RE: [Haskell-cafe] Haskell newbie indentation query.
Thanks Jules and Daniel.
That was very helpful.
Regards
-Vivek Ramaswamy-
-Original Message-
From: Jules Bean [mailto:[EMAIL PROTECTED]
Sent: 15 October 2008 18:19
To: Ramaswamy, Vivek
Cc: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] Haskell newbie indentation query.
Ramaswamy, Vivek wrote:
Hello All~
I have just started with Haskell, and I must confess; I am in love
with it.
However one area that I am really confused about is indentation.
Lets take a look at if-else if- else block.
Important point 1.
There are two contexts in haskell programs. Layout and non-layout.
In a non-layout context you can do whatever you like with indentation.
You can put the newlines wherever you want.
In practice, almost everyone uses layout for the 'top-level' of a
module. That means that anything flush to the left margin starts a new
declaration. However, making sure we are not flush to the left margin,
the following are all fine
x = if True then 1 else 2
x = if True
then 1
else 2
x =
if True
then 1
else 2
x =
if True
then 1
else 2
because, layout is not relevant in expressions.
Now, do blocks are layout blocks. So what you really want us to look
at is the use of if/then/else in do blocks.
x =
do
if True
then (return 1)
else (return 2)
The first line in the do block defines the left margin for this block.
In this example, the first line is the if line, that defines the left
margin. Since the then and the else are also both on the left
margin, they are new statements. So, the layout interprets as:
do {if True; then (return 1); else (return 2)}
...which is a parse error, because a statement cannot begin with 'then'
or 'else'.
any pattern of indentation which keeps the if expression indented
further to the right will be OK, such as
x =
do
if True
then (return 1)
else (return 2)
x =
do
if True
then (return 1)
else (return 2)
..are both fine.
Does that help?
Jules
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Re: [Haskell-cafe] Re: What I wish someone had told me...
Derek Elkins wrote: All you need is a T-shirt: http://www.cafepress.com/skicalc Or http://www.cafepress.com/l_revolution Paul. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Terminal-like Application Design
Hi Jeff
It sounds like maybe you just want an application that works a bit like 'cabal'.
So with cabal the first argument is taken as the 'command' and then the rest
are based on that:
cabal build --some other --options --which may --or --may --not have --arguments
Yi has a simple template for a script which should get you started, please find
it attached.
So here instead of processOptions, you might want, processCommand
processCommand :: [ String ] - IO ()
processCommand (build : args) = processBuildCommand args
processCommand (play : args) = processPlayCommand args
processCommand [] = putStrLn You must supply a command
processCommand _= putStrLn Sorry I don't understand your
command --Probably out put help here as well
processBuildCommand :: [ String ] - IO ()
processBuildCommand = similar to the processOptions except now you are sure you
are in a 'build' command
you *might* even have a separate set of option descreptions for each command.
hth
allan
Jeff Wheeler wrote:
Hi,
I'm a slight Haskell newbie, but I'm trying to write a terminal-like
application that accepts simple commands with optional arguments, and
can then execute them. Most of these commands will need IO, as later I
will want to communicate over USB for most of them.
I was hoping, though, that I could get some comments on the initial
architecture I've been playing with [1].
I suspect I should be using some sort of monad to represent the
commands, but I don't fully understand monads, and am not sure how it
would apply in this context.
Should I be using a monad here, and if so, how?
Thanks in advance,
Jeff Wheeler
[1] http://media.nokrev.com/junk/cli/
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--
The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.
{-
-}
module Main
( main )
where
{- Standard Library Modules Imported -}
import System.Console.GetOpt
( getOpt
, usageInfo
, ArgOrder( .. )
, OptDescr( .. )
, ArgDescr( .. )
)
import System.Environment
( getArgs
, getProgName
)
{- External Library Modules Imported -}
{- Local Modules Imported -}
{- End of Imports -}
data CliFlag =
CliHelp
| CliVersion
deriving Eq
options :: [ OptDescr CliFlag ]
options =
[ Option h [ help ]
(NoArg CliHelp)
Print the help message to standard out and then exit
, Option v [ version ]
(NoArg CliVersion)
Print out the version of this program
]
helpMessage :: String - String
helpMessage progName =
usageInfo progName options
versionMessage :: String - String
versionMessage progName =
progName ++ : This is version 0.001
-- | The main exported function
main :: IO ()
main = getArgs = processOptions
processOptions :: [ String ] - IO ()
processOptions cliArgs =
case getOpt Permute options cliArgs of
(flags, args, []) -
processArgs flags args
(_flags, _args, errors) -
do progName - getProgName
ioError $ userError (concat errors ++ helpMessage progName)
-- We assume all of the arguments are files to process
processArgs :: [ CliFlag ] - [ String ] - IO ()
processArgs flags files
| elem CliHelp flags= getProgName = (putStrLn . helpMessage)
| elem CliVersion flags = getProgName = (putStrLn . versionMessage)
| otherwise = mapM_ processFile files
-- Our processing of a file is to simply count the words
-- in the file and output the number as a line.
processFile :: FilePath - IO ()
processFile file =
do contents - readFile file
putStrLn (show $ length $ words contents)
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Re: [Haskell-cafe] Re: Ubuntu Haskell
On Thu, Oct 16, 2008 at 8:12 PM, Duncan Coutts [EMAIL PROTECTED] wrote: On Tue, 2008-10-14 at 11:25 +0100, Magnus Therning wrote: Playing the devil's advocate I'd say that cabal (not the library Cabal, but the tool cabal in cabal-install) is only needed on systems with pacakge managers that are broken or completely missing (e.g. Windows). As such cabal is a waste of time and shouldn't have been written at all; on many systems it's of no use, on the ones where it is useful it's a fix at the wrong level. Somewhat harsh, and not completely in line with my own opinion, but it can be argued that way. I think they're actually complementary. Sure on Windows it's needed in place of a native packaging system, but even on systems like debian it's still needed in places. It's needed for packages that are too new or are not sufficiently mature or popular to have been packaged yet for the distro. There will always be such packages. Of course many ordinary users would be able to make do with the subset of packages that are provided by the distro and that's great. Yes, that would actually be my opinion as well, when my devil hat is off. When putting it back on again though I'd say that the distro's own package manager should be used also in this case, e.g. new versions of stuff from Hackage should go into Debian experimental. Taking the hat off again, cabal-install strikes me as a lighter approach and more suited for bleeding-edge things. On top of that it's X-distro and X-platform. All good things! Enough of this schizophrenia. /M -- Magnus Therning(OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Terminal-like Application Design
2008/10/17 allan [EMAIL PROTECTED]: Hi Jeff It sounds like maybe you just want an application that works a bit like 'cabal'. So with cabal the first argument is taken as the 'command' and then the rest are based on that: cabal build --some other --options --which may --or --may --not have --arguments Yi has a simple template for a script which should get you started, please find it attached. So here instead of processOptions, you might want, processCommand processCommand :: [ String ] - IO () processCommand (build : args) = processBuildCommand args processCommand (play : args) = processPlayCommand args processCommand [] = putStrLn You must supply a command processCommand _= putStrLn Sorry I don't understand your command --Probably out put help here as well processBuildCommand :: [ String ] - IO () processBuildCommand = similar to the processOptions except now you are sure you are in a 'build' command you *might* even have a separate set of option descreptions for each command. I wanted to throw in another idea, something I didn't come up with myself but used in omnicodec[1]. Now I don't remember where I picked up the idea: 1. Keep all option values as members of a type T 2. Define an instance of T with default values, dT 3. When using getOpt let the type of the options be something like [OptDescr (T - IO T)] 4. To get the final set of values fold (=) over all the list of arguments returned from getOpt and using dT as the start value. Something like 'effectiveT - foldl (=) dT arguments' In a tool I recently started working on I decided not to work in IO and instead I ended up with something like 'effectiveT = (foldl (.) id arguments) dT'. /M [1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/omnicodec -- Magnus Therning(OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [fun] HaskellDB Talk trailer
On Fri, Oct 17, 2008 at 1:54 AM, Don Stewart [EMAIL PROTECTED] wrote: kyagrd: There is an impressive HaskellDB Talk trailer on the web. http://www.vimeo.com/1983774 Cheers to the HaskellDB developers :-) AWESOME! Trailers for talks, eh? The bar has been raised. -- Don Sweet! There wouldn't happen to be a video or slides from the actual talk? I couldn't find anything about it on the PDXPUG homepage. Björn A HaskellDB developer ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Invalid declaration names in TH that typecheck
[I posted this on the Template Haskell list, but nobody seemed interested. Thus, I'm re-posting it here for higher coverage.] Apparently, I can create a function declaration (FunD) with a name containing an invalid sequence of characters. It passes typechecking and compiles. It even shows up in GHCi! The function name is ep:#:, and the :#: is derived from an infix type constructor. In GHCi (6.8.3), I type ep, hit Tab to get the list of completions, and, sure enough, there it is! When I try to use it, GHCi interprets it as two distinct symbols, ep and :#:, as it should. Template Haskell's claim to fame is that it allows you to do type-safe compile-time meta-programming, and while this issue is perhaps considered lexical rather than type-level, I'm wondering if there should be some additional error checking (somewhere) to account for this. I admit though, that I wouldn't know how to do this easily within the bounds of the TH API. The Name type is common to functions, infix operators, and everything else. That which is valid for one is not necessarily so for the other. Regards, Sean ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] howto mix - within do?
The attached code produces error: -- cut here -- runghc -dcore-lint do_with_assignment.proto.hs do_with_assignment.proto.hs:30:2: Couldn't match expected type `[]' against inferred type `IO' Expected type: [t] Inferred type: IO () In the expression: putStr v0= In a 'do' expression: putStr v0= -- cut here-- However, similar code here: http://www.nabble.com/List-as-input-p19987726.html apparently does work. Is there some compiler option I need to use? The compiler is ghc: ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] OPPS, missing attachment (was Re: howto mix - within do?
On 10/17/08 07:39, Larry Evans wrote:
The attached code produces error:
-- cut here --
[snip]
{-
Purpose:
Explore how to mix 'assignments' inside do.
Motivation:
Instead of:
let
{ v0 = e0
; v1 = e1
}
in do
{ print v0
; print v1
}
which is hard to read, recode above as:
do
{ v0 = e0
; print v0
; v1 = e1
; print v1
}
which is easier to read and more intuitive for
c++ programmers.
Example_code_suggesting_should_work:
http://www.nabble.com/List-as-input-p19987726.html
-}
module Main where
import Data.List
main = do
{ v0 - [999]
; putStr v0=
; print v0
}
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Re: [Haskell-cafe] Terminal-like Application Design
2008/10/17 Magnus Therning [EMAIL PROTECTED]: I wanted to throw in another idea, something I didn't come up with myself but used in omnicodec[1]. Now I don't remember where I picked up the idea: This method is described in http://www.haskell.org/haskellwiki/High-level_option_handling_with_GetOpt. I have used it in the past and eventually end up with huge Config data structures and not much separation of unrelated parts of the computation. Of course, this is probably just me ;-) Alternatively (and this is where I have had more success) you can take several passes over the command arguments, using different parsers. Each parser just ignores what it doesn't recognise: parseArgs names = do args - getArgs let info = parseWith infoOptions [] let options = parseWith argOptions defaultOptArgs let query = parseWith queryOptions defaultQuery return (info, options, query) where parseWith os z = case getOpt Permute os args of (o,_,_) - foldr id z o Note that this method tends to ignore user error at the command line, but I'm sure a hybrid could be constructed that was more chatty but still quite clean at the back end. See http://www.dougalstanton.net/code/buses/ for this code in its wider context. Cheers, D -- Dougal Stanton [EMAIL PROTECTED] // http://www.dougalstanton.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] OPPS, missing attachment (was Re: howto mix - within do?
Every statement in a do-expression has to be in the same monad. You are mixing the list monad (v0 - [999]) with the IO monad (putStr v0=). Instead of v0 - [999] try: let v0 = [999] Hope this helps, Martijn. Larry Evans wrote: On 10/17/08 07:39, Larry Evans wrote: The attached code produces error: -- cut here -- [snip] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] howto mix - within do?
On Fri, Oct 17, 2008 at 1:39 PM, Larry Evans [EMAIL PROTECTED] wrote:
do_with_assignment.proto.hs:30:2:
Couldn't match expected type `[]' against inferred type `IO'
Expected type: [t]
Inferred type: IO ()
In the expression: putStr v0=
In a 'do' expression: putStr v0=
Prelude let f = do { v - [999] ; return () }
Prelude :t f
f :: [()]
Prelude :t putStr
putStr :: IO ()
So v - [999] is in the list monad, but putStr is in the IO monad.
The two cannot mix in that way.
This looks like it might be homework so I'll refrain from saying any
more for now. Good luck!
D
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Re: [Haskell-cafe] OPPS, missing attachment (was Re: howto mix - within do?
Am Freitag, 17. Oktober 2008 14:42 schrieb Larry Evans:
On 10/17/08 07:39, Larry Evans wrote:
The attached code produces error:
-- cut here --
[snip]
{-
Purpose:
Explore how to mix 'assignments' inside do.
Motivation:
Instead of:
let
{ v0 = e0
; v1 = e1
}
in do
{ print v0
; print v1
}
which is hard to read, recode above as:
do
{ v0 = e0
; print v0
; v1 = e1
; print v1
}
That would have to be
do let v0 = e0
print v0
let v1 = e1
print v1
or
do v0 - return e0
print v0
v1 - return e1
print v1
(better (?):
do print $ e0
print $ e1
)
which is easier to read and more intuitive for
c++ programmers.
intuitive for c++ programmers is dangerous, the languages are very
different, and one shouldn't gloss over that. The different syntax should
help to not confound the languages.
Example_code_suggesting_should_work:
http://www.nabble.com/List-as-input-p19987726.html
-}
module Main where
import Data.List
main = do
{ v0 - [999]
; putStr v0=
; print v0
}
remember that in
do value - action
statements -- using value (or not)
action and statements must belong to the same monad. In your code above, the
'action' [999] has type Num a = [a], so the monad is [], but putStr v0=
has type IO (), the monad is IO. So the binding v0 - [999] and the statement
putStr v0= can't appear in the same do-block.
If what you want is for every member of some list, do some monadic action,
you need mapM/mapM_ resp forM(_):
mapM(_) (\v - putStr v0= print v) [999]
or
forM(_) [999] $ (putStr v0= ) . print
especially
forM_ list $ \v - do
some
actions
with v
looks pretty imperative.
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Re: [Haskell-cafe] List as input
So, what's the solution? This one: (l::[Ord]) - readLn doesn't work (because Ord isn't a type constructor). It doesn't even comply to Haskell 98 standard. I want to be able to read any list of ordered elements. -- View this message in context: http://www.nabble.com/List-as-input-tp19987726p20033244.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] List as input
On Fri, Oct 17, 2008 at 7:21 AM, leledumbo [EMAIL PROTECTED] wrote: So, what's the solution? This one: (l::[Ord]) - readLn doesn't work (because Ord isn't a type constructor). It doesn't even comply to Haskell 98 standard. I want to be able to read any list of ordered elements. What you're after is not possible that easily. What you want to do is to read a bunch of elements, and based on what you read in, determine their type. This is contrary to static typing. A number of ways are possible, but they all involve some sort of enumeration of all the possible types to read in. Way #1: make a data type which represents any of the types of things you can accept, and define Ordering on it. data Thing = ThingInt Int | ThingString String | ThingList [Thing] | ... deriving Ord And then define a parser to get from a string to a Thing (pretty easy using ReadP or Parsec or something). Note that this would accept a heterogeneous list like 1, foo, 2, [3,bar,4], and it would sort it as 1, 2, foo, [3,bar,4], because of the way Ord deriving works (all Ints are less than all Strings are less than all Lists ...). You can customize this by not using deriving, and instead manually declaring the instance. Way #2: use the parser from way #1 to parse the first one and determine what kind of thing you're reading, and then read a bunch of those. sortThings :: (Read a) = IO [a] main = do first - getLine case first of ThingInt _ - print = (sortThings :: IO [Int]) ThingString _ - print = (sortThings :: IO [String]) ... There are some other more clever ways. But you will not be able to get Haskell to make the choice for you. In particular, there are some ambiguous choices, for example is read 42 an Int, an Integer, a Double ? So you need to write an algorithm which resolves the ambiguities. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] List as input
On Fri, Oct 17, 2008 at 2:21 PM, leledumbo [EMAIL PROTECTED] wrote: So, what's the solution? This one: (l::[Ord]) - readLn doesn't work (because Ord isn't a type constructor). It doesn't even comply to Haskell 98 standard. I want to be able to read any list of ordered elements. I hope to be enlightened, but I'm pretty sure this is not possible. Your readLn has to present a list of some specific type which can be inferred at compile time. Which type is it? String? Int? Something else? Also, neither Show nor Read relate to Ord, so you cannot ever be sure that all Readable/Showable types are Ordered, or that all Ordered types can be Read/Shown. D ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] List as input
I can't think of a language that lets you do this; that is, allow you to input a list of any type as text. Some languages effectively encode the types in the parsing, for example in LISP, you know that 'foo is a symbol. It has a very limited set of data types and new types are described entirely in terms of those simple types, which makes parsing simple. But lets say you have data Color = Red | Green | Blue deriving (Read,Show,Eq,Ord) Now you suddenly expect readLn to detect the word Green and interpret it differently from 1.0, restricting the type at runtime? Do you realize how difficult this is? What if Green is also used in a type in another module? You need to specify the type to read, or provide a parser that works for every type you care about. -- ryan On Fri, Oct 17, 2008 at 2:21 PM, leledumbo [EMAIL PROTECTED] wrote: So, what's the solution? This one: (l::[Ord]) - readLn doesn't work (because Ord isn't a type constructor). It doesn't even comply to Haskell 98 standard. I want to be able to read any list of ordered elements. -- View this message in context: http://www.nabble.com/List-as-input-tp19987726p20033244.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] List as input
On Fri, Oct 17, 2008 at 6:21 AM, leledumbo [EMAIL PROTECTED]wrote: So, what's the solution? This one: (l::[Ord]) - readLn doesn't work (because Ord isn't a type constructor). It doesn't even comply to Haskell 98 standard. I want to be able to read any list of ordered elements. The problem is one of decoding data from a data source. This is usually handled with parsers. Neither static nor dynamic typing could really save you here, unless the file is written out in a format that could be automatically parsed by the input. (like S-expressions perhaps for a lisp read call...) If it was as easy as your code would like it to be, people wouldn't have bothered with things like XDR, XML, ASN.1 (BER) etc. They'd just send binary data everywhere. How can you tell a 32bit value from four 8 bit bytes for instance? You must parse I think. -- View this message in context: http://www.nabble.com/List-as-input-tp19987726p20033244.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Implementing pi-calculus using STM
Hi, (Note: assumes knowledge of pi-calculus.) I am playing with writing a simple interpreter for the pi-calculus using STM. The implementation of most of the operators of the pi- calculus is straightforward, but I am unsure on how to implement the replication operator. The interpretation of !P should be the interpretation of the infinite number of parallel processes P | P | P ... . I am implementing parallel processes using the fork operation, but spawning an infinite amount of processes -- even if the (infinite) majority of them all immediately lock -- seems like a bad idea. So I would prefer to spawn one P, and as soon as the thread that interprets P makes any progress at all, spawn another, and so on. I'm not too sure however how to achieve this. Any ideas? Edsko ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monadic Floating Point [was: Linking and unsafePerformIO]
It is an interesting question: can IEEE floating point be done purely while preserving the essential features. I've not looked very far so I don't know how far people have looked into this before. Not sure. My doubts are mainly on interference between threads. If a thread can keep its FP state changes 'local' then maybe it could be done. I still think FP operations should be combined in a monad though --- after all, they depend on the evaluation order. Haskell currently only supports a subset of the IEEE FP api. One can assume that that's mainly because the native api for the extended features is imperative. But need it be so? Rounding modes sound to me like an implicit parameter. Traps and exception bits sound like the implementation mechanism of a couple standard exception handling strategies. The interesting thing here is that the exception handling strategy is actually an input parameter. Reader? =) So part of the issue is a functional model of the FP api but the other part is what compiler support would be needed to make a functional api efficient. For example if the rounding mode is an implicit parameter to each operation like + - * etc then we need some mechanism to make sure that we don't have to actually set the FP rounding mode before each FP instruction, but only at points where we know it can change, like passing a new value for the implicit parameter, or calling into a thunk that uses FP instructions. This one seems like a tough one to figure. Again, I'd vouch for a solution like STM --- composition of FP operations is allowed at a certain level (maybe enforcing some settings to remain constant here), while it takes a stronger level to connect them to their surroundings. There's also the issue that if the Float/Double operations take an implicit parameter, can they actually be instances of Num? Is that allowed? I don't know. Technically I guess they could, just like (Num a) = (b-a) can be made an instance. It would look more like State though, IMO. Or Cont. Doesn't even look like Oleg-fu is needed. Should they? That's a horse of a different colour. There are certain properties most programmers come to expect of such instances (regardless of whether the Report demands them or not), such as associativity of (+) and (==) being an equivalence that break miserably for floating point. Floating point is a hairy area for programing in general, but I think it's also one where Haskell can shine with an elegant, typesafe model. -- Ariel J. Birnbaum ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: OPPS, missing attachment (was Re: howto mix - within do?
On 10/17/08 08:12, Daniel Fischer wrote:
Thanks very much Daniel.
Am Freitag, 17. Oktober 2008 14:42 schrieb Larry Evans:
On 10/17/08 07:39, Larry Evans wrote:
The attached code produces error:
-- cut here --
[snip]
{-
Purpose:
Explore how to mix 'assignments' inside do.
Motivation:
Instead of:
let
{ v0 = e0
; v1 = e1
}
in do
{ print v0
; print v1
}
which is hard to read, recode above as:
do
{ v0 = e0
; print v0
; v1 = e1
; print v1
}
That would have to be
do let v0 = e0
print v0
let v1 = e1
print v1
or
do v0 - return e0
print v0
v1 - return e1
print v1
(better (?):
do print $ e0
print $ e1
)
At first, the |let v = e| combination looked best (less typing).
However, my actual e0 was more complicated:
array_complete
[ Expr == Op0 GramOne
, Term == Op0 GramOne
, Factor == Op0 GramOne
]
where == was a binary operator which just produced a tuple pair.
I wanted to keep the expression on multiple lines because it's
easier to read. The == was defined to make the expression
look more like grammar productions (the Op0 GramOne is not
the final rhs. That's only for testing purposes.).
Unfortunately, using let with the multiline rhs failed to
compile (due, I guess, to some layout rules, which I find
difficult to fathom). So, I tried the | v - return e|
combination. This worked although I had to surround e with
parenthesis to enable compilation with multiple lines:
; gram_eqs::GramEqs ArithInp ArithVar - return
(array_complete
[ Expr == Op0 GramOne
, Term == Op0 GramOne
, Factor == Op0 GramOne
])
; putStr gram_eqs=
; print gram_eqs
I didn't use the last combination |print $ e0|
because e1 may use v0. IOW:
v0 - return e0
v1 - return ...v0...
which is easier to read and more intuitive for
c++ programmers.
intuitive for c++ programmers is dangerous, the languages are very
different, and one shouldn't gloss over that. The different syntax should
help to not confound the languages.
OK, but I'm finding it very difficult to figure out how to do
something that's very simple in c++:
; calculate e0
; store e0 in v0
; print v0
; calculate e1 using possibly v0
; store e1 in v1
; print v1
However, apparently, in haskell, the operands on each side of the ';'
must be compatible. Hmmm, I remember seeing a translation of
more complex haskell statments/expression to simpler ones...
looking...
http://haskell.org/onlinereport/exps.html#sect3.14
Mea culpa :( I should have read that first. Then
a would have realized:
(v0 - e0) putStr v0
would not have worked because, I guess:
e0 putStr v0
would not work. OOPS, sect3.14 has a specialization on the pattern p-e.
That specialization is harder to understand. I'll work on it :)
[snip]
remember that in
do value - action
statements -- using value (or not)
action and statements must belong to the same monad. In your code above, the
'action' [999] has type Num a = [a], so the monad is [], but putStr v0=
has type IO (), the monad is IO. So the binding v0 - [999] and the statement
putStr v0= can't appear in the same do-block.
If what you want is for every member of some list, do some monadic action,
you need mapM/mapM_ resp forM(_):
Nope. I just want to trace through various stages in translation of
a language grammar(a set of grammar equationss) to a corresponding
set of equations defining the lookahead sets for that grammar.
So, I want the do{...} to just be a sequence of (calculate value; print
value} where calculate value may use previously calculated values).
Thanks for you help.
-regards,
Larry
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Re: [Haskell-cafe] Invalid declaration names in TH that typecheck
On 2008 Oct 17, at 7:33, Sean Leather wrote: Apparently, I can create a function declaration (FunD) with a name containing an invalid sequence of characters. It passes typechecking and compiles. It even shows up in GHCi! This may be a feature. Consider that internal functions use # as an alphanumeric (via an extension); it can be useful to have names that can't be used in ordinary code. This is an inevitable aspect of being able to program at a level which permits symbol table manipulation. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: OPPS, missing attachment (was Re: howto mix - within do?
Am Freitag, 17. Oktober 2008 17:56 schrieb Larry Evans:
On 10/17/08 08:12, Daniel Fischer wrote:
At first, the |let v = e| combination looked best (less typing).
However, my actual e0 was more complicated:
array_complete
[ Expr == Op0 GramOne
, Term == Op0 GramOne
, Factor == Op0 GramOne
]
where == was a binary operator which just produced a tuple pair.
I wanted to keep the expression on multiple lines because it's
easier to read. The == was defined to make the expression
look more like grammar productions (the Op0 GramOne is not
the final rhs. That's only for testing purposes.).
Unfortunately, using let with the multiline rhs failed to
compile (due, I guess, to some layout rules, which I find
difficult to fathom).
It would be no problem if you used only layout. The problem is that you used
braces and semicolons for the do-block, but not for the let.
You would have to write it like
do { let { v0 = e0 }
; print v0
}
, i.e. also use braces for the let block.
If you write
do { let v0 = e0
; print v0
}
it would be parsed as
do { let { v0 = e0
; print v0
}
}
Either braces and semicolons everywhere or all layout, anything else invites
misparsings.
HTH,
Daniel
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[Haskell-cafe] Temlpate Haskell: [d| ... |]
[d| ... |], where the ... is a list of top-level declarations; the quotation has type Q [Dec]. (http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html) Can someone elaborate on what a list means here? Neither declarations = [d| foo = bar bar = foo |] nor declarations = [d| [ foo = bar , bar = foo ] |] work. I'm trying to automatically generate a bunch of functions out of an xml file. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Temlpate Haskell: [d| ... |]
On Fri, Oct 17, 2008 at 9:26 AM, Achim Schneider [EMAIL PROTECTED] wrote: [d| ... |], where the ... is a list of top-level declarations; the quotation has type Q [Dec]. ( http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html ) Can someone elaborate on what a list means here? Neither declarations = [d| foo = bar bar = foo |] nor declarations = [d| [ foo = bar , bar = foo ] |] work. I'm trying to automatically generate a bunch of functions out of an xml file. Works for me. Note that -fth is needed. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: What I wish someone had told
Richard O'Keefe wrote: On 17 Oct 2008, at 9:53 am, Daryoush Mehrtash wrote: So does this mean that the reason for complexity of generics is the Java inheritance? No. The reason for the complexity of generics in Java is that they weren't designed into the language in the first place. It took several attempts and quite a lot of work to come up with a version of generics that was 100% interoperable with earlier JVMs *and* still worth having. You could have something very like Java generics but without the strangeness if you designed it into the language from the beginning, as Betrand Meyer did with Eiffel. Of course, what _he_ didn't design in from the beginning was lambdas, now present as agents. Eiffel has its own kinds of strangeness. Or C# generics, which are built into the IL (in .Net 2.0 and above). In many ways, C# is a very nice language, and having generics fully supported in reflection is pretty cool. I do have some words to say about various ASP.Net libraries, however... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Temlpate Haskell: [d| ... |]
Philip Weaver [EMAIL PROTECTED] wrote: On Fri, Oct 17, 2008 at 9:26 AM, Achim Schneider [EMAIL PROTECTED] wrote: [d| ... |], where the ... is a list of top-level declarations; the quotation has type Q [Dec]. ( http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html ) Can someone elaborate on what a list means here? Neither declarations = [d| foo = bar bar = foo |] nor declarations = [d| [ foo = bar , bar = foo ] |] work. I'm trying to automatically generate a bunch of functions out of an xml file. Works for me. Note that -fth is needed. -fth doesn't make a difference here, I'm using -XTemplateHaskell with ghc 6.8.3 Now if I wasn't sure that it somehow should work ([d| ... |]'s type is Q [Dec], after all), I would write myself an concatM :: [m [a]] - m [a] -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Pandoc questions
John MacFarlane wrote: +++ Andrew Coppin [Oct 12 08 11:21 ]: There doesn't seem to be any option to make Pandoc produce actual MathML output. Is there a reason for this? 1. Nobody has written the LaTeX - MathML code yet, and I've been too lazy. Anyone who is interested in doing this should get in touch. Well, I'd certainly be interested. I use mathematics *a lot* in my writing. Presumably modifying a large program like Pandoc is intractably difficult though? It strikes me that perhaps using LaTeX to enter mathematical markup is rather against the spirit of Markdown. Surely there should be an option to include raw LaTeX, but a more natural encoding that covers most mathematics would be nice also. Of course, that means somebody has to design it first... 2. Not all browsers can process MathML. The current system (using the LaTeXMathML.js javascript) has the advantage of falling back to raw LaTeX in browsers that don't support MathML. It's been a while since I looked, but I believe the spec provides a way to provide an alternative block of XML, similar to the 'alt' tag in the img element, for precisely this reason. (And if there was a math converter, rather than raw LaTeX you could provide something a little easier on the eyes given what raw Unicode + plain HTML can do...) MathML has the advantage that it's machine-readable as well as human-readable. That probably doesn't matter right now, but maybe it will someday. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Temlpate Haskell: [d| ... |]
--- On Fri, 10/17/08, Achim Schneider [EMAIL PROTECTED] wrote:
declarations = [d|
foo = bar
bar = foo
|]
-fth doesn't make a difference here, I'm using
-XTemplateHaskell with
ghc 6.8.3
The following lines, verbatim, pasted into a file, work for me with 6.8.3 with
no command line options:
{-# LANGUAGE TemplateHaskell #-}
module TH where
import Language.Haskell.TH
declarations = [d|
foo = bar
bar = foo
|]
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[Haskell-cafe] Re: Temlpate Haskell: [d| ... |]
Robert Greayer [EMAIL PROTECTED] wrote:
--- On Fri, 10/17/08, Achim Schneider [EMAIL PROTECTED] wrote:
declarations = [d|
foo = bar
bar = foo
|]
-fth doesn't make a difference here, I'm using
-XTemplateHaskell with
ghc 6.8.3
The following lines, verbatim, pasted into a file, work for me with
6.8.3 with no command line options:
{-# LANGUAGE TemplateHaskell #-}
module TH where
import Language.Haskell.TH
declarations = [d|
foo = bar
bar = foo
|]
Argh. Yes. They do. While I certainly am in favour of layout rules, I'm
not in favour of one that gives raises an error if you don't indent the
closing |] by at least one whitespace.
declarations =
[d|
foo = bar
bar = foo
|]
doesn't work, either, as
declarations =
[d| foo = bar
bar = foo
|]
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[Haskell-cafe] Troubles getting isA of ArrowIf from HXT 8.10 to work as expected
I'm attempting to use ifA to conditionalize generated XHTML.
ifA (arr test_status arr (/= Running))
(mkelem a [attr href (arr test_log_URL
mkText)] [txt log])
(mkelem a [attr href (arr test_log_URL
mkText)] [txt no log])
The arrow type at the point of the ifA is (I think) XmlArrow a = a
XmlTree TestRun where TestRun is defined as:
data TestRun = TestRun
{
test_log_URL:: String,
test_status :: String
}
The issue I'm seeing is that regardless of the predicate arrow the
result is always the same: The XmlTree for the True branch is always
generated. In other words, I get the same behavior using this code:
ifA (arr (const False))
(mkelem a [attr href (arr test_log_URL
mkText)] [txt log])
(mkelem a [attr href (arr test_log_URL
mkText)] [txt no log])
Which, I thought, the arr (const False) would force the ifA to
always descend the False path.
This is my first attempt at using HXT and Arrows (Which is otherwise
going great!) so I figure I'm missing something. Any ideas?
Cheers,
Corey O'Connor
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[Haskell-cafe] How to deal with pointers to pointers in the FFI
I have the following functions in C: OGRErr OGR_G_CreateFromWkb (unsigned char *, OGRSpatialReferenceH, OGRGeometryH *, int) OGRErr OGR_G_CreateFromWkt (char **, OGRSpatialReferenceH, OGRGeometryH *) voidOGR_G_DestroyGeometry (OGRGeometryH) OGRGeometryHOGR_G_CreateGeometry (OGRwkbGeometryType) The normal sequence of calls is OGRGeometryH handle = OGR_G_CreateGeometry(SOME_TYPE); // do stuff OGR_G_DestroyGeometry(handle); OR OGR_G_CreateFromWkb(blob, ref, handle, 0); // do stuff OGR_G_DestroyGeometry(handle); As you can see, in one case, I have to pass in a pointer to the returned handle, and not just the handle. How can I accomplish this feat using a single type? I had data OGRGeometryH type Geometry = Ptr OGRGeometryH but then can I declare that a function returns a Ptr (Geometry) ? -- I try to take things like a crow; war and chaos don't always ruin a picnic, they just mean you have to be careful what you swallow. -- Jessica Edwards ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to deal with pointers to pointers in the FFI
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Jefferson Heard wrote: I have the following functions in C: OGRErrOGR_G_CreateFromWkb (unsigned char *, OGRSpatialReferenceH, OGRGeometryH *, int) OGRErrOGR_G_CreateFromWkt (char **, OGRSpatialReferenceH, OGRGeometryH *) void OGR_G_DestroyGeometry (OGRGeometryH) OGRGeometryH OGR_G_CreateGeometry (OGRwkbGeometryType) Are these Ogre3D functions? If so, I hope you are planning on releasing these bindings! But I don't have an answer to your question, sorry. :\ - - Jake -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.9 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iEYEARECAAYFAkj45dwACgkQye5hVyvIUKmRoQCfTOnlxfRCUY/9SY9qA29NNIJL AvYAn1m6h8b36amu+DBqd/r1dT5iXb/H =Smjr -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to deal with pointers to pointers in the FFI
Sadly, nothing so awesome... OGR is part of GDAL, an open-source geographic information system suite. On Fri, Oct 17, 2008 at 3:22 PM, Jake McArthur [EMAIL PROTECTED] wrote: -BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Jefferson Heard wrote: I have the following functions in C: OGRErrOGR_G_CreateFromWkb (unsigned char *, OGRSpatialReferenceH, OGRGeometryH *, int) OGRErrOGR_G_CreateFromWkt (char **, OGRSpatialReferenceH, OGRGeometryH *) void OGR_G_DestroyGeometry (OGRGeometryH) OGRGeometryH OGR_G_CreateGeometry (OGRwkbGeometryType) Are these Ogre3D functions? If so, I hope you are planning on releasing these bindings! But I don't have an answer to your question, sorry. :\ - - Jake -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.9 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iEYEARECAAYFAkj45dwACgkQye5hVyvIUKmRoQCfTOnlxfRCUY/9SY9qA29NNIJL AvYAn1m6h8b36amu+DBqd/r1dT5iXb/H =Smjr -END PGP SIGNATURE- -- I try to take things like a crow; war and chaos don't always ruin a picnic, they just mean you have to be careful what you swallow. -- Jessica Edwards ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to deal with pointers to pointers in the FFI
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Jefferson Heard wrote: Sadly, nothing so awesome... OGR is part of GDAL, an open-source geographic information system suite. On Fri, Oct 17, 2008 at 3:22 PM, Jake McArthur [EMAIL PROTECTED] wrote: Are these Ogre3D functions? If so, I hope you are planning on releasing these bindings! Aw. As I understand it, there is no plain C interface to Ogre3D right now, so I was wondering where these were coming from anyway. Oh well, I can dream. -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.9 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iEYEARECAAYFAkj46lIACgkQye5hVyvIUKlHGQCg0gE5QmoB51ZwGZ8i/dMgR1nJ ny8An3HrwdoKQkS2jLoGf0KooFsOuaEz =4kC8 -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANNOUNCE: Glob 0.1, globbing library
Greetings to all, I hereby announce the release of Glob 0.1, a small library for glob-matching purposes based on a subset of zsh's syntax. Web page at: http://iki.fi/matti.niemenmaa/glob/index.html Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Glob Simple example: match (compile foo*baz) foobarbaz == True. It can also group the contents of a directory given a list of patterns to match: globDir [compile foo*baz] a directory containing foobarbaz and joe gives ([dir/foobarbaz],[dir/joe]). Comments, complaints, feature requests, bug reports are all welcome. It should work on both GHC 6.8 and 6.10, but I've only tested the latter. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: How to deal with pointers to pointers in the FFI
Jefferson Heard wrote: I have the following functions in C: OGRErrOGR_G_CreateFromWkb (unsigned char *, OGRSpatialReferenceH, OGRGeometryH *, int) OGRErrOGR_G_CreateFromWkt (char **, OGRSpatialReferenceH, OGRGeometryH *) void OGR_G_DestroyGeometry (OGRGeometryH) OGRGeometryH OGR_G_CreateGeometry (OGRwkbGeometryType) The normal sequence of calls is OGRGeometryH handle = OGR_G_CreateGeometry(SOME_TYPE); // do stuff OGR_G_DestroyGeometry(handle); OR OGR_G_CreateFromWkb(blob, ref, handle, 0); // do stuff OGR_G_DestroyGeometry(handle); As you can see, in one case, I have to pass in a pointer to the returned handle, and not just the handle. How can I accomplish this feat using a single type? I had data OGRGeometryH type Geometry = Ptr OGRGeometryH Almost: Assuming that OGRGeometryH is some sort of pointer, e.g. typedef struct OGRGeometry *OGRGeometryH you could write: data OGRGeometry type OGRGeometryH = Ptr OGRGeometry where OGRGeometryH is the type that you API exports. foreign import ccall oGR_G_CreateFromWkb :: ...whatever... - Ptr OGRGeometryH - OGRErr Note that Ptr OGRGeometryH is in fact a pointer to a pointer, just as the C type demands. When calling such a procedure you must first allocate space. This is most elegantly done using alloca (I have added some basic error handling, otherwise the value returned might be undefined): createFromWbk :: ...argtypes... - Either OGRErr OGRGeometryH createFromWbk ...args... = alloca $ \ptr - do -- in this code block you can peek and poke the ptr; -- space will be deallocated after block exits res - oGR_G_CreateFromWkb ...args... ptr if checkResultOK res then do -- this can be shortened to: liftM Right $ peek ptr -- or: peek ptr = return . Right h - peek ptr return (Right h) else return (Left res) (Instead of returning an Either type you could throw exceptions, but you didn't ask about error handling, did you ?-) Cheers Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Compiling regex-posix-0.93.2 on windows
I am not sure what is going wrong. I have not been using Haskell on windows. I am also copying this reply to haskell-cafe and libaries mailing lists. Does anyone know? Parnell Flynn wrote: I am having a terrible time compiling the 0.93.2 version of the regex-posix library on windows XP. I have Cygwin and MinGW installed and I have a version of the regex.h header in C:/cygwin/usr/include what shared libraries do I need? Here is a copy of the error it finds the include but is unable to link. This links to the standard c library version of regular expressions. The corresponding c header file is regex.h and there is a chance you will need to edit the end of the regex-posix.cabal file to find the include directory and/or library. Preprocessing library regex-posix-0.93.2... dist/build/Text/Regex/Posix/Wrap_hsc_make.o(.text+0x85):Wrap_hsc_make.c: undefined reference to `_impure_ptr' dist/build/Text/Regex/Posix/Wrap_hsc_make.o(.text+0xc9):Wrap_hsc_make.c: undefined reference to `_impure_ptr' dist/build/Text/Regex/Posix/Wrap_hsc_make.o(.text+0xf1):Wrap_hsc_make.c: undefined reference to `_impure_ptr' dist/build/Text/Regex/Posix/Wrap_hsc_make.o(.text+0x125):Wrap_hsc_make.c: undefined reference to `_impure_ptr' dist/build/Text/Regex/Posix/Wrap_hsc_make.o(.text+0x159):Wrap_hsc_make.c: undefined reference to `_impure_ptr' dist/build/Text/Regex/Posix/Wrap_hsc_make.o(.text+0x18d):Wrap_hsc_make.c: more undefined references to `_impure_ptr' follow collect2: ld returned 1 exit status linking dist\build\Text\Regex\Posix\Wrap_hsc_make.o failed command was: y:\ghc\ghc-6.8.2\bin\ghc.exe -optl-LC:/cygwin/lib dist\build\Text\Regex\Posix\Wrap_hsc_make.o -o dist\build\Text\Regex\Posix\Wrap_hsc_make.exe bash-3.2$ Thanks, Parnell Parnell Flynn • 230 South Lasalle Street • Suite 400 • Chicago, IL 60604 • 312.244.5317 (o) • [EMAIL PROTECTED] mailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Fwd: [Haskell-beginners] HaXml.SAX successfully parses a malformed XML document
I asked this question on the haskell-beginners list, but I didn't get a
response, so I am forwarding it on to the cafe list.
David
--- Original Message ---
Date: 10/16/2008
From: David Frey [EMAIL PROTECTED]
Subject: [Haskell-beginners] HaXml.SAX successfully parses a malformed
XML document
It seems that saxParse from Text.XML.HaXml.SAX will successfully parse a
malformed XML document.
Notice that the SubElem opening and closing tag are not matched in the
XML document below.
--- input.xml ---
RootNode
ElemOne
SubElem attr=fooelement data/SubElemBroken
NoDataElem/
ElemOne
/RootNode
--- end input.xml ---
The Haskell code below runs without error. It prints out the type of
elements found during the parse.
--- Haskell Code ---
module Main where
import qualified Text.XML.HaXml.SAX as SAX
import Data.Maybe
main = let inputFilename = input.xml in
do content - readFile inputFilename
let (elements, error) = SAX.saxParse inputFilename content
if isNothing error
then mapM_ putStrLn (summarizeElements elements)
else putStrLn $ ERROR: ++ (fromJust error)
summarizeElements :: [SAX.SaxElement] - [String]
summarizeElements elements = map summarizeElement elements
summarizeElement :: SAX.SaxElement - String
summarizeElement element = case element of
(SAX.SaxDocTypeDecl d) - DocType
(SAX.SaxProcessingInstruction p) - Processing Instruction
(SAX.SaxComment s) - Comment
(SAX.SaxElementOpen name attrs) - Element Open
(SAX.SaxElementClose name) - Element Close
(SAX.SaxElementTag name attrs) - No Content Element
(SAX.SaxCharData charData) - Character Data
(SAX.SaxReference reference) - Reference
--- End Haskell Code ---
The Python code below throws an exception when parsing the same input
document.
--- Python Code ---
from xml.sax import make_parser
from xml.sax.handler import ContentHandler
def main():
c = ContentHandler()
p = make_parser()
p.setContentHandler(c)
p.parse(open('input.xml'))
if __name__ == '__main__':
main()
-- End Python Code ---
Is this a bug in the HaXml SAX parser?
Thanks,
David
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[Haskell-cafe] Glut using freeglut and non starndard header / lib location?
Which is the way to install the glut library with non standard header / lib location? I've tried setting CFLAGS before running ./configure ./setup configure and adding the include directory this way include-dirs: include, /nix/store/rz4nfm5qcrjrk0jsr1lxnjwamgxmgip8-freeglut-2.4.0/include All results in checking for GL/glut.h... (cached) no checking GLUT/glut.h usability... no checking GLUT/glut.h presence... no checking for GLUT/glut.h... no configure: error: no GLUT header found, so this package cannot be built The configure script is meant to be run by cabal only? Marc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] package question/problem
Hello, I am trying to cabal install HSQL. I am using ghc 6.8.2. I get the following error about a non-visible/hidden package (old-time-1.0.0.0): [EMAIL PROTECTED]:~$ cabal install hsql Resolving dependencies... 'hsql-1.7' is cached. [1 of 1] Compiling Main ( Setup.lhs, dist/setup/Main.o ) Linking dist/setup/setup ... Configuring hsql-1.7... Warning: No 'build-type' specified. If you do not need a custom Setup.hs or ./configure script then use 'build-type: Simple'. Preprocessing library hsql-1.7... Building hsql-1.7... Database/HSQL.hsc:66:7: Could not find module `System.Time': it is a member of package old-time-1.0.0.0, which is hidden cabal: Error: some packages failed to install: hsql-1.7 failed during the building phase. The exception was: exit: ExitFailure 1 [EMAIL PROTECTED]:~$ I don't see (old-time-1.0.0.0) with parens around it so I am confused: [EMAIL PROTECTED]:~$ ghc-pkg list /usr/local/lib/ghc-6.8.2/package.conf: Cabal-1.2.3.0, GLUT-2.1.1.1, HUnit-1.2.0.0, OpenGL-2.2.1.1, QuickCheck-1.1.0.0, array-0.1.0.0, base-3.0.1.0, bytestring-0.9.0.1, cgi-3001.1.5.1, containers-0.1.0.1, directory-1.0.0.0, fgl-5.4.1.1, filepath-1.1.0.0, (ghc-6.8.2), haskell-src-1.0.1.1, haskell98-1.0.1.0, hpc-0.5.0.0, html-1.0.1.1, mtl-1.1.0.0, network-2.1.0.0, old-locale-1.0.0.0, old-time-1.0.0.0, packedstring-0.1.0.0, parallel-1.0.0.0, parsec-2.1.0.0, pretty-1.0.0.0, process-1.0.0.0, random-1.0.0.0, readline-1.0.1.0, regex-base-0.72.0.1, regex-compat-0.71.0.1, regex-posix-0.72.0.2, rts-1.0, stm-2.1.1.0, template-haskell-2.2.0.0, time-1.1.2.0, unix-2.3.0.0, xhtml-3000.0.2.1 /home/vigalchin/.ghc/i386-linux-6.8.2/package.conf: Cabal-1.4.0.1, GLFW-0.3, HDBC-1.1.5, HTTP-3001.0.4, MonadRandom-0.1.1, Stream-0.2.6, Takusen-0.8.3, X11-1.4.2, arrows-0.4, binary-0.4.2, category-extras-0.53.5, chp-1.1.0, dlist-0.4.1, event-list-0.0.7, ipc-0.0.3, midi-0.0.6, mtl-1.1.0.1, network-bytestring-0.1.1.2, non-negative-0.0.3, posix-realtime-0.0.0.1, posix-realtime-0.0.0.2, probability-0.2.1, quantum-arrow-0.0.4, unix-2.4.0.0, zlib-0.4.0.4 ?? What is the history of old-time package? Regards, Vasili ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] mysql server on Linux
Hello, I want to use the mysql server (mysqld) on Linux. What Haskell packages must I install in order to write a Haskell mysql package?? Thank you, Vasili ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] package question/problem
Hi Vasili, have a look at all options of ghc-pkg. There is one command to hide a package. Hidden packages will be shown by ghc-pkg list in parenthesis. The second way to hide packages is passing a special option to ghc (you can look it up in the ghc documentation or by running ./setup build -v3). Probably here it's another reason: Maybe the build-depends fields of the package are wrong. So just open the .cabal file and add it. pay attention to if flag(...) build-depends : else build-depends : so that you add it to the right one. What is the history of old-time package? Hmm. It's superseded by the new time library found on hackage.. I hope that this message will help you. Sincerly Marc Weber ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mysql server on Linux
On Fri, Oct 17, 2008 at 06:27:43PM -0500, Galchin, Vasili wrote: Hello, I want to use the mysql server (mysqld) on Linux. What Haskell packages must I install in order to write a Haskell mysql package?? What do you mean by mysql package? Do you want to write either an application or an library connecting to the mysqld-server executing some queries? Marc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Pandoc questions
1. Nobody has written the LaTeX - MathML code yet, and I've been too lazy. Anyone who is interested in doing this should get in touch. Well, I'd certainly be interested. I use mathematics *a lot* in my writing. Presumably modifying a large program like Pandoc is intractably difficult though? Just write a separate library that parses LaTeX input and returns MathML output. Pandoc could then use this library. So you wouldn't need to know anything about pandoc's internals. Just write a function teXMathToMathML :: String - String. This would be a great contribution! You could get a head start by looking at the LaTeXMathML.js code. It strikes me that perhaps using LaTeX to enter mathematical markup is rather against the spirit of Markdown. Surely there should be an option to include raw LaTeX, but a more natural encoding that covers most mathematics would be nice also. Of course, that means somebody has to design it first... I think it makes good sense to use LaTeX, which is already designed to be natural but flexible, and is already known by most mathematicians. My guess is that in designing a more natural format, one would eventually reinvent something like LaTeX... John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Implementing pi-calculus using STM
On Fri, 2008-10-17 at 14:56 +0100, Edsko de Vries wrote: Hi, (Note: assumes knowledge of pi-calculus.) I am playing with writing a simple interpreter for the pi-calculus using STM. The implementation of most of the operators of the pi- calculus is straightforward, but I am unsure on how to implement the replication operator. The interpretation of !P should be the interpretation of the infinite number of parallel processes P | P | P ... . I am implementing parallel processes using the fork operation, but spawning an infinite amount of processes -- even if the (infinite) majority of them all immediately lock -- seems like a bad idea. So I would prefer to spawn one P, and as soon as the thread that interprets P makes any progress at all, spawn another, and so on. I'm not too sure however how to achieve this. Typically replication is restricted to replicated input for reasons akin to this. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
