 'Vector', but
its type is '[Point]'.

Does that help?

Antoine

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Re: [Haskell-cafe] Lifted Spine View

[Antoine Latter]

2011/11/20 bob zhang :
> Hi, all
>    I tried to follow the program of the paper "Scrap your boilerpolate"
> Revolutions. Unfortunately,
> I found the program in the section lifted spine view does not compile in my
> GHC, could anybody
>  point out where I am wrong? Many Thanks
>
> My code is posted here http://hpaste.org/54357
>

Hello!

What error do you get? Which version of GHC are you using?

Thanks,
Antoine

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