Re: [Haskell-cafe] ANN: fix-imports-0.1.2

[Ivan Lazar Miljenovic]

On 12 July 2011 16:09, Evan Laforge  wrote:
> Not sure if anyone else is using this, but I've fixed some bugs and
> bumped fix-imports by a few more versions.
>
> Here's copy paste from the package description:
>
> A small standalone program to manage the import block of a haskell
> program. It will try to add import lines for qualified names with no
> corresponding import, remove unused import lines, and sort the import
> block according to some heuristic you can define. This only works for
> qualified imports! Unqualified imports are left untouched.
>
> It's most convenient if bound to an editor key.

This sounds interesting; it would be helpful if you could provide an
example if you could provide an example in the README or description
of what exactly this program does however.

-- 
Ivan Lazar Miljenovic
ivan.miljeno...@gmail.com
IvanMiljenovic.wordpress.com

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[Haskell-cafe] ANN: fix-imports-0.1.2

[Evan Laforge]

Not sure if anyone else is using this, but I've fixed some bugs and
bumped fix-imports by a few more versions.

Here's copy paste from the package description:

A small standalone program to manage the import block of a haskell
program. It will try to add import lines for qualified names with no
corresponding import, remove unused import lines, and sort the import
block according to some heuristic you can define. This only works for
qualified imports! Unqualified imports are left untouched.

It's most convenient if bound to an editor key.

Changes since last announcement:

Mon Jul 11 23:00:19 PDT 2011  qdun...@gmail.com
  * version 0.1.2

Mon Jul 11 22:58:56 PDT 2011  qdun...@gmail.com
  * fix but where multiple comments above an import are lost

Fri Jul  1 15:16:40 PDT 2011  qdun...@gmail.com
  * don't reload file if no changes were made

Fri Jul  1 14:43:02 PDT 2011  qdun...@gmail.com
  * sort and format the import block even if imports weren't added or removed

Wed Jun 15 12:39:12 PDT 2011  qdun...@gmail.com
  * version 0.1.1

Wed Jun 15 12:14:31 PDT 2011  qdun...@gmail.com
  * upgrade to haskell-src-exts 1.11, which removes the mandatory fixities

Wed Jun 15 12:11:27 PDT 2011  qdun...@gmail.com
  * fix bug causing qualified vs. non-qualified imports to be sorted
inconsistently

Thu Jun  2 12:10:08 PDT 2011  qdun...@gmail.com
  * more specific versions for base

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Re: [Haskell-cafe] The Typeable class is changing

[dm-list-haskell-cafe]

At Mon, 11 Jul 2011 21:18:30 +0300,
Yitzchak Gale wrote:
> 
> Please respond to this thread if you own a package
> that will be affected by this change.

iterIO uses mkTyCon for the simple reason that ((Typeable t, Typeable
m) => Iter t m) is Typeable1 and there is no automatic way of deriving
Typeable1.

David

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Re: [Haskell-cafe] Call for GUI examples - Functional Reactive Programming

[Paul Liu]

The original SOE had some FAL (functional animation) stuff, but I
don't think it used specifically the word FRP. Certainly the concept
of a first-class signal (behavior and event) was present.

Regards,
Paul Liu

On Mon, Jul 11, 2011 at 3:22 PM, Yves Parès  wrote:
>> 1. Paul Hudak is writing a new book.
>> http://plucky.cs.yale.edu/cs431/reading.htm
>
> Wow... this is going to be my bedside reading.
>
> I haven't read the original Haskell School of Expression, did it use FRP
> back then?
>
> 2011/7/11 Paul Liu 
>>
>> You guys might want to checkout the recent work on Euterpea at Yale
>> CS. In particular:
>>
>> 1. Paul Hudak is writing a new book.
>> http://plucky.cs.yale.edu/cs431/reading.htm
>> 2. It uses FRP and arrows for sound synthesis.
>> 3. It combines FRP signals with monadic (which recently gets
>> re-written in arrows) GUI composition.
>> 4. New novel techniques is being developed to handle I/O within arrows
>> framework.
>>
>> The code can be obtained through darcs, details at
>> http://plucky.cs.yale.edu/cs431/software_resources.htm
>>
>> Notably, the way it handles GUI is that the composition of widgets are
>> static, but the signals flowing between them are dynamic. This closely
>> follows Conal Elliott's Phooey approach, and greatly reduces the
>> complexity of GUI programming.
>>
>> Disclaimer: I was an ex-student who worked on this project.
>>
>> Regards,
>> Paul Liu
>>
>> On Thu, Jul 7, 2011 at 11:08 PM, Heinrich Apfelmus
>>  wrote:
>> > Dear Haskellers,
>> >
>> > Can GUI programming be liberated from the IO monad?
>>
>>
>> --
>> Regards,
>> Paul Liu
>>
>> ___
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>
>



-- 
Regards,
Paul Liu

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Re: [Haskell-cafe] External system connections

[Daniel Gorín]

On Jul 11, 2011, at 10:48 PM, Alistair Bayley wrote:

> 12 July 2011 05:49, Michael Snoyman  wrote:
>> 
>> As for Bryan's resource-pool: currently I would strongly recommend
>> *against* using it for any purpose. It is based on
>> MonadCatchIO-transformers[2], which is a subtly broken package. In
>> particular, when I tried using it for pool/persistent in the first
>> place, I ended up with double-free bugs from SQLite.
> 
> Do you have a reference explaining this brokenness? e.g. a mailing
> list message? I wasn't aware of this. Are the other MonadCatchIO-*
> packages also broken?
> 

http://www.haskell.org/pipermail/haskell-cafe/2010-October/084890.html


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Re: [Haskell-cafe] Call for GUI examples - Functional Reactive Programming

[Yves Parès]

 > 1. Paul Hudak is writing a new book.
http://plucky.cs.yale.edu/cs431/reading.htm

Wow... this is going to be my bedside reading.

I haven't read the original Haskell School of Expression, did it use FRP
back then?

2011/7/11 Paul Liu 

> You guys might want to checkout the recent work on Euterpea at Yale
> CS. In particular:
>
> 1. Paul Hudak is writing a new book.
> http://plucky.cs.yale.edu/cs431/reading.htm
> 2. It uses FRP and arrows for sound synthesis.
> 3. It combines FRP signals with monadic (which recently gets
> re-written in arrows) GUI composition.
> 4. New novel techniques is being developed to handle I/O within arrows
> framework.
>
> The code can be obtained through darcs, details at
> http://plucky.cs.yale.edu/cs431/software_resources.htm
>
> Notably, the way it handles GUI is that the composition of widgets are
> static, but the signals flowing between them are dynamic. This closely
> follows Conal Elliott's Phooey approach, and greatly reduces the
> complexity of GUI programming.
>
> Disclaimer: I was an ex-student who worked on this project.
>
> Regards,
> Paul Liu
>
> On Thu, Jul 7, 2011 at 11:08 PM, Heinrich Apfelmus
>  wrote:
> > Dear Haskellers,
> >
> > Can GUI programming be liberated from the IO monad?
>
>
> --
> Regards,
> Paul Liu
>
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Re: [Haskell-cafe] Call for GUI examples - Functional Reactive Programming

[Paul Liu]

You guys might want to checkout the recent work on Euterpea at Yale
CS. In particular:

1. Paul Hudak is writing a new book. http://plucky.cs.yale.edu/cs431/reading.htm
2. It uses FRP and arrows for sound synthesis.
3. It combines FRP signals with monadic (which recently gets
re-written in arrows) GUI composition.
4. New novel techniques is being developed to handle I/O within arrows
framework.

The code can be obtained through darcs, details at
http://plucky.cs.yale.edu/cs431/software_resources.htm

Notably, the way it handles GUI is that the composition of widgets are
static, but the signals flowing between them are dynamic. This closely
follows Conal Elliott's Phooey approach, and greatly reduces the
complexity of GUI programming.

Disclaimer: I was an ex-student who worked on this project.

Regards,
Paul Liu

On Thu, Jul 7, 2011 at 11:08 PM, Heinrich Apfelmus
 wrote:
> Dear Haskellers,
>
> Can GUI programming be liberated from the IO monad?


-- 
Regards,
Paul Liu

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Re: [Haskell-cafe] The Typeable class is changing

[Gregory Crosswhite]

On 7/11/11 11:18 AM, Yitzchak Gale wrote:

The standard way to create a Typeable instance is
just to derive it. If you do that, you will not be affected
by this change.


This is only the "standard way" if one is willing to sacrifice Haskell98 
or Haskell2010 compatibility by using the non-standard 
DeriveDataTypeable extension.



But it seems that many packages create Typeable
instances by explicitly using mkTyCon. If your package
does this, it will eventually break, after a deprecation
period.


I have done this in at least one of my packages 
(type-level-natural-number) because I wanted to keep my library 
compatible with Haskell2010 rather than relying on GHC-specific extensions.


To be clear, I think that the new Typeable is an improvement and don't 
mind migrating my package to use it, I just get the sense after reading 
the linked exchange that those discussing this have just assumed that 
the standard way to use Typeable *should be* to use "deriving Typeable" 
and are missing what I suspect is a major reason why people avoid using 
it, which is to reduce dependence on GHC-specific extensions.


Again, this not a complaint or an objection at all, I'm just sharing my 
own thoughts on the matter.  :-)


Cheers,
Greg

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Re: [Haskell-cafe] The Typeable class is changing

[Carl Howells]

This will affect snap-core and heist, of the things I've provided
Typeable instances for.

In snap-core, deriving makes it use the internal module name, rather
than the canonical location of the type.  This causes issues with the
Hint library, so it's worked around by using a manual instance of
Typeable.

Alternatively, heist involves a monad transformer.  "deriving
Typeable" fails for monad transformers, because it can't handle TyCons
with arguments of kind other than *.

So, this change will hit me for two different reasons, and sadly
involve using CPP to control how things are compiled.

Carl

On Mon, Jul 11, 2011 at 11:18 AM, Yitzchak Gale  wrote:
> Simon Marlow has announced[1] on the Haskell Libraries
> list that the Typeable class is changing.
>
> The standard way to create a Typeable instance is
> just to derive it. If you do that, you will not be affected
> by this change.
>
> But it seems that many packages create Typeable
> instances by explicitly using mkTyCon. If your package
> does this, it will eventually break, after a deprecation
> period.
>
> Please respond to this thread if you own a package
> that will be affected by this change.
>
> Can someone who has quick access to the entire contents
> of Hackage please do a grep and find out exactly which
> packages on Hackage will be affected? Thanks.
>
> -Yitz
>
> [1] http://www.haskell.org/pipermail/libraries/2011-July/016546.html
>
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Re: [Haskell-cafe] Fwd: The Typeable class is changing

[Thomas DuBuisson]

Alberto G. Corona  wrote:
> What to do when the data has been defined in other package and provides no
> Typeable instance?

You'd have to use standalone deriving, which I hope gets into Haskell 201X.


module A where

data A = A

{-# LANGUAGE StandaloneDeriving, DeriveDataTypeable #-}
module B where

import A
import Data.Typeable

deriving instance Typeable A


Cheers,
Thomas



>
> 2011/7/11 Yitzchak Gale 
>>
>> Simon Marlow has announced[1] on the Haskell Libraries
>> list that the Typeable class is changing.
>>
>> The standard way to create a Typeable instance is
>> just to derive it. If you do that, you will not be affected
>> by this change.
>>
>> But it seems that many packages create Typeable
>> instances by explicitly using mkTyCon. If your package
>> does this, it will eventually break, after a deprecation
>> period.
>>
>> Please respond to this thread if you own a package
>> that will be affected by this change.
>>
>> Can someone who has quick access to the entire contents
>> of Hackage please do a grep and find out exactly which
>> packages on Hackage will be affected? Thanks.
>>
>> -Yitz
>>
>> [1] http://www.haskell.org/pipermail/libraries/2011-July/016546.html
>>
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>
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Re: [Haskell-cafe] External system connections

[Alistair Bayley]

 12 July 2011 05:49, Michael Snoyman  wrote:
>
> As for Bryan's resource-pool: currently I would strongly recommend
> *against* using it for any purpose. It is based on
> MonadCatchIO-transformers[2], which is a subtly broken package. In
> particular, when I tried using it for pool/persistent in the first
> place, I ended up with double-free bugs from SQLite.

Do you have a reference explaining this brokenness? e.g. a mailing
list message? I wasn't aware of this. Are the other MonadCatchIO-*
packages also broken?

Alistair

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[Haskell-cafe] Fwd: The Typeable class is changing

[Alberto G. Corona]

What to do when the data has been defined in other package and provides no
Typeable instance?


2011/7/11 Yitzchak Gale 

> Simon Marlow has announced[1] on the Haskell Libraries
> list that the Typeable class is changing.
>
> The standard way to create a Typeable instance is
> just to derive it. If you do that, you will not be affected
> by this change.
>
> But it seems that many packages create Typeable
> instances by explicitly using mkTyCon. If your package
> does this, it will eventually break, after a deprecation
> period.
>
> Please respond to this thread if you own a package
> that will be affected by this change.
>
> Can someone who has quick access to the entire contents
> of Hackage please do a grep and find out exactly which
> packages on Hackage will be affected? Thanks.
>
> -Yitz
>
> [1] http://www.haskell.org/pipermail/libraries/2011-July/016546.html
>
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Re: [Haskell-cafe] The Typeable class is changing

[Thomas DuBuisson]

On Mon, Jul 11, 2011 at 1:22 PM, jutaro  wrote:
> I hope that typeRepKey is no longer in the IO monad (for the simple reason to
> teach me that the key can change between session).

If it's implementation dependent then I see no reason for it to be in
IO (this was mentioned on another ML).  On the other hand, something
that changes run-to-run strikes me as good to keep in IO, otherwise
I'd be too tempted to serialize & save it for some purpose and get
confused while a later read was corrupt.

Cheers,
Thomas

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Re: [Haskell-cafe] The Typeable class is changing

[jutaro]

I hope that typeRepKey is no longer in the IO monad (for the simple reason to
teach me that the key can change between session). This forced me to use
unsafePerformIO to make a typeable boxed type of instances of Ord an
instance of Ord (to put it in a Map). Or did I miss something and I can do
better anyhow?

e.g.:
class (Ord s, Typeable s) => Selector s
data GenSelector = forall s . Selector s =>  GS s

instance Ord GenSelector where
compare (GS a) (GS b) = if typeOf a == typeOf b then
compare (fromJust (cast a)) b
else compare (unsafePerformIO $ typeRepKey $ typeOf a)
 (unsafePerformIO $ typeRepKey $ typeOf b)

Jürgen Nicklisch
 

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Re: [Haskell-cafe] External system connections

[Bryan O'Sullivan]

On Mon, Jul 11, 2011 at 10:49 AM, Michael Snoyman wrote:

> I did email Bryan about this a bit ago, but he didn't get back [...]
>

Thanks for jogging my memory. I've released an updated version of
resource-pool that drops the dependency on that package.

http://hackage.haskell.org/package/resource-pool
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Re: [Haskell-cafe] External system connections

[Michael Orlitzky]

On 07/11/11 13:49, Michael Snoyman wrote:
> 
> Author of the package speaking. I agree, that's a problem :). I've
> just uploaded a new version[1] that is properly documented, though it
> will take a bit for Hackage to generates the Haddocks. As Brandon
> said, the reason this wasn't documented in the first place is that it
> was an internally used module for Persistent that I decided after the
> fact to release separately.

Thanks for humoring me. I guess I'll begin tracking down other authors
and making them feel guilty!

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[Haskell-cafe] The Typeable class is changing

[Yitzchak Gale]

Simon Marlow has announced[1] on the Haskell Libraries
list that the Typeable class is changing.

The standard way to create a Typeable instance is
just to derive it. If you do that, you will not be affected
by this change.

But it seems that many packages create Typeable
instances by explicitly using mkTyCon. If your package
does this, it will eventually break, after a deprecation
period.

Please respond to this thread if you own a package
that will be affected by this change.

Can someone who has quick access to the entire contents
of Hackage please do a grep and find out exactly which
packages on Hackage will be affected? Thanks.

-Yitz

[1] http://www.haskell.org/pipermail/libraries/2011-July/016546.html

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Re: [Haskell-cafe] External system connections

[Michael Snoyman]

On Mon, Jul 11, 2011 at 6:27 PM, Michael Orlitzky  wrote:
> On 07/10/11 15:37, Felipe Almeida Lessa wrote:
>> You don't need to do it, it is already done =).  See the pool
>> package by Michael Snoyman on Hackage [1].  More specifically, see
>> createPoolCheckAlive [2].
>>
>> Cheers,
>>
>> [1] http://hackage.haskell.org/package/pool [2]
>> http://hackage.haskell.org/packages/archive/pool/0.1.0.2/doc/html/Data-Pool.html#v:createPoolCheckAlive
>>
>>
>
> How do people use this stuff? The README is empty, there's no
> documentation, and none of the functions are commented. I'm forced to
> conclude that there are people capable of looking at this,
>
>  createPoolCheckAlive :: MonadControlIO m
>                          => IO a
>                          -> (a -> IO ())
>                          -> Int
>                          -> (Pool a -> m b)
>                          -> (a -> IO Bool)
>                          -> m b
>
> and knowing immediately what it does.
>
> I'm still a beginner, and I don't think I have a use for this package
> (what does it do?), but I've run into similar situations with other
> packages and would genuinely like to know (as opposed to just complain
> about it).
>

Author of the package speaking. I agree, that's a problem :). I've
just uploaded a new version[1] that is properly documented, though it
will take a bit for Hackage to generates the Haddocks. As Brandon
said, the reason this wasn't documented in the first place is that it
was an internally used module for Persistent that I decided after the
fact to release separately.

As for Bryan's resource-pool: currently I would strongly recommend
*against* using it for any purpose. It is based on
MonadCatchIO-transformers[2], which is a subtly broken package. In
particular, when I tried using it for pool/persistent in the first
place, I ended up with double-free bugs from SQLite. As an abridged
history, I ended up writing a replacement called MonadInvertIO, which
later was superceded by MonadPeelIO[3] and MonadControlIO[4]. The
latter two packages are both much more correct that MonadCatchIO, and
either one should be used in its place.

I did email Bryan about this a bit ago, but he didn't get back (he
*is* a very busy guy). Ideally, I think that we don't need to have
both pool and resource-pool, but such is the situation right now.

Michael

[1] http://hackage.haskell.org/package/pool-0.1.0.3
[2] http://hackage.haskell.org/package/MonadCatchIO-transformers-0.2.2.2
[3] http://hackage.haskell.org/package/monad-peel
[4] http://hackage.haskell.org/package/monad-control-0.2.0.1

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Re: [Haskell-cafe] Inconsistent trailing comma in export list and record syntax

[Henning Thielemann]

On Mon, 11 Jul 2011, Johan Tibell wrote:


On Mon, Jul 11, 2011 at 3:54 PM, Henning Thielemann
 wrote:

Johan Tibell wrote:

I've found this quite annoying, especially when using CPP to
conditionally include something in a list, as it might force you to
reorder the list to make the commas appear correctly when the
conditional section is enabled/disabled.


In this case I use the colon like a terminator.

http://www.haskell.org/haskellwiki/List_notation


Sorry, I wasn't being clear. I meant for import/export lists.


But in import/export lists you can use the trailing comma, and thus you 
can conditionally export or import identifiers at any position. However, 
the trailing comma is forbidden in enumerations of constructors. E.g.


import M (R, S, T, )

is allowed, but not

import M (T(A,B,C,))

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[Haskell-cafe] Monad.Reader #19 call for copy -- special issue on parallelism and concurrency

[Brent Yorgey]

Call for Copy: The Monad.Reader - Issue 19
SPECIAL ISSUE on parallelism and concurrency


Whether you're an established academic or have only just started
learning Haskell, if you have something to say, please consider
writing an article for The Monad.Reader! Issue 19 will be a special
issue focusing on parallelism and concurrency, construed broadly. The
submission deadline for Issue 19 will be:

**Tuesday, August 16**

The Monad.Reader


The Monad.Reader is a electronic magazine about all things Haskell. It
is less formal than journal, but somehow more enduring than a wiki-
page. There have been a wide variety of articles: exciting code
fragments, intriguing puzzles, book reviews, tutorials, and even
half-baked research ideas.

Submission Details
~~

Get in touch with me if you intend to submit something -- the sooner
you let me know what you're up to, the better.

Please submit articles for the next issue to me by e-mail (byorgey at
cis.upenn.edu).

Articles should be written according to the guidelines available from

http://themonadreader.wordpress.com/contributing/

Please submit your article in PDF, together with any source files you
used. The sources will be released together with the magazine under a
BSD license.

If you would like to submit an article, but have trouble with LaTeX
please let me know and we'll work something out.

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Re: [Haskell-cafe] IterIO: How to write use my inumReverse

[Maciej Wos]

Sorry, my previous message got truncated.

I was trying to say that many iteratees like iterReverse can be
defined nicely using combinators from Control.Applicative. You end up
with much cleaner code.

Also, iterLines doesn't work as the name would suggest. It only
consumes one line from the input stream and returns it inside a
singleton list.

Instead, iterLines can be defined like this:

iterLines :: (Monad m) => Iter L.ByteString m [L.ByteString]
iterLines = do
el <- tryBI lineI
case el of
-- no more "full" lines left, return the remaining data
Left (e :: SomeException) -> (:[]) <$> dataI
-- read one line; add it to the list and read more
Right line -> (line:) <$> iterLines

enumPure "line1\nline2\nline3" |$ iterLines
[Chunk "line1" Empty,Chunk "line2" Empty,Chunk "line3" Empty]

However, the above returns the resulting list only after consuming the
whole stream, which is something to avoid.

In case of iterReverse it is better to read and accumulate characters
until \n is found and then return the reversed string. One way to do
this is to read one character at a time:

reverseLineSlow :: Iter L.ByteString IO L.ByteString
reverseLineSlow = iter ""
where
iter acc = do
c <- headI
case c of
10 -> return $ L.reverse acc
_  -> iter (acc `mappend` L.singleton c)

But this will be really slow. Instead, the data should be read one
chunk at a time:

reverseLine :: Iter L.ByteString IO L.ByteString
reverseLine = iter ""
where
iter acc = do
-- read the data from the stream one chunk at a time
Chunk c eof <- chunkI
-- check if there is any \n, i.e. if we read a whole line
let (a,b) = L.break (==10) c
if b == ""
-- haven't found any \n yet; append all data to the accumulator
then iter (acc `mappend` a)
-- have found \n
else do
-- put the data after \n back (while removing \n itself)
ungetI $ L.tail b
-- return reversed accumulator plus the data up to \n
return $ L.reverse $ acc `mappend` a

Hope this helps!

-- Maciej

On Tue, Jul 12, 2011 at 12:47 AM, Maciej Wos  wrote:
> Don't forget Applicative instance!
> iterReverse = L.reverse <$> lineI
>
> On Monday, 4 July 2011 at 22:54, dm-list-haskell-c...@scs.stanford.edu
> wrote:
>
> At Mon, 4 Jul 2011 20:36:33 +1000,
> John Ky wrote:
>
> Hi Haskell Cafe,
>
>       enum |$ inumLines .| inumReverse .| inumUnlines .| iter
> ...
>
> iterLines :: (Monad m) => Iter L.ByteString m [L.ByteString]
> iterLines = do
>   line <- lineI
>   return [[line]
>
> iterUnlines :: (Monad m) => Iter [L.ByteString] m L.ByteString
> iterUnlines = (L.concat . (++ [C.pack "\n"])) `liftM` dataI
>
> iterReverse :: (Monad m) => Iter [L.ByteString] m [L.ByteString]
> iterReverse = do
>   lines <- dataI
>   return (map L.reverse lines)
>
> inumLines = mkInum iterLines
> inumUnlines = mkInum iterUnlines
> inumReverse = mkInum iterReverse
>
> It all works fine.
>
> My question is: Is it possible to rewrite inumReverse to be this:
>
> iterReverse :: (Monad m) => Iter L.ByteString m L.ByteString
> iterReverse = do
>   line <- dataI
>   return (L.reverse line)
>
> inumReverse = mkInum iterReverse
>
> And still be able to use it in the line:
>
> enum |$ inumLines .| {-- inumReverse goes in here somehow --} .|
> inumUnlines .| iter
>
> The reason I ask is that the Haskell function reverse has the type [a] ->
> [a],
> not  [[a]] -> [[a]].
>
> I thought perhaps the alternative inumReverse is cleaner than the original
> as
> it behaves more similarly to Haskell's own reverse function.
>
> I'm not sure what you are trying to achieve. If you want an iter that
> works on L.ByteStrings, then you can say:
>
> iterReverse :: (Monad m) => Iter L.ByteString m L.ByteString
> iterReverse = do
> line <- lineI
> return (L.reverse line)
>
> In that case you don't need inumLines and inumUnlines. If, however,
> you want the type to be [L.ByteString], and you would rather do this
> one line at a time, instead of calling map, then you could do
> something like the following:
>
> iterReverse :: (Monad m) => Iter [L.ByteString] m [L.ByteString]
> iterReverse = do
> line <- headI
> return [L.reverse line]
>
> But the code you have above should also work, so it all depends on
> what you are trying to achieve.
>
> David
>
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Re: [Haskell-cafe] External system connections

[Michael Orlitzky]

On 07/11/11 11:41, Felipe Almeida Lessa wrote:
> 
> Yes, there's a documentation problem.  But you can guess by the types =).
> 
> IO a: opens a new resource a.
> a -> IO (): disposes the given resource.
> Int: maximum number of open resources.
> Pool a -> m b: the action you want to execute; after it executes, the
> pool is destroyed.
> a -> IO Bool: check if the resource is still alive.
> m b: the result of running your given action with a new pool.
> 
> The key to understanding all of this is looking backwards from the result.
> 
> 1) The result is m b.  Where did this come from?
> 2) Well, there's a 'Pool a -> m b'.  So it runs the action for me.  So
> this is like a 'withFile :: FilePath -> (Handle -> IO a) -> IO a'
> function.
> 3) So if it is a 'Pool a', then the resource is of type 'a'.
> 4) The only function returning 'a' is 'IO a', so this creates the resource.
> 5) By the same reason, 'a -> IO ()' disposes it.
> 6) 'a -> IO Bool' is confusing, but there's a doc for that.
> 7) 'Int' is also confusing, but since this is a pool, this must be the
> maximum number of open resources.

Magic, got it =)

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Re: [Haskell-cafe] External system connections

[Ozgun Ataman]

Have you looked at Bryan O'Sullivan's resource-pool[1] library?

I've used it with MySQL, Redis and Riak connection pooling. It's been robust
and easy to use. It seems like it would be a good fit for your application.

Cheers,
Ozgun

[1] http://hackage.haskell.org/package/resource-pool

On Sun, Jul 10, 2011 at 3:04 PM, Richard Wallace <
rwall...@thewallacepack.net> wrote:

> Hello all,
>
> I'm trying to understand how to properly structure a connection to an
> external system.  I'm writing an application that processes requests
> (it's an IRC bot - ya, I've looked at lambabot but it's a bit beyond
> my current understanding and I'm really trying to learn this stuff and
> find the best way to do that is to write it myself) and sends requests
> to an external system, via SOAP.  The SOAP requests should be sent on
> separate threads.  The SOAP server requires an initial request for
> authentication be made to obtain a token used on subsequent requests.
>
> There are three ways I can structure this.  One is to do the
> authentication with the SOAP server when the application starts up and
> just use that token for all subsequent requests.  This will fall over
> if the token times out and I'd have to restart the application.
> Another way is to do authenticate with the server and get a new token
> for each request.  This is obviously really inefficient.
>
> What I'd like to do is something a bit smarter.  When a request to the
> SOAP server is to be made, if we have a token then we try and use it.
> If it fails, we reauthenticate and get a new token.  When establishing
> a new token, other threads trying to send SOAP requests should block
> until a new token is available.  There are other conditions that
> should be handled - such as an exponential back-off when the SOAP
> server can't be reached - but I feel like reestablishing the
> authentication token is really the key concern here.  Unfortunately, I
> have no idea right now how to write this in Haskell.
>
> Any pointers would be awesome.  Thanks!
>
> Rich
>
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Re: [Haskell-cafe] External system connections

[Felipe Almeida Lessa]

On Mon, Jul 11, 2011 at 12:27 PM, Michael Orlitzky  wrote:
> How do people use this stuff? The README is empty, there's no
> documentation, and none of the functions are commented. I'm forced to
> conclude that there are people capable of looking at this,
>
>  createPoolCheckAlive :: MonadControlIO m
>                          => IO a
>                          -> (a -> IO ())
>                          -> Int
>                          -> (Pool a -> m b)
>                          -> (a -> IO Bool)
>                          -> m b
>
> and knowing immediately what it does.

Yes, there's a documentation problem.  But you can guess by the types =).

IO a: opens a new resource a.
a -> IO (): disposes the given resource.
Int: maximum number of open resources.
Pool a -> m b: the action you want to execute; after it executes, the
pool is destroyed.
a -> IO Bool: check if the resource is still alive.
m b: the result of running your given action with a new pool.

The key to understanding all of this is looking backwards from the result.

1) The result is m b.  Where did this come from?
2) Well, there's a 'Pool a -> m b'.  So it runs the action for me.  So
this is like a 'withFile :: FilePath -> (Handle -> IO a) -> IO a'
function.
3) So if it is a 'Pool a', then the resource is of type 'a'.
4) The only function returning 'a' is 'IO a', so this creates the resource.
5) By the same reason, 'a -> IO ()' disposes it.
6) 'a -> IO Bool' is confusing, but there's a doc for that.
7) 'Int' is also confusing, but since this is a pool, this must be the
maximum number of open resources.

HTH,

-- 
Felipe.

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Re: [Haskell-cafe] External system connections

[Brandon Allbery]

On Mon, Jul 11, 2011 at 11:27, Michael Orlitzky  wrote:
> How do people use this stuff? The README is empty, there's no
> documentation, and none of the functions are commented. I'm forced to
> conclude that there are people capable of looking at this,

I can *almost* work it out just from the type and knowing what it
should be doing.  But in this case, the main problem is that it was
split off of yesod (http://www.yesodweb.com/), and it was only
documented in the context of that.

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

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Re: [Haskell-cafe] External system connections

[Michael Orlitzky]

On 07/10/11 15:37, Felipe Almeida Lessa wrote:
> You don't need to do it, it is already done =).  See the pool
> package by Michael Snoyman on Hackage [1].  More specifically, see 
> createPoolCheckAlive [2].
> 
> Cheers,
> 
> [1] http://hackage.haskell.org/package/pool [2]
> http://hackage.haskell.org/packages/archive/pool/0.1.0.2/doc/html/Data-Pool.html#v:createPoolCheckAlive
>
> 

How do people use this stuff? The README is empty, there's no
documentation, and none of the functions are commented. I'm forced to
conclude that there are people capable of looking at this,

  createPoolCheckAlive :: MonadControlIO m  
  => IO a   
  -> (a -> IO ())   
  -> Int
  -> (Pool a -> m b)
  -> (a -> IO Bool) 
  -> m b

and knowing immediately what it does.

I'm still a beginner, and I don't think I have a use for this package
(what does it do?), but I've run into similar situations with other
packages and would genuinely like to know (as opposed to just complain
about it).

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Re: [Haskell-cafe] Inconsistent trailing comma in export list and record syntax

[Johan Tibell]

On Mon, Jul 11, 2011 at 3:54 PM, Henning Thielemann
 wrote:
> Johan Tibell wrote:
>> I've found this quite annoying, especially when using CPP to
>> conditionally include something in a list, as it might force you to
>> reorder the list to make the commas appear correctly when the
>> conditional section is enabled/disabled.
>
> In this case I use the colon like a terminator.
>
> http://www.haskell.org/haskellwiki/List_notation

Sorry, I wasn't being clear. I meant for import/export lists.

Johan

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Re: [Haskell-cafe] Inconsistent trailing comma in export list and record syntax

[Malcolm Wallace]

> That just shifts the problem, I think? Now you can no longer comment out the 
> first line.

If you are using to-end-of-line comments with --, then the likelihood of 
noticing a leading ( or { on the line being commented, is much greater than the 
likelihood of noticing a trailing comma on the end of the line _before_ the one 
you are commenting.

Regards,
Malcolm
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Re: [Haskell-cafe] Inconsistent trailing comma in export list and record syntax

[Joachim Breitner]

Hi,

Am Montag, den 11.07.2011, 10:49 +0200 schrieb L Corbijn:
> You could of course say that I'm using a bad style, but it remains
> that it seems to be inconsistent to allow a trailing comma in one
> place and not in the other. So is there an reason for this?

there is actually a bug report against the ghc package in Debian from a
user, requesting a more liberal comma separated list parsing. We did not
patch ghc in Debian to that end, though :-)
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=511834

Greetings,
Joachim

-- 
Joachim "nomeata" Breitner
  m...@joachim-breitner.de  |  nome...@debian.org  |  GPG: 0x4743206C
  xmpp: nome...@joachim-breitner.de | http://www.joachim-breitner.de/



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Re: [Haskell-cafe] Inconsistent trailing comma in export list and record syntax

[Johan Tibell]

On Mon, Jul 11, 2011 at 11:51 AM, Sjoerd Visscher  wrote:
>
> On Jul 11, 2011, at 11:42 AM, Jack Henahan wrote:
>
>> Well, for your example frustration, the leading comma style would sort your 
>> problem nicely. As for the particulars… hmm, not sure. I use leading commas 
>> for both, so I never really noticed.
>
> That just shifts the problem, I think? Now you can no longer comment out the 
> first line.

I've found this quite annoying, especially when using CPP to
conditionally include something in a list, as it might force you to
reorder the list to make the commas appear correctly when the
conditional section is enabled/disabled.

Johan

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Re: [Haskell-cafe] Inconsistent trailing comma in export list and record syntax

[Sjoerd Visscher]

On Jul 11, 2011, at 11:42 AM, Jack Henahan wrote:

> Well, for your example frustration, the leading comma style would sort your 
> problem nicely. As for the particulars… hmm, not sure. I use leading commas 
> for both, so I never really noticed.

That just shifts the problem, I think? Now you can no longer comment out the 
first line.

--
Sjoerd Visscher





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Re: [Haskell-cafe] Inconsistent trailing comma in export list and record syntax

[Christopher Done]

On 11 July 2011 10:49, L Corbijn  wrote:
> You could of course say that I'm using a bad style, but it remains that it
> seems to be inconsistent to allow a trailing comma in one place and not in
> the other. So is there an reason for this?

I've also noticed that I can write

data X = X deriving Show

But not

import Prelude hiding print

But we can write

x :: Foo x => …
x :: (Foo x) => …

Can write

class X

But not

module X

I wouldn't expect that there are particular argued reasons for these
inconsistencies. It's difficult to balance a consistent syntax in a
language while trying to be convenient. I don't think it matters too
much.

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Re: [Haskell-cafe] Inconsistent trailing comma in export list and record syntax

[Jack Henahan]

Well, for your example frustration, the leading comma style would sort your 
problem nicely. As for the particulars… hmm, not sure. I use leading commas for 
both, so I never really noticed.

It may be that since modules simply expose functions to other programs, the 
form is syntactically irrelevant except when the module is being loaded.

I am quite curious about it now, though, so I hope there are some more 
knowledgeable folks with some input.

On Jul 11, 2011, at 4:49 AM, L Corbijn wrote:

> Hello,
> 
> I'm wondering why the trailing comma is allowed in export syntax, but not in 
> record syntax, here an example
> module Foo (
> export1, -- is allowed
> ) where
> 
> data Type = Type  {
> record1 :: Foo, -- is not allowed
> }
> 
> To me this seems quite inconsistent and sometimes quite frustrating, imagine 
> the case that you want to temporarily remove the last record:
> data Type = Type  {
> record1 :: Foo,
> --record2 :: Bar
> }
> this would fail due to an extra comma that has to be commented out. 
> 
> You could of course say that I'm using a bad style, but it remains that it 
> seems to be inconsistent to allow a trailing comma in one place and not in 
> the other. So is there an reason for this?
> 
> Lars Corbijn
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[Haskell-cafe] Inconsistent trailing comma in export list and record syntax

[L Corbijn]

Hello,

I'm wondering why the trailing comma is allowed in export syntax, but not in
record syntax, here an example
module Foo (
export1, -- is allowed
) where

data Type = Type  {
record1 :: Foo, -- is not allowed
}

To me this seems quite inconsistent and sometimes quite frustrating, imagine
the case that you want to temporarily remove the last record:
data Type = Type  {
record1 :: Foo,
--record2 :: Bar
}
this would fail due to an extra comma that has to be commented out.

You could of course say that I'm using a bad style, but it remains that it
seems to be inconsistent to allow a trailing comma in one place and not in
the other. So is there an reason for this?

Lars Corbijn
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Re: [Haskell-cafe] Call for GUI examples - Functional Reactive Programming

[Yves Parès]

> Can GUI programming be liberated from the IO monad?

>From a layman's point of view, aren't structures like enumerators/iteratees
more suited to deal with IO?
Or maybe they just wouldn't apply in the case of GUI...


2011/7/8 Heinrich Apfelmus 

> Dear Haskellers,
>
> Can GUI programming be liberated from the IO monad? Functional Reactive
> Programming (FRP) promises as much, and I'm trying to make this dream a
> reality with my [reactive-banana][] library. Having released version
> 0.4.0.0, I am now looking for example programs to direct the future
> evolution of the library.
>
> Do you know any *small GUI programs* that you would *like* to see
> *implemented with Functional Reactive Programming?*
>
> I'm sure that some of you
>
> * are interested in FRP and would like to learn from a few simple but
> beautiful examples
> * or have written small GUI programs that they are unhappy with because
> they were not purely functional
> * or have thought about FRP before and concocted a few examples that are
> very tricky to implement with FRP
> * or have written a nice little GUI application that simply makes a great
> example.
>
> I would love to hear your examples, so that I can try to convert them to
> FRP style and test my library against them!
>
> Strictly speaking, it doesn't have to be an example with a graphical
> interface, I'm also interested in audio and animation examples, though I'm
> currently focusing on GUIs.
>
> Here a few examples of what I am looking for:
>
> * I think that Tim Docker's minimal step sequencer [hbeat][] simply makes a
> great example. I hope that wxHaskell offers a platform-independent way to
> play sound.
>
> * While editing the [Haskell Weekly News][hwn], Brent Yorgey wrote a little
> command-line [program to gather newsworthy items][zipedit]. But wouldn't a
> GUI be great? Since I'm only interested in the GUI, someone would have to
> supply the feed parser for this example to be viable. Maybe the current
> editor, Daniel Santa Cruz, might be interested?
>
> * Notes of a musical performance can be modeled as event streams (MIDI), as
> Henning Thielemann has [done with great effect][midi streams]. Surely,
> reactive-banana should be up to the task, but writing an arpeggiator seems
> impossible at the moment.
>
>
>  [reactive-banana]: 
> http://hackage.haskell.org/**package/reactive-banana
>  [zipedit]: 
> http://byorgey.wordpress.com/**2008/06/21/zipedit/
>  [hbeat]: http://dockerz.net/twd/hBeat
>  [midi streams]: 
> http://hackage.haskell.org/**package/streamed
>  [hwn]: 
> http://www.haskell.org/**haskellwiki/Haskell_Weekly_**News
>
>
>
> Best regards,
> Heinrich Apfelmus
>
> --
> http://apfelmus.nfshost.com
>
>
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