- Haskell-Cafe - Haskell.org

The values of infinite lists
by Deokhwan Kim 25 May '06

25 May '06

Are the values of infinite lists _|_ (bottom)? In section 1.3, the Haskell 98 report said as follows: Errors in Haskell are semantically equivalent to _|_. Technically, they are not distinguishable from nontermination, so the language includes no mechanism for detecting or acting upon errors. Therefore, the value of the following infinity is _|_. Right? data Nat = Zero | Succ Nat infinity = Succ infinity What about infinite lists? For example, is the value of [1 ..] also _|_? Thanks.

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Re: Editors for Haskell
by Jeremy O'Donoghue 25 May '06

25 May '06

Hi Walt, > I'm using Haskell (GHC and Hugs) on several different platforms. > Windows, OS X and Linux systems. Assuming that you want your students to be able to use any of the above platforms, the only options I know of which work well on all of the platforms are Emacsen, Vim, hIDE, Eclipse and JEdit. > I'd like to have an IDE that works well for medium to large size > projects. I know of Eclipse and hIDE. > Vim works fine but I'd like more. hiDE seems to be "in process." Much as I love Emacs, I can possibly imagine that you'd prefer to spend the course teaching Haskell, rather than how to use your editor. I personally found Eclipse very difficult to get on with, although YMMV. Although I don't use it much any more, I found JEdit to be very straightforward to use (very good menu-based GUI, plenty of features, syntax highlighting for most languages any sane person could want, and generally very easy to pick up). It's very underrated IMHO. The main reason I switched is that it does slow down with lots (>20, some very large) of files open. Emacs is better with very large projects (I mean > 10,000 files here), which is why I use it. I'd recommend it as a very good editor / simple IDE for people who don't want to spend their life learning to use their editor... HTH Regards Jeremy

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QuickChecking IO
by Mike Gunter 25 May '06

25 May '06

I'd like to use QuickCheck on IO code. For instance, I'd like to check a property of type String -> IO Bool. Using unsafePerformIO seems straightforward (though I haven't written the code, so I may be wrong about that) and it might be possible to make a solution involving unsafeInterleaveIO work. Short of rewriting QuickCheck, is there any way to check IO code "safely"? Barring that, I suspect it's possible modify QuickCheck to accommodate IO code (or perhaps, general monadic code). Has anyone done this? thanks -m

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Linking to third party libraries in windows
by SevenThunders 25 May '06

25 May '06

Has anyone actually seen ghc link successfully to third party libraries on windows? While I have been able to link to C object code compiled by ghc (and thus gcc by proxy), I have not been able to actually link against any substantial third party library or dll in windows. I am currently attempting to link to an amd linear algebra library that was compiled, in fact via gcc (libacml.a) . As soon as I include the following statement in my haskell code foreign import ccall "acml.h daxpy" daxpy:: CInt -> CDouble -> Ptr CDouble -> CInt -> Ptr CDouble -> CInt -> IO () and then link against the acml library using ghc via flags of the form ghc -O2 -fglasgow-exts main.hs -L. -llibacml my executable becomes a "do nothing" routine. That is inclusion of the foreign import prevents any output from occurring even though it compiles just fine. In fact this happens even if I don't use daxpy in the code. Removal of the foreign import allows the program to run. The same problem occurs whether or not I'm linking against a static library or a library providing stubs for a .dll. -- View this message in context: http://www.nabble.com/Linking+to+third+party+libraries+in+windows-t1681736.… Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

2 1

UTF-8 Strings and GHC
by Dmitry V'yal 25 May '06

25 May '06

Hi, all. I'm writing a GUI app using Haskell and Gtk2HS. All goes well besides one thing. I need to display some messages in russian and I can't figure out, how to handle that. Gtk uses UTF-8 internally, so i have to pass UTF-8 strings to it somehow. But how to define them in source file? I get "lexical error in string/character literal" message then compiling using GHC-6.4.1. I tried to bypass it by using koi8-r in sources and converting strings to UTF-8 on the fly using ffi binding to iconv I found in MissingH (iirc). Works fine on Linux though is a little awkward, but I get wried results on Win32. Is there any way to use unicode strings in sources?

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TypeCasting in Haskell
by Christophe Poucet 24 May '06

24 May '06

Dear all, I typically use indirect composite for making AST's. It allows me to easily make new types with other annotations without having to duplicate all elements but only those that actually change. It also allows a whole amalgam of other possibilities. Recently I have observed a type error which seems to be fixable only by doing a "typecast". What do I call a typecast, you may ask? Basically a noop that changes the type. Here attached you will find the code that demonstrates this. > module TypeCast where > > data FooBar foo bar = --- Indirect composite > Foo { unFoo :: foo} > | Bar { unBar :: bar} > > --- Assume some PFoobar (parsed) > data PFooBar = PF {unPF :: FooBar String String} > --- Assume some TFoobar (typed) > data TFooBar = TF {unTF :: FooBar Int String } > > -- Merrily we write our conversion, using binding to optimize slightly > typer :: PFooBar -> TFooBar > typer pFooBar = > case unPF pFooBar of > f@Foo { unFoo = foo} -> -- We only need to change foo > TF $ f{unFoo = 1} > b@Bar { unBar = bar} -> -- We don't need to change this string > TF $ b -- So we just return b > > --- Nice little main to make this a full module: > main :: IO () > main = do > print . typer . PF . Foo $ "Hello" > > --- Type error: > -- TypeCast.hs:19:11: > -- Couldn't match `Int' against `String' > -- Expected type: FooBar Int String > -- Inferred type: FooBar String String > -- In the second argument of `($)', namely `b' > -- In a case alternative: (b@Bar {unBar = bar}) -> TF $ b > > --- what is the fix? Basically do a noop on b > --- b@Bar {unBar = bar} -> > --- TF $ b{unBar = bar} > Cheers, Christophe -- Christophe Poucet Ph.D. Student Phone:+32 16 28 87 20 E-mail: Christophe (dot) Poucet (at) imec (dot) be Website: http://notvincenz.com/ IMEC vzw – Register of Legal Entities Leuven VAT BE 0425.260.668 – Kapeldreef 75, B-3001 Leuven, Belgium – www.imec.be *****DISCLAIMER***** This e-mail and/or its attachments may contain confidential information. It is intended solely for the intended addressee(s). Any use of the information contained herein by other persons is prohibited. IMEC vzw does not accept any liability for the contents of this e-mail and/or its attachments. **********

2 1

GHC wish
by Christophe Poucet 24 May '06

24 May '06

Dear all, I recently stumbled upon something that I think should be added to the GHC wishlist. Not knowing where to put it, I will put it here. As there is deriving(Show) for most basic data types, wouldn't it be possible to have something similar but then deriving(Pretty). When printing ASTs it could make it much easier to look at the output. With regards, Christophe -- Christophe Poucet Ph.D. Student Phone:+32 16 28 87 20 E-mail: Christophe (dot) Poucet (at) imec (dot) be Website: http://notvincenz.com/ IMEC vzw – Register of Legal Entities Leuven VAT BE 0425.260.668 – Kapeldreef 75, B-3001 Leuven, Belgium – www.imec.be *****DISCLAIMER***** This e-mail and/or its attachments may contain confidential information. It is intended solely for the intended addressee(s). Any use of the information contained herein by other persons is prohibited. IMEC vzw does not accept any liability for the contents of this e-mail and/or its attachments. **********

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How to automatically free memory allocated by malloc? and how to reliably realloc such buffer?
by Bulat Ziganshin 24 May '06

24 May '06

Hello Haskell-Cafe, my program uses datastructure that contains plain Ptr, this Ptr points to the memory area allocated by 'malloc': createRawMemBuf size = do buf <- mallocBytes (fromIntegral size) bufRef <- newURef buf ... return (Mem bufRef ...) i need to free this memory buffer on GC if there are no more references to Mem structure. how i can accomplish this? i can't allocate this buffer in GHC heap because later i can use 'realloc' on it. i think that i should use ForeignPtr what points to nothing and performs 'readURef bufRef >>= free' in it's finalizer? something like this: createRawMemBuf size = do buf <- mallocBytes (fromIntegral size) bufRef <- newURef buf ... fin <- mkFinalizer (\_ -> readURef bufRef >>= free) fptr <- newForeignPtr fin nullPtr return (Mem bufRef ... fptr) type Finalizer a = Ptr a -> IO () foreign import ccall "wrapper" mkFinalizer :: Finalizer a -> IO (FinalizerPtr a) ... well, i implemented this and it seems to work - at least memory freed at performGC. another question is how to free 'fin' - i should apply 'freeHaskellFunPtr' to it, but i think i can't do it in finalizer itself The second question is how to make buffer reallocation reliable. Currently i use the following code: reallocBuffer (Mem bufRef ...) newsize = do buf <- readURef bufRef writeURef bufRef nullPtr newbuf <- reallocBytes buf newsize writeURef bufRef newbuf First 'writeURef' is used to prevent repetitive memory deallocation by finalizer i this routine will be interrupted just after 'reallocBytes' operation. will it be enough to use 'block' instead? i.e.: reallocBuffer (Mem bufRef ...) newsize = do buf <- readURef bufRef block $ do newbuf <- reallocBytes buf newsize writeURef bufRef newbuf -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Re: [Haskell-cafe] GHC wish
by Christophe Poucet 24 May '06

24 May '06

Hello Chris, It seems that your reply only replied to me and not to the haskell-cafe list. I hope you do not mind that I reply to the list. Anyways, regarding how the AST is pretty printed. I was not thinking of any highlevel pretty printing of an AST (at least not derived automatically). The type of pretty printing I was thinking of would be synonymous to what deriving Show would give you for any type, except that it would be layed out a bit more cleanly with indentation and newlines. Here is an example: data Foo = Foo {unFoo :: [Bar]} data Bar = Bar {unBar :: Int} prettyPrint . Foo. map Bar $ [1..10] Foo { unFoo = [ Bar {unBar = 1}, Bar {unBar = 2}, .. Bar {unBar =10} ] } Jared: Thank you for mentioning DrIFT. Will certainly take a look at it :) Cheers, Christophe Christopher Brown wrote: > Christophe, > > But that depends on what you think is the correct way of displaying > an AST. I can personally think of a few ways an AST could be pretty > printed. > > Chris. > > > On 24 May 2006, at 00:05, Christophe Poucet wrote: > >> Dear all, >> >> I recently stumbled upon something that I think should be added to >> the GHC wishlist. Not knowing where to put it, I will put it here. >> As there is deriving(Show) for most basic data types, wouldn't it be >> possible to have something similar but then deriving(Pretty). When >> printing ASTs it could make it much easier to look at the output. >> >> With regards, >> Christophe >> >> -- >> Christophe Poucet >> Ph.D. Student >> Phone:+32 16 28 87 20 >> E-mail: Christophe (dot) Poucet (at) imec (dot) be >> Website: http://notvincenz.com/ IMEC vzw – Register of Legal >> Entities Leuven VAT BE 0425.260.668 – Kapeldreef 75, B-3001 Leuven, >> Belgium – www.imec.be >> *****DISCLAIMER***** >> This e-mail and/or its attachments may contain confidential >> information. It is intended solely for the intended addressee(s). >> Any use of the information contained herein by other persons is >> prohibited. IMEC vzw does not accept any liability for the contents >> of this e-mail and/or its attachments. >> ********** >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > > > -- Christophe Poucet Ph.D. Student Phone:+32 16 28 87 20 E-mail: Christophe (dot) Poucet (at) imec (dot) be Website: http://notvincenz.com/ IMEC vzw – Register of Legal Entities Leuven VAT BE 0425.260.668 – Kapeldreef 75, B-3001 Leuven, Belgium – www.imec.be *****DISCLAIMER***** This e-mail and/or its attachments may contain confidential information. It is intended solely for the intended addressee(s). Any use of the information contained herein by other persons is prohibited. IMEC vzw does not accept any liability for the contents of this e-mail and/or its attachments. **********

1 0

Re: [Haskell-cafe] GHC wish
by Jared Updike 23 May '06

23 May '06

Sorry, meant to reply all: --------------------------------------------------- This tool (DrIFT) can derive these instances for you, if you care to make rules for it, for Pretty: http://repetae.net/john/computer/haskell/DrIFT/ Jared. -- http://www.updike.org/~jared/ reverse ")-:"

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Problem trying to get class Bounded to work
by Brian Hulley 23 May '06

23 May '06

Hi - I've got the following function which doesn't compile: createMonoMultiFont :: (MonadException m, Enum i, Bounded i) => [(i, Font, Colour)] -> m (MonoMultiFont i) createMonoMultiFont elements = liftIO . block $ do mmfRaw <- duma_MonoMultiFont_create (fromEnum (maxBound::i)) mmfPtr <- newForeignPtr duma_MonoMultiFont_Release mmfRaw return $ MonoMultiFont mmfPtr The problem is that ghc complains that there is no instance for Bounded i even though I've written "Bounded i" in the type declaration for the function. I was expecting that I could use the same type variables in the function body as in the type signature so that I could use maxBound::i to find the max bound of whatever type is instantiated to i. What am I doing wrong here, and is it possible to create such a function? Thanks, Brian.

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calling haskell from C++
by Vladimir Portnykh 23 May '06

23 May '06

Hello ALL, I am very new to Haskell and my question might appear very basic sorry about it in advance. I have a C++ application and I would like to be able to launch Haskell interpreter (lets say GHC) supplying the Haskell code (in a file ) to be interpreted from it. The Haskell program generates some output and I would like to use this output in my C++ program. Do you have any simple example to help me, please? Many thanks, Vladimir _________________________________________________________________ Are you using the latest version of MSN Messenger? Download MSN Messenger 7.5 today! http://join.msn.com/messenger/overview

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Trouble with Cabal
by Scherrer, Chad 22 May '06

22 May '06

Thanks to all who responded about the benefits of using Cabal. After trying out Bulat's Streams library, I thought I would download ByteString (aka FPS) to compare the wc demos. To compile something using Streams, I need to use a -i flag (thanks Bulat). When I followed the README that comes with ByteString, I get this: % ./Setup.hs install Installing: /f/g/lib/fps-0.7 & /f/g/bin fps-0.7... *** Exception: /f: createDirectory: permission denied (Permission denied) I don't have root on this machine, and it's not obvious (to me) how to specify that everything should be installed locally. Any suggestions? Thanks, Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

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QuickCheck Fun with Phantom Types
by Dominic Steinitz 22 May '06

22 May '06

I was doing Exercise 5 of Ralf's Fun with Phantom Types and naturally thought I'd check my solution with QuickCheck. The best I could was this. Is there something better? Can you somehow generate random types as well as random values in those types? Thanks, Dominic. PS the full source for my solution with tests is attached. prop_Idem t x = x == uncompress t (compress t x) instance Arbitrary Char where arbitrary = oneof (map return ['A'..'z']) class Reflect a where reflect :: Type a instance Reflect Int where reflect = RInt instance Reflect Char where reflect = RChar instance (Reflect a, Reflect b) => Reflect (a,b) where reflect = RPair reflect reflect instance Reflect a => Reflect [a] where reflect = RList reflect type Test1 = Int -> Bool type Test2 = (Int,Int) -> Bool type Test3 = String -> Bool type Test4 = (String,String) -> Bool main = do quickCheck ((prop_Idem reflect) :: Test1) quickCheck ((prop_Idem reflect) :: Test2) quickCheck ((prop_Idem reflect) :: Test3) quickCheck ((prop_Idem reflect) :: Test4)

1 1

Re: Writing an extensible interpreter
by Jonathan Tang 22 May '06

22 May '06

> Message: 2 > From: "David House" <dmhouse(a)gmail.com> > Subject: [Haskell-cafe] Writing an extensible interpreter > I'm in the process of writing an interpreter for the self-modifying > automaton languages SMATINY [1], designed by ihope (whose name some of > you may recognise from IRC). The current effort is online [2], but > it's at a rather early stage and there's a few modifications I'd like > to make to it. [snip] However, the remaining issue is step 1): how would the metacode handle > additional constructors to the Step ADT? Type classes won't work here: > the intepreter has a type of [Step] -> String. With type classes this > would have a type of something like Step a => [a] -> String, but by > the homogeneity of lists you'd only be able to have one type of step > in your program. > > You can get around the homogeneity of lists problem with existential types: http://www.haskell.org/hawiki/ExistentialTypes http://halogen.note.amherst.edu/~jdtang/scheme_in_48/tutorial/evaluator2.ht… Another solution might be to eliminate the algebraic data type entirely and have the Parsec actions return a Smatiny action. Instead of dispatching on the constructor of Step, package up any information you need as a closure and make Step a type synonym for 'Smatiny ()' or 'Smatiny a' or whatever type you need. Then the interpreter main loop just executes the action returned by the parser. - Jonathan

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Writing an extensible interpreter
by David House 22 May '06

22 May '06

Hi all. I'm in the process of writing an interpreter for the self-modifying automaton languages SMATINY [1], designed by ihope (whose name some of you may recognise from IRC). The current effort is online [2], but it's at a rather early stage and there's a few modifications I'd like to make to it. To begin with, extensibility is key. Such tiny languages are designed to be easily extendable, and I'd like my interpreter to follow suit. At the moment, adding a new command is as follows: 1) Add a new constructor for the Step type to represent your command. 2) Write a Parsec parser for your command in Smatiny.hs. 3) Add said parser to the 'inputs' list in Smatiny.hs. 4) Write a 'handler' for your command; a function in State that will take the appropriate action. This is a bit of an ordeal; I'd like new commands to be addable without touching Smatiny.hs, which will form the base of the interpreter. One solution I was thinking about was to write a new module for each command that exports a "parser" and "handler" function. A bit of metacode would read the Commands directory then import qualified all the modules it finds in there. Subsequently, it would call parseSmatiny (the parser; in Smatiny.hs) with all the parsers it found as an argument, then send the output of this into runSmatiny (the interpreter; also in Smatiny.hs). However, the remaining issue is step 1): how would the metacode handle additional constructors to the Step ADT? Type classes won't work here: the intepreter has a type of [Step] -> String. With type classes this would have a type of something like Step a => [a] -> String, but by the homogeneity of lists you'd only be able to have one type of step in your program. Thanks in advance for your input. Let me know if I didn't explain something too well; I have a tendancy toward brevity :) [1]: http://esoteric.voxelperfect.net/wiki/SMATINY [2]: http://esoteric.voxelperfect.net/files/smatiny/impl/smatiny.tar.gz -- -David House, dmhouse(a)gmail.com

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parsing machine-generated natural text
by Evan Martin 21 May '06

21 May '06

For a toy project I want to parse the output of a program. The program runs on someone else's machine and mails me the results, so I only have access to the output it generates, Unfortunately, the output is intended to be human-readable, and this makes parsing it a bit of a pain. Here are some sample lines from its output: France: Army Marseilles SUPPORT Army Paris -> Burgundy. Russia: Fleet St Petersburg (south coast) -> Gulf of Bothnia. England: 4 Supply centers, 3 Units: Builds 1 unit. The next phase of 'dip' will be Movement for Fall of 1901. I've been using Parsec and it's felt rather complicated. For example, a "location" is a series of words and possibly parenthesis, except if the word is SUPPORT. And that "Supply centers" line ends up being code filled with stuff lie "char ':'; skipMany space". I actually have a separate parser that's Javascript with a bunch of regular expressions and it's far shorter than my Haskell one, which makes sense as munging this sort of text feels to me more like a regexp job than a careful parsing job. I'm considering writing a preprocessing stage in Ruby or Perl that munges those output lines into something a bit more "machine-readable", but before I did that I thought I'd ask here if anyone had any pointers, hints, or better ideas.

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Re: [Haskell] installing streams library
by Chad Scherrer 19 May '06

19 May '06

Thanks Jared, but already tried those. I was able to download the Streams library, but I have know idea how to install it. On 5/19/06, Jared Updike <jupdike(a)gmail.com> wrote: > > I found this in an old post (gotta love GMail search): > > > You can find further information about the library at the > > page http://haskell.org/haskellwiki/Library/Streams and > > download it as http://freearc.narod.ru/Streams.tar.gz > > from a thread in February entitled: > > Streams: the extensible I/O library (Feb 21) > > Google site:haskell.org bulat extensible I/O library for the thread, etc. > > Hope that helps, > Jared. > > On 5/19/06, Chad Scherrer <chad.scherrer(a)gmail.com> wrote: > > It sounds like Bulat has gotten some impressive I/O speedups with his > > Streams library. I'd like to try this out, but I'm having some trouble > > installing it. I'm using GHC on Linux. > > > > My first attempt was looking around on this page: > > http://www.haskell.org/haskellwiki/Library/Streams > > > > There's a really nice description, but no signs of where to actually get > the > > library. Eventually (thanks to Google) I tracked down this message: > > http://article.gmane.org/gmane.comp.lang.haskell.general/13625 > > > > From here I was able to download it, but there's no information > regarding > > how this needs to be set up. There are directories "Data", "Examples", > and > > "System", which I assume are supposed to be plugged into the > hierarchical > > module structure, but how do I do that? I thought this might have > something > > to do with Cabal (I've not yet used that), but the Cabal manual talks > about > > a .cabal file, which doesn't exist here. > > > > Does this follow some standard approach that I'm not familiar with? > Where > > should I look to learn more? > > > > Thanks, > > -- > > > > Chad Scherrer > > > > "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx > > _______________________________________________ > > Haskell mailing list > > Haskell(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell > > > > > > > > > -- > http://www.updike.org/~jared/ > reverse ")-:" > -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

1 0

Troubles with FFI
by Matthew Bromberg 19 May '06

19 May '06

I am a newcomer to Haskell and I'm offering my perspective on the troubles I endured trying to make Haskell interface to some simple C programs. It is my hope, that by documenting this, someone else will benefit from my mistakes and errors and climb the learning curve somewhat faster than me. I was attracted to Haskell, after playing around with OCAML, and realizing that functional programming offers some nice advantages to scientific programming. Mainly because the notation and thinking is very mathematical. But first, in order to evaluate the suitability of this language for my purposes, I had to verify that the foreign function interface (FFI) was suitable. There are a number of numerical libraries, all in imperative languages, that I want to link to. I quickly discovered that the documentation on Haskell's FFI is rather poor currently. This topic is simply not of interest to the academics I suppose. It took many hours of searching to find this document: http://www.cs.caltech.edu/courses/cs11/material/haskell/papers/tackling_the… This little tutorial was a goldmine of information about monads and an introduction to the FFI. (No template solutions for my problem sorry to say.) It was certainly dismaying to see that even the simplest IO requires at least a minimal understanding of monads, and adds a bunch of extra semantics to the problem. However none of the solutions offered, even the automated ones, seemed to handle the case of passing numerical arrays to C, and having them get updated and passed back. That is my primary interest in the FFI and yet no obvious solution or example could be found after days of internet searching and pouring over tutorials etc. My conclusion, after hours of studying the hierarchical libraries was that I needed to use the StorableArray type which led me to this site, http://www.haskell.org/haskellwiki/Arrays#StorableArray_.28module_Data.Arra… This site proposes the following curious piece of code: {-# OPTIONS_GHC -fglasgow-exts #-} import Data.Array.Storable import Foreign.Ptr import Foreign.C.Types main = do arr <- newArray (1,10) 37 :: IO (StorableArray Int Int) readArray arr 1 >>= print withStorableArray arr $ \ptr -> memset ptr 0 40 readArray arr 1 >>= print foreign import ccall unsafe "string.h" memset :: Ptr a -> CInt -> CSize -> IO () After having freshly read the tutorial on monads I thought I was ready for this, but I ran into some parsing problems. I don't have a full grasp of the precedence rules for the operators, >>= , $ and function evaluation are, and whether they group left to right or right to left. A little type parsing on the inputs and outputs allowed me to resolve all but the line withStorableArray arr $ \ptr -> memset ptr 0 40 Which should resolve to a monad if I understand the do notation correctly. My first assumption was that $ has lower precedence than function application, but a higher precendence than >> or >>=, so I assumed the grouping, (withStorableArray arr) $ (\ptr -> (memset ptr 0 40)) Now withStorableArray has the type specification StorableArray i e -> (Ptr e -> IO a) -> IO a , so it should be obvious how to use it (Not). At any rate it takes two arguments so I might assume that (withStorableArray arr) curries the first argument so that this expression is a function of type (Ptr e -> IO a) -> IO a . Because of the type of the array, it appears that a = Int. However (\ptr -> (memset ptr 0 40)) should be a function of type Ptr a -> IO(a) in order to qualify as the second argument of withStorableArray. This almost fits, but not quite. memset should return type IO(Int) for this to work correctly, however according to it's type declaration it returns IO (). Shouldn't the compiler complain about this? So after about 10 hours of wrestling with syntax, including messing up the initial capitalization of module names, indenting in do lists, forgetting to put commas in tuples and lists I finally got the following example to compile without error or warning Matrix.hs __________ import Foreign import Foreign.C import Foreign.C.Types (CInt, CDouble ) import Data.Array.Storable foreign import ccall "matrix_c.h sumarr" sumarr :: Ptr CDouble -> CDouble main = do arr <- newListArray (1 , 3) [3,2,1]:: IO (StorableArray Int CDouble) -- extract the pointer to arr dsum <- withStorableArray arr (\ptr -> return (sumarr ptr )) print dsum matrix_c.c ------------------ #include <math.h> double sumarr(double *in) { return(in[0] + in[1]) ; } This is how I compiled it using ghc 6.4.2 > ghc -c matrix_c.o matrix_c.c > ghc -O -o Matrix.exe -fglasgow-exts Matrix.hs matrix_c.o It runs correctly > Matrix > 5.0 Notes: ---------------------------------------------- Matrix.exe fails to work correctly if the header file matrix_c.h is not included in the foreign import statement The hack? where the second argument of withStorableArray is a -> IO() instead of a -> IO(a) allows the do clause to be simplified to main = do arr <- newListArray (1 , 3) [3,2,1]:: IO (StorableArray Int CDouble) withStorableArray arr $ print . sumarr If one changes the array in[] in the C code, e.g. double sumarr(double *in) { in[0] = -10.0 ; return(in[0] + in[1]) ; } The array is changed inside the haskell program as a side effect. main = do arr <- newListArray (1 , 3) [3,2,1]:: IO (StorableArray Int CDouble) -- extract the pointer to arr withStorableArray arr $ print . sumarr (readArray arr 1 ) >>= print I suppose this is one way to 'pass' arrays back to Haskell.

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Space leak when returning pairs?
by Shin-Cheng Mu 19 May '06

19 May '06

Dear members, I am experiencing a space leak, which I suspect to be an instance of the problem addressed by Wadler before. I'd appreciate if someone here would take a look. Given the following datatype: data XMLEvent = StartEvent String | EndEvent String | TextEvent String deriving Show where the three constructors represent the start tag (<a> where a is a string), end tag (</a>), and text data, respectively, of an XML stream. (They are actually from the library HXML). The following function simply returns the same stream while doing a minimal amount of validation (ignoring the closing tag). idX :: [XMLEvent] -> ([XMLEvent], [XMLEvent]) idX [] = ([], []) idX (StartEvent a : strm) = let (ts, strm') = idX strm (us, strm'') = idX strm' in (StartEvent a [] : ts ++ EndEvent a : us, strm'') idX (EndEvent _: strm) = ([], strm) idX (TextEvent s : strm) = let (ts, strm') = idX strm in (TextEvent s : ts, strm') The function idX returns a pair, where the first component is the processed stream, while the second component is the rest of the input. The intention is to thread the input and release processed events. If the function is used in a pipelined manner: print . fst . idX . parseInstance $ input where parseInstance :: String -> [XMLEvent] is a lexical analyser, I would expect that the input stream will be consumed one by one as the output is produced, and the program would use a constant amount of heap space (while keeping a program stack proportional to the depth of the XML tree). This was not the case, however. For some reason the heap grew linearly. The GHC profiler showed that most of the thunks are created by parseInstance, which implies that the entire input stream somehow still resides in memory. I was wondering where the space leak came from and suspected that it's the leak described in one of Philip Wadler's early paper Fixing Some Space Leaks With a Garbage Collector (1987). Consider idX (StartEvent a : strm) = let (ts, strm') = idX strm (us, strm'') = idX strm' in (StartEvent a [] : ts ++ EndEvent a : us, strm'') The body of the let clause might have actually been treated as (StartEvent a [] : ts ++ EndEvent a : fst (idX (snd strm)), snd (idX (snd strm))) Therefore strm will not be freed until the whole evaluation finishes. But since Wadler has addressed this problem a long time ago, I think the fix he proposed should have been implemented in GHC already. Was that really the source of the space leak? If so is there a way to fix it? Or is there another reason for the leak? * * * The function idX above actually resulted from fusing two functions: buildF parses a stream into a forest, while idF flattens the tree. data ETree = Element String [ETree] | Text String buildF :: [XMLEvent] -> ([ETree], [XMLEvent]) buildF [] = ([],[]) buildF (StartEvent a : strm) = let (ts, strm') = buildF strm (us, strm'') = buildF strm' in (Element a ts : us, strm'') buildF (EndEvent _ : strm) = ([], strm) buildF (TextEvent s : strm) = let (ts, strm') = buildF strm in (Text s : ts, strm') idF :: [ETree] -> [XMLEvent] idF [] = [] idF (Element a ts : us) = StartEvent a : idF ts ++ EndEvent a : idF us idF (Text s : ts) = TextEvent s : idF ts My original program was like print . idF . fst . buildF . parseInstance $ input and it had the same space leak. By accident (assuming that the input is always a single tree), I discovered that if I added a "wrap . head" into the pipe: print . idF . wrap . head . fst . buildF . parseInstance $ input where wrap a = [a], the heap residency would reduce by half! The output remains the same. My explanation is that applying a "head" means that (in the definition of buildF): buildF (StartEvent a : strm) = let (ts, strm') = buildF strm (us, strm'') = buildF strm' in (Element a ts : us, strm'') the "us" part , which contains a reference to strm, need not be kept. Thus the reduced heap residency. This seems to support that the space leak resulted from the same problem Wadler addressed before. But isn't that solved already in GHC? * * * I'd appreciate if someone could look into it. The actual program is available at http://www.psdlab.org/~scm/hx.tar.gz It actually uses HXML, a library by Joe (Joe English?) to do the parsing. The main program is hxpc.hs. There is a 1 MB sized sample input file size1m.xml. There are two simple scripts "recompile" and "runhxpc" for compiling and running the program. Thank you! sincerely, Shin-Cheng Mu

4 6

Parsec memory behavior
by Juan Carlos Arevalo Baeza 19 May '06

19 May '06

Try this simple program: import Text.ParserCombinators.Parsec ppAny = tokenPrim show (\p _ _ -> p) (\t -> Just t) ppTest = many ppAny p s = case runParser ppTest True "" s of Left error -> show error Right result -> result main = do let fname = "C:/main.i" f <- readFile fname let tokens = p f putStrLn . show $ head tokens This is the simplest expression of using Parsec, it just returns the input unchanged. But it already seems to have a big memory leak. When parsing a 2 MB file, I'm seeing the program grow in size up to about 160 MB before it ends. I assume this is known behavior. I believe it's because of the way Parsec deals with errors: when you chain two parsers in sequence, the result of the first one (in this case the head of the list) is not available until it knows if the second one failed or not. Is there any known alternative that doesn't exhibit this behavior? It would have to somehow return errors inline or on a "side channel". I'll be toying with this sort of thing for a while. Thanx! JCAB

4 5

shared local definitions
by Alberto Ruiz 19 May '06

19 May '06

Hi all, I have a question about optimization of shared values in local definitions. I frequently use this scheme: fun a b c d x = r where q = costly computation depending only on a, b, c, and d r = depends only on q and x g1 = fun 1 2 3 4 g2 = fun 5 4 2 7 (etc.) When I compute (using ghc -O) things like map g1 [1 .. 1000] the common q is evaluated only once, which is very nice. But the problem is that in some strange cases this kind of optimization is not applied, and the same q is evaluated 1000 times. Curiously, this happens if I add just: module Main where to one of my programs. Optimization is also lost if in the same program I use two partially applied functions: map g1 [1 .. 1000] map g2 [1 .. 1000] And in some cases optimization is only applied if the local definition is "easy" enough. For example: --------------------------------------------------- fun1 :: Int -> Int -> Int fun1 a x = q*x where q = {-# SCC "easy" #-} a+1+a^2 -- OK fun2 :: Int -> Int -> Int fun2 a x = q*x where q = {-# SCC "hard" #-} a+1+a^2 +a^3+(2*a) -- NO fun3 :: Int -> Int -> Int fun3 a x = r where q = local a r = q*x local u = {-# SCC "local easy" #-} u+1 -- OK fun4 :: Int -> Int -> Int fun4 a x = r where q = local a r = q*x local u = {-# SCC "local hard" #-} u+1+u^2 -- NO test h = print $ sum $ map h [1 .. 100] main = do test (fun1 3) test (fun2 3) test (fun3 3) test (fun4 3) -------------------------------------------------- COST CENTRE MODULE no. entries MAIN MAIN 1 0 main Main 154 101 fun4 Main 166 300 local hard Main 167 100 NO fun3 Main 163 100 fun2 Main 161 100 hard Main 162 100 NO fun1 Main 157 100 test Main 155 4 CAF Main 148 6 fun4 Main 168 0 main Main 158 0 fun3 Main 164 1 local easy Main 165 1 OK fun1 Main 159 0 easy Main 160 1 OK test Main 156 0 CAF System.IO 103 1 CAF GHC.Handle 101 3 ------------------------------------------------------------ Where can I find information about this topic? I have made some searches but probably using wrong keywords. Should I use some individual optimization flags? Thanks, Alberto

4 3

Controlling scope using monadic classes
by oleg＠pobox.com 18 May '06

18 May '06

Daniel McAllansmith wrote: > I'm trying to control the scope within which functions can be used by > putting them in a type class. Unfortunately I can't seem to figure > out how to get it done. Any advice would be much appreciated. Hopefully the following is close to what you wanted. The idea is to describe each appropriate function (like getInfo) with a label and to identify each monadic scope with a label, and then define the relationship which tells which labeled functions can be used within which scopes. If we attempt to obtain InfoC within the scope B, we get an error ``no instance (LabelOK ScopeC ScopeB)'' -- which seems clear. The more elaborate version of similar example can be found here: http://pobox.com/~oleg/ftp/Haskell/types.html#monadic-regions Using labels to enforce well-formedness term constraints (content model constraints for HTML/XML terms) can be found here: http://www.haskell.org/pipermail/haskell/2006-March/017656.html http://www.haskell.org/pipermail/haskell/2006-March/017684.html {-# OPTIONS -fglasgow-exts #-} module Scopes where import Control.Monad.Trans -- Our scopes newtype ScopeA m a = ScopeA{runScopeA :: m a} deriving (Monad, MonadIO) newtype ScopeB m a = ScopeB{runScopeB :: m a} deriving (Monad, MonadIO) newtype ScopeC m a = ScopeC{runScopeC :: m a} deriving (Monad, MonadIO) runScopeBA (ScopeB x) = ScopeA x runScopeCB (ScopeC x) = ScopeB x -- The deriving Show part is just to make the example better instance Show (ScopeA m a) where show _ = "ScopeA" instance Show (ScopeB m a) where show _ = "ScopeB" instance Show (ScopeC m a) where show _ = "ScopeC" -- Here we define the relationship that tells which labeled functions -- (identified by label') can appear within which scope (identified by label) class LabelOK (label' :: ( * -> * ) -> * -> * ) (label :: ( * -> * ) -> * -> * ) -- If we use overlapping isntances extension, the number of instances -- below can be significantly reduced. instance LabelOK ScopeA ScopeA instance LabelOK ScopeA ScopeB instance LabelOK ScopeA ScopeC instance LabelOK ScopeB ScopeB instance LabelOK ScopeB ScopeC instance LabelOK ScopeC ScopeC -- The function getInfo is labeled. The constraint LabelOK tells -- the the function can be used within any (monadic) scope where -- it is OK... getInfo :: (Monad m, Monad (label m), Show (label' [] ()), LabelOK label' label) => label' [] () -> label m String getInfo l = return (show l) -- The latter functions are to be defined analogously -- putInfo :: LabelOK label' label => label' -> String -> m () -- updateInfo :: UpdateOK label' label => label' -> Int -> m () main = runScopeA aScoped >>= print aScoped = do bResult <- runScopeBA bScoped -- updateAInfo bResult return "done" -- Inferred type: bScoped :: ScopeB IO Int bScoped = do i1 <- b1 i2 <- b2 return (i1 + i2) b1 :: (Monad m, MonadIO m) => ScopeB m Int b1 = do -- Within B scope, we can request AInfo and BInfo getInfo (ScopeA [()]) >>= liftIO . print getInfo (ScopeB [()]) >>= liftIO . print -- But we can't request CInfo -- If we uncomment the following, we get -- no instance (LabelOK ScopeC ScopeB) -- I think the error message is quite clear -- getInfo (ScopeC [()]) >>= liftIO . print return 2 b2 :: (Monad m, MonadIO m) => ScopeB m Int b2 = runScopeCB cScoped >>= return . fromEnum cScoped :: (Monad m, MonadIO m) => ScopeC m Char cScoped = do -- Within C scope, we can request AInfo and BInfo and CInfo getInfo (ScopeA [()]) >>= liftIO . print getInfo (ScopeB [()]) >>= liftIO . print getInfo (ScopeC [()]) >>= liftIO . print return '('

2 1

Haskell DB and XML libs: one user's experience
by Robert Dockins 18 May '06

18 May '06

Hello all, I recently found myself needing to do some data manipulation; I needed to take some data from a database and generate a series of XML files from it. In the past I've done most of this sort of work in Java, but this time I decided I'd take the opportunity to explore the state of the art of Haskell DB and XML libraries. As to DB, I tried using HDBC first. I was actually a little surprised how straightforward it was. My database (PostgreSQL) is directly supported, and the compile/install went smoothly. My first test connection program that typechecked worked as expected (!) and I was soon executing queries doing useful work with the results. I'd just like to take a moment to congratulate John Goerzen for creating a product with a low barrier of entry for using databases in Haskell. As I didn't really do anything beyond simple queries, can't comment on more advanced features. For XML, I wavered between HaXml and HXT (the Haskell XML Tookbox). I initially decided to use HXT because it has support for xml namespaces, which I was going to need, and because it just seems to be the most complete and advanced package available. The HXT install suffers a little bit from transitive-closureitis, but, overall wasn't too bad. However, I had a really hard time using it! The API is _really_ intimidating, and I couldn't find any basic tutorial-style documentation. The API docs are a little hard to use because related definitions are spread out over a bunch of modules, and the links don't always work. Also, the theses are nice, but they read like theses ;-) That's not what I want when I have a job to complete. Long story made short; I couldn't figure out how to create and XML document and serialize it to disk. I was reasonably motivated and I'm a pretty experienced Haskell programmer, but I had to call it quits after about 3 hours of struggling with it. Most of my programs would mysteriously fail to produce output OR errors! It was really frustrating. I ended up using HaXml instead and shoehorning in the namespaces by using attributes named "xmlns:xyz" etc. on the document root element (which is OK, but not ideal). The HaXml API was also tough to work with but was less mystifying than HXT's, and I eventually got it to work. I was a little disappointed by the results, because the pretty printer does some fairly bizarre things to ensure that it doesn't introduce extra whitespace into the DOM. I also had to do some futzing to make HaXml correctly escape literal text. Finally, the using the HaXml API to generate XML results in verbose code that's hard to read. I was hoping that I'd get results comparable to using xmlenc (http://xmlenc.sourceforge.net/) in Java, but I was disappointed by fairly low signal-to-noise ratio (although in all fairness, its probably comparable to using the DOM or SAX Java APIs). Overall, HaXml works, but feels a bit awkward, at least for this use case. Now taking a slightly closer look at HXML, I see that it may be the best choice for what I was attempting to do (although it also doesn't support namespaces). The simplified representation looks particularly nice for building XML from scratch. I may try rewriting with HXML and see how that goes. So that's it. I don't have any deep conclusions, but I thought I'd share my experiences in the hopes that they will be helpful to somebody. Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG

2 1

RE: [Haskell-cafe] develop new Haskell shell?
by Bayley, Alistair 17 May '06

17 May '06

> From: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Graham Klyne > > Did you see [http://nellardo.com/lang/haskell/hash/] ? > > Google also finds some links to code. > > #g > -- > > Marc Weber wrote: > > Hi. > > > > Who wants to try devloping a new shell with me? Also: http://www.cse.unsw.edu.au/~dons/h4sh.html ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

14 24

Controlling scope using monadic classes
by Daniel McAllansmith 17 May '06

17 May '06

Hi. I'm trying to control the scope within which functions can be used by putting them in a type class. Unfortunately I can't seem to figure out how to get it done. Any advice would be much appreciated. What I want is to start out in a certain scope, which restricts me to using functions in that scope or opening up a subsidiary scope at which point I'm restricted to functions in that scope or opening up an even deeper scope. Hopefully a failed attempt will help explain what I'm trying to achieve... the following has trouble with the inScope{B,C} functions. type AInfo = String type BInfo = String type CInfo = String type BResult = Int type CResult = Char class (Monad m) => WithinA m where askAInfo :: m AInfo class (WithinA m) => WithinB m where askBInfo :: m BInfo class (WithinB m) => WithinC m where askCInfo :: m CInfo class (WithinA m) => ScopeA m where getAInfo :: m AInfo putAInfo :: AInfo -> m () updateAInfo :: BResult -> m () inScopeB :: (ScopeB m2) => m2 BResult -> m BResult class (WithinB m) => ScopeB m where getBInfo :: m BInfo putBInfo :: BInfo -> m () inScopeC :: (ScopeC m2) => m2 CResult -> m CResult class (WithinC m) => ScopeC m where getCInfo :: m CInfo putCInfo :: CInfo -> m () aScoped :: (ScopeA m) => m String aScoped = do bResult <- inScopeB bScoped updateAInfo bResult return "done" bScoped :: (ScopeB m) => m BResult bScoped = do i1 <- b1 i2 <- b2 return (i1 + i2) b1 :: (ScopeB m) => m Int b1 = return 2 b2 :: (ScopeB m) => m Int b2 = inScopeC cScoped >>= return . fromEnum cScoped :: (ScopeC m) => m Char cScoped = return '(' Thanks Daniel

1 0

transactional cache
by Alberto G. Corona 16 May '06

16 May '06

Finally i did it. The transactional cache uses STM, no blocking MVars. I created has a simple interface, the transactional "plumbing" is hidden in the TransactCacheSTM module. The usage is simple: withResources [object1, object2,..] process where process[maybeObject1,maybeObject2.....]= [newObject1,newObect2...] newObject1=..... newObject2=.... "process" run atomically, object1,object2.. are partially defined, just with the fields neccesary to retrieve them. newObjects are the resulting objects that replace the previous content in the cache. Non existing objects are created. the objects must implement "IResource" type class procedures for reading, writing and obtaining a unique key so that the objects can be read to, write from and do search from the cache. No other asumptions are made about the object types or storages. In the following example, client Alberto buy a computer. the total amount due by Alberto is increased with the computer price. The quantity of the item "computer" in stock is decreased. withResources[Client{name="Alberto"}, Item{iname="computer"} buy where buy[Nothing,_]= error "Client does not exist" buy[_,Nothing]= error "Item does not exist" buy[Just client, Just item]= [client{total client=total client+ price item}, item{quantity=quantity-1}] The resulting code is transactional and clean. The object is read if not already in the cache using the user defined procedure for that object (either a sql query or read from a file). The write is delayed depending on object usage in the cache and the cache size. I also defined ordinary, non transactional object retrieval (getResources :: [Resource]->IO[Resource] ) To have real ACID transactions, I also defined a procedure for atomic write of the entire cache to disk, in order to have coherent content in permanent storage, (I´m using it on a portal 100% coded in Haskell (I love Haskell since i knew it five months ago). this trhansactional cache makes the data handling in my portal a lot easier. I use XML files, no boring database tables are necessary. I will put online the transactional cache sometime later, but by now I will send te code by mail to anyone interested. Message: 8 Date: Sat, 13 May 2006 12:26:42 +0900 From: "Evan Martin" <martine(a)danga.com> Subject: Re: [Haskell-cafe] RE: transactional cache To: "Alberto G. Corona" <agocorona(a)gmail.com> Cc: haskell-cafe(a)haskell.org Message-ID: <3a6f89fc0605122026y6727d55cl61e60a101c73e93e(a)mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed On 5/13/06, Alberto G. Corona <agocorona(a)gmail.com> wrote: > > notglobal = newIORef True > > main = do a <- notglobal > > b <- notglobal > > Thanks. I got the poit more or less; Each invocation creates a new > IORef instance. Another way of looking at this, that might be more instructive, is that notglobal is defined to be the action of creating a new IO ref. You can see that in its type: > :t newIORef True newIORef True :: IO (IORef Bool) I read that type as "an IO operation that produces an IORef Bool when executed". Then the code in main "executes" notglobal twice. Another way of looking at this is that you can always substitute the right side of an equals sign in for the left side. If you do that on this code this makes it plain that a and b will be different. (unsafePerformIO breaks this substitution rule.)

1 0

Newbie:Debugging and Overgeneralization
by Aditya Siram 15 May '06

15 May '06

I have been working with a Haskell text for the past couple of months or so and I have some general problem solving questions. 1. Is there a way to output intermediate values of a calculation? As an imperative programmer I have become used to using "System.out"'s or 'cout's to check that my function works as intended. I can see no easy way to do that in Haskell. The only solution I could come up with was to avoid using nested functions except for the most trivial expressions and test all expressions in the interpreter. This approach seems to make my code ugly and less readable. 2. Haskell is great because it makes abstracting from problem very easy. For example, if the problem asks for the area of a square, why not write a function to compute the area of all polygons? find myself falling into the trap of generalizing to the point that a simple problem becomes quite a bit harder . From a general design perspective should I concentrate on abstracting away just enough to solve the problem or solve the harder problem in the hoping of reusing that code to make life easier in the future? Thanks.... Deech

3 2

dumb monad syntax question
by mvanier 15 May '06

15 May '06

I've been reading Phil Wadler's monad papers from the early '90s, and it's been interesting to see how the monad concept evolved over the course of those years. But I haven't been able to track down the first use of the "do" notation for monads. Can anyone tell me where that came from? I'd appreciate paper citations if it was presented initially in a paper. Thanks in advance, Mike

4 3

Linspire/Freespire Core OS Team and Haskell
by Clifford Beshers 15 May '06

15 May '06

The OS team at Linspire, Inc. would like to announce that we are standardizing on Haskell as our preferred language for core OS development. We are redoing a bunch of our infrastructure using Haskell as our common standard language. Our first task is redoing our Debian package builder (aka autobuilder) in Haskell. Other tools such as ISO builders, package dependency checkers are in progress. The goal is to make a really tight simple set of tools that will let developers contribute to Freespire, based on Debian tools whenever possible. Our hardware detector, currently in OCaml, is on the block to be rewritten as well. There are four of us using Haskell, all CCed on this message. All of us have been using functional languages for quite some time. At Linspire, our choices have been OCaml and Haskell. David Fox wrote the hardware detector in OCaml and is now porting it to Haskell. Jeremy Shaw has been doing various utilities in Haskell for several years. Sean Meiners recently wrote an application for managing his recipe collection and is now hooked. I am porting our CD build procedure from OCaml to Haskell. We are interested in many other uses of Haskell. The recent discussion about Haskell as a shell interests greatly, for example, as we have all suffered through years of bash code. We'd also like to make some Haskell bindings for Qt and KDE, though at the moment we don't have a good plan to tackle that problem efficiently. To date, Linspire (formerly Lindows) has focused on polishing Linux for the consumer market. I mentioned Freespire, above. We announced Freespire recently (www.freespire.org) Essentially it is a more open, developer friendly version of Linspire. http://freespire.org/about/vision and http://freespire.org/support/faqs have good overviews. Access through apt, open-source CNR client and many other good things. I mention Freespire because some of our colleagues were concerned that using Haskell would isolate us from the larger community of developers and make it hard to find new employees skilled in Haskell, should we need to. From our perspective, functional programming makes us more effective and we think that getting even a few people who know Haskell hacking with us is a better combination than lots of Perl and bash. I'm not sure I expect anyone on this list to disagree, but still I'd love to hear your thoughts on the subject. Also, Linspire is based on Debian. We've talked a little with John Goerzen who announced his missingH library here a while back. We've imported it and expect to pass updates back to him as well as any other libraries and tools that he would be interested in includng in the Debian archive. Also, it seems there are quite a few other libraries out there which are either not debianized or stale. We are looking into helping the folks on the debian-haskell list with that, if possible, documenting and automating wherever possible. Clifford Beshers <clifford.beshers(a)linspire.com> OS Team Lead Linspire, Inc. (I'm subscribed to this list from my personal account. Feel free to use either address.)

2 1

Existentially-quantified constructors: Hugs is fine, GHC is not?
by Otakar Smrz 15 May '06

15 May '06

Dear all, while WinHugs (20051031) lets me match against an existentially quantified constructor data ... = ... | forall b . FMap (b -> a) (Mapper s b) ... where FMap qf qc = stripFMap f q the GHC compiler as well as GHCi (6.4.2 and earlier) issue an error My brain just exploded. I can't handle pattern bindings for existentially-quantified constructors. Let me give the whole (non-practical) code, which is well typed and compiles in Hugs, and then show the change I had to do to make it work in GHC, too. The question is why there is a difference. Am I misusing something? The point of the complexFun below is to explore the Mapper data structure, taking care of the :&: constructor and quickly (transitively) skipping the FMap constructors, only accumulating and composing the tranformation functions that these provide. ----------------------------------------------------------- module Problem where import Data.Map as Map hiding (map) type Labels a = [a] data Mapper s a = Labels a :&: Map.Map s (Mapper s a) | forall b . FMap (b -> a) (Mapper s b) stripFMap :: Ord s => (a -> c) -> Mapper s a -> Mapper s c stripFMap k (FMap f p) = stripFMap (k . f) p stripFMap k x = FMap k x complexFun :: Ord s => (b -> a) -> Mapper s b -> s -> [a] complexFun f c y = case c of FMap t q -> complexFun qf qc y where FMap qf qc = stripFMap (f . t) q -- !!! r :&: k -> case Map.lookup y k of Just q -> let FMap qf qc = stripFMap f q -- !!! in case qc of ([] :&: _) -> complexFun qf qc y (xs :&: _) -> map qf xs _ -> error "Never matching" Nothing -> error "Irrelevant" ----------------------------------------------------------- Even though GHC does not let me pattern-match on FMap, I can use a case expression in complexFun instead -- then it compiles alright: ------ complexFun f c y = case c of FMap t q -> case stripFMap (f . t) q of -- !!! FMap qf qc -> complexFun qf qc y -- !!! _ -> error "No option" -- !!! r :&: k -> case Map.lookup y k of Just q -> case stripFMap f q of -- !!! FMap qf qc -> case qc of -- !!! ([] :&: _) -> complexFun qf qc y (xs :&: _) -> map qf xs _ -> error "Never matching" _ -> error "No option" -- !!! Nothing -> error "Irrelevant" ------ If I wanted to make this auxiliary case on stripFMap local, there would be a problem for both Hugs and GHC: Hugs: Existentially quantified variable in inferred type! *** Variable : _48 *** From pattern : FMap xf xc *** Result type : (_48 -> _32,Mapper _30 _48) GHC: Inferred type is less polymorphic than expected Quantified type variable `b' is mentioned in the environment: qc :: Mapper s1 b (bound at Problem.hs:65:27) qf :: b -> a1 (bound at Problem.hs:65:23) When checking an existential match that binds xf :: b -> a xc :: Mapper s b The pattern(s) have type(s): Mapper s1 a1 The body has type: (b -> a1, Mapper s1 b) In a case alternative: FMap xf xc -> (xf, xc) ------ complexFun f c y = case c of FMap t q -> complexFun qf qc y where (qf, qc) = case stripFMap (f . t) q of -- !!! FMap xf xc -> (xf, xc) _ -> error "No option" r :&: k -> case Map.lookup y k of Just q -> let (qf, qc) = case stripFMap f q of -- !!! FMap xf xc -> (xf, xc) _ -> error "No option" in case qc of ([] :&: _) -> complexFun qf qc y (xs :&: _) -> map qf xs _ -> error "Never matching" Nothing -> error "Irrelevant" ------ Many thanks for your comments or advice! Best, Otakar Smrz

4 3

CDouble type coercion
by SevenThunders 14 May '06

14 May '06

I am new to Haskell and found myself in a bind concerning the use of the C types, CDouble in particular. I extract a CDouble via it's pointer from a StorableArray. Since the array must interface with C the elements of the array must be CDouble. Now I'd like to use Text.Printf to format print statements of elements of the array, but Text.Printf requires Doubles as inputs and so far I have not found an obvious way to coerce CDoubles into Doubles. Additionally, withStorableArray returns a monad containing a CDouble instead of Double as peek I suppose does. Because I cant coerce my CDouble into a Double, printf chokes. Thus the following code fails to compile on GHC import Foreign import Foreign.C import Foreign.C.Types (CInt, CDouble ) import Data.Array.Storable import Text.Printf foreign import ccall "matrix_c.h sumarr" sumarr :: Ptr CDouble -> IO (CDouble) main = do arr <- newListArray (1 , 3) [3,2,1]:: IO (StorableArray Int CDouble) -- extract the pointer to arr withStorableArray arr sumarr >>= (\x -> printf "15.7f\n" x) The error message is, test2.hs:13:44: No instance for (PrintfArg CDouble) arising from use of `printf' at test2.hs:13:44-49 Probable fix: add an instance declaration for (PrintfArg CDouble) In a lambda abstraction: \ x -> printf "15.7f\n" x In the second argument of `(>>=)', namely `(\ x -> printf "15.7f\n" x)' In the result of a 'do' expression: (withStorableArray arr sumarr) >>= (\ x -> printf "15.7f\n" x) -- View this message in context: http://www.nabble.com/CDouble-type-coercion-t1615450.html#a4378492 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

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[newbie] processing large logs
by Eugene Crosser 14 May '06

14 May '06

Hello gentlemen, I am exposed to functional programming for less than a month, and just trying to understand the concepts, so please bear with me. I tried to use Haskell for a simple task on my dayjob, that involved parsing mail system logs and counting distinct addresses (I work for a relatively big ISP). Reducing unnecessary details, let's say that we have a file of N lines, some of them repeating, so that there are only M distinct lines, where M << N. The task is to count the number of times each distinct line appears in the file, and print the most frequent one with its count. This is my program: ======== module Main where import Data.Map main = printMax . (foldr processLine empty) . lines =<< getContents processLine line map = insertWith (\new old -> new + old) line 1 map printMax map = putStrLn $ show $ foldWithKey (\key val accum -> if val > (snd accum) then (key,val) else accum) ("",0) map ======== The thing kinda works on small data sets, but if you feed it with 250,000 lines (1000 distinct), the process size grows to 200 Mb, and on 500,000 lines I get "*** Exception: stack overflow" (using runhaskell from ghc 6.2.4). (For comparison, a perl script does the same job (using global hash) an order of magnitude faster, consumes 3 Mb RAM and can process billion lines without a problem). The question is: what I am doing wrong? Thanks Eugene crosser@pccross:~/src$ perl genlist.pl 500000 1000|time runhaskell distinct.hs *** Exception: stack overflow Command exited with non-zero status 1 27.46user 0.49system 0:29.75elapsed 93%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+34676minor)pagefaults 0swaps crosser@pccross:~/src$ perl genlist.pl 250000 1000|time runhaskell distinct.hs ("a0000000531",300) 36.66user 0.72system 0:54.82elapsed 68%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (522major+50361minor)pagefaults 0swaps crosser@pccross:~/src$ perl genlist.pl 250000 1000|time perl distinct.pl a0000000531: 300 0.22user 0.00system 0:01.14elapsed 20%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (15major+472minor)pagefaults 0swaps

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foldl to foldl' , ok ; but with foldM ?
by minh thu 14 May '06

14 May '06

Hi, I know the question is raised often but i cannot find the answer. In a previous mail, I was said to use foldl' instead of foldl and it worked really well : acc1 values = foldl1' (+) values How is foldl' defined ? I have two other related questions : What if I want to write something like this : foldl' f emptyArray values where f accumulates a value into an array. What's the best representation for an array here ? Also, given the fact that I though to represent my array with a c-like Ptr Word8 array, i tried this code, where the accumulator is referenced by an IORef : acc2 ints = do accRef <- newIORef 0 foldM add accRef ints acc <- readIORef accRef putStrLn $ show acc return acc where add accRef i = do modifyIORef accRef (+ i) return accRef but it's far worse than the code with foldl'. I guess someone will tell me to try to strictify something... but I don't know what and how. I think trying to strictify the accRef is worthless : it's the referenced acc that needs to be evaluated. Is-it correct ? Thanks !! VO Minh Thu

1 0

develop new Haskell shell?
by Marc Weber 13 May '06

13 May '06

Hi. Who wants to try devloping a new shell with me? The main goals: try adding some haskell scriting instead of bash/zsh, history dependend on a) executing program b) current dir c) last commands d) workspaces which should mean that the shell should save at least the last 10 commands of a,b,c,d. So you can do emerge (lookup parameters in history) even if you haven't used emerge for ages.. :) Nice, isn't it? d) Workspaces should mean: You can define some kind of workspace like workspace=haskellproject, wash, apache to add these tags together with the commands to the history.. So when working only in the "wash" workspace you can easily find those commands.. Perhpas it's even useful to attach commands or even scripts to those workspaces? eg the startApache script may be attached to admin, apache, ..., the cd /etc/init.d command only to admin.. I also would like to have some advanced kind of directory matching, defining aliases for directories. eg just type cd /usl to get a list of diretories looking like this: /UserShupportLocales /usr/src/linux /usl ? Using tab and bash is nice but it might be done better? Any suggestions? One would have to think about how to run processes in background and so on ... adding files as parameters the way it's possible in mc ( select them and add them to the command line ) perhaps even implement cp/mv/ ... for virtual file systems like zip files/ ftp/ ... ? and last but not least: on windows add all Programs beeing found in Start-> Programs to the path list... I wish I could just do word/ Enterprise Manager at a shell and not searching for the menu entries over and over again.. ;) I know I can add them the to the path.. but that would be some work, too.. and not desirable in any case. I could imagine adding a small prefix to each cmd eg. eb (execute bash cmd) ez (execute zsh cmd) r (remove file list) efs (execute from windows start menu) bg <cmd> run in background like bashs & feature. Perhaps even introduce some new syntax ? or use ghci or hugs with a preprocessor to translate these commands to haskell commands? What do you think? Marc Weber

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RE: transactional cache
by Alberto G. Corona 13 May '06

13 May '06

Albert Lai: Thanks. I got the poit more or less; Each invocation creates a new IORef instance. UnsafePerformIO appears to generate a unique IORef that can be shared (sorry for my imperative vocabulary, I´m sill contaminated by al these evil languages ;). I tried with "usafePerformIO NewTVar v" but the program fails miserably in a memory fault. I finally did it well usign a IORef than contains the TVar: refcache =unsafePerformIO $ (do c <- atomically $ newTVar emptyFM newIORef c) and then dereferencing refcache in the IO Monad I get ever the same context no matter where i do it: do tvcache <- readIORef refcache atomically $ do finiteMap <- readTVar tvcache (useful code here at last)... --------------------- Who said that Haskell is difficult?. Jokes apart, STM is powerful. I will share the transactional cache when I have it tested. -----------referred message:------- Message: 1 Date: 12 May 2006 00:19:28 -0400 From: Albert Lai <trebla(a)vex.net> Subject: Re: [Haskell-cafe] Re: Haskell-Cafe Digest, Vol 33, Issue 9 To: haskell-cafe(a)haskell.org Message-ID: <4u3bffq3hr.fsf(a)shell.vex.net> Content-Type: text/plain; charset=us-ascii "Alberto G. Corona " <agocorona(a)gmail.com> writes: > stmcache= newTVar 0 I will explain what this doesn't with an analogy. import Data.IORef notglobal = newIORef True main = do a <- notglobal b <- notglobal writeIORef a False x <- readIORef b print x To better show what's going on, I also provide this for contrast: import Data.IORef import System.IO.Unsafe global = unsafePerformIO (newIORef True) main = do x <- readIORef global print x writeIORef global False x <- readIORef global print x

2 1

(no subject)
by rami makram 12 May '06

12 May '06

Hi, Thx for replying and answering my question.. But, The idea is that i have to implement it using this kind of type generateTruthTable->[[(Char,Bool)]] and we didn't take the Shell and libraries in common, so i have to implement in a simpler way.. Can you give me another way or even some hints.. I have already implemented the And,Or,Impl,Neg,and other functions.. Thx.. _________________________________________________________________ Don't just search. Find. Check out the new MSN Search! http://search.msn.click-url.com/go/onm00200636ave/direct/01/

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Haskell + Windows API = wuh?
by ihope 12 May '06

12 May '06

Ello. I'm looking for a way to interface the Windows API with Haskell in order to write a Win32 console handler thingy. I don't know anything about the Haskell Foreign Function Interface beyond what its name implies and that it's an extension to Haskell 98 (or is it?). So just where do I start? --ihope

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Re: Haskell-Cafe Digest, Vol 33, Issue 9
by Alberto G. Corona 12 May '06

12 May '06

Hi, I´m trying to make a searchable transactional cache using STM. The whole idea is to use indexed TVar variables using a FiniteMap. another TVar holds the finitemap . This last TVar has to be a global variable. I found that when handled as global, a TVar does not keep the state. For example: stmcache= newTVar 0 main= do v<- val v<- val print v val = atomically $! do cache <- stmcache x <- readTVar cache writeTVar cache (x+1) return x this code print 0 instead of 1 as the result of incrementing cache and retrieving it the second time The same happens with: iocache= atomically $ newTVar 0 `debug` "newTVar" main= do v<- val v<- val print v getChar val = do cache<- iocache atomically $! do x <- readTVar cache writeTVar cache (x+1) return x here i used a IO TVar global instead of a STM TVar This other version , that passes TVar as parameter, does work: iocache= atomically $ newTVar 0 `debug` "newTVar" main= do cache<-iocache v<- val cache v<- val cache print v val cache= atomically $! do x <- readTVar cache writeTVar cache (x+1) I need the global version since the cache acces may be in any location on the program.... Anyone know a solution using STM? The MVar version works OK, but blocking the entire index is not the best solution....

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RE: Template Haskell Person/Paper entry
by Simon Peyton-Jones 11 May '06

11 May '06

Can anyone help Christoph with uploading a Postscript file to a haskellwiki page? Thanks | -----Original Message----- | From: Ch. A. Herrmann [mailto:herrmann@uni-passau.de] | Sent: 11 May 2006 14:29 | To: Ian Lynagh | Cc: Simon Peyton-Jones | Subject: Re: Template Haskell Person/Paper entry | | Hi, | | I was able to edit | | http://haskell.org/haskellwiki/Template_Haskell | | and add a paragraph to it, but there are still two problems. | | (1) When I try to upload a Postscript file (597453 bytes) I get | the error message "file contains no data" |

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Porting GHC to OSX86?
by Deling Ren 10 May '06

10 May '06

Hi there, Has anyone made any attempt to port GHC to Mac OS X on x86? Wolfgang Thaller’s binary package runs over Rosetta but slow (not surprising). It can not be used to compile a native version either (I got some errors related to machine registers). I tried to do a bootstrap but can't find the ".HC" files mentioned in the manual. They don't seem to be on the download page of GHC. Any ideas? Thanks. Deling

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Re: [Haskell] FP-style vs. OO-style AST structure
by Robert Dockins 10 May '06

10 May '06

[moved to haskell-cafe] On Wednesday 10 May 2006 11:09 am, Doug Kirk wrote: > Hi, > > I'm a Haskell newbie, but not new to programming, and I have a > question regarding style (I think). > > I'm writing a parser for OMG's OCL, and have two ways of defining the > AST model of a constraint. Each constraint in OCL has the following 4 > characteristics: > > 1. name :: Maybe String > 2. context :: UmlElement > 3. expr :: OclExpression > 4. type :: OclConstraintType > > Now, having come from an O-O background, this looks right; however, in > an FPL, it may not be. OclConstraintType is essentially an enumeration > of the values: > > Invariant > > | Precondition > | Postcondition > | InitialValue > | Derivation > | Body > > The question is this: is it better to create a single type as above > with a 'type' attribute, or would it be better to use the types as > separate constructors of a Constraint, each constructor taking the > same attributes? Opinions may differ on this, bit I'm going to go out on a limb and say that using separate constructors with the same field names is the more idiomatic approach in Haskell. Having a product type or a big record with an enumerated tag field is usually just a way to simulate sum types. Why do that if the language supports them directly? > I'm looking to avoid any pitfalls that could occur with either > decision, and at this point I don't know the benefits of doing it one > way vs. the other. So long as every constraint has exactly the same fields, there isn't much practical difference. If later you discover that different kinds of constraints need different fields, you'll be better off with the separate constructors. > BTW, I am using UUST Parser Combinator AG for the parser definition, > in case that makes a difference. > > Thanks! > --doug

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GHC 6.4.1 x86-64 does not compile
by Dusan Kolar 09 May '06

09 May '06

Hello all, I've install "universal binary" for x86_64 of GHC 6.4.1. The installation was done on AMD dual core machine. Uname for the machine gives: Linux <machine name> 2.6.16.5 #1 SMP Thu Apr 13 09:08:22 CEST 2006 x86_64 x86_64 x86_64 GNU/Linux While ghci was running some tests well, the ghc ended compilation with a long list of errors starting like this: Chasing modules from: queen-main Compiling QueensDK ( ./QueensDK.hs, ./QueensDK.o ) /tmp/ghc27286.s: Assembler messages: /tmp/ghc27286.s:26: Error: bad register name `%r12' /tmp/ghc27286.s:27: Error: bad register name `%r15' /tmp/ghc27286.s:29: Error: bad register name `%r12)' /tmp/ghc27286.s:30: Error: bad register name `%r12)' /tmp/ghc27286.s:31: Error: bad register name `%r12)' /tmp/ghc27286.s:32: Error: bad register name `%rax' /tmp/ghc27286.s:33: Error: bad register name `%r13)' /tmp/ghc27286.s:34: Error: bad register name `%rbp' /tmp/ghc27286.s:37: Error: bad register name `%rbx)' /tmp/ghc27286.s:38: Error: bad register name `%rbx)' /tmp/ghc27286.s:45: Error: bad register name `%rbp)' /tmp/ghc27286.s:46: Error: bad register name `%r14' /tmp/ghc27286.s:48: Error: bad register name `%rbp)' /tmp/ghc27286.s:49: Error: bad register name `%r13' ... The list is quite long. I've truncated it. Well, OK, the x386 version is running, but it won't compile GHC-6.4.2 and, moreover, it's not optimized for the HW. ;-) Is there any way out, or what may I be doing wrong way? Regards, Dusan

2 2

Predicate Library?
by Neil Mitchell 08 May '06

08 May '06

Hi, I'm trying to manipulate predicates in Haskell, and was wondering if there was a predicate library available that I could use? My predicates consist of `and` and `or` over some constraints. I have various rules which can be used to collapse certain constraints. Currently I have written my own predicate code, but its not great, and the simplification is overly hacky. The main facility I want out of the predicate library is some simplification. Thanks Neil

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Partial Derivatives
by Gerhard Navratil 08 May '06

08 May '06

Hi, I need a library that provides partial derivatives for functions. The solution I came up with is based on a datatype using reversed polish notation to store the function: Type VarName = String data Fkt a = Val a | Var VarName | Add (FktElem a) (FktElem a) | Sub (FktElem a) (FktElem a) | Mul (FktElem a) (FktElem a) | ... I can then use pattern matching to compute the derivative: derivative :: VarName -> Fkt b -> Fkt b derivative name (Var n) = if n == name then Val 1.0 else Val 0.0 derivative name (Add a b) | (containsVar name a) && (containsVar name b) = Add (derivative name a) (derivative name b) | (containsVar name a) && (not(containsVar name b)) = (derivative name a) | (not(containsVar name a)) && (containsVar name b) = (derivative name b) | otherwise = Val 0.0 derivative name (Sub a b) | (containsVar name a) && (containsVar name b) = Sub (derivative name a) (derivative name b) | (containsVar name a) && (not(containsVar name b)) = (derivative name a) | (not(containsVar name a)) && (containsVar name b) = (derivative name b) | otherwise = Val 0.0 derivative name (Mul a b) | (containsVar name a) && (containsVar name b) = Add (Mul (derivative name a) b) (Mul a (derivative name b)) | (containsVar name a) && (not(containsVar name b)) = Mul (derivative name a) b | (not(containsVar name a)) && (containsVar name b) = Mul a (derivative name b) | otherwise = Val 0.0 ... Where the function containsVar :: VarName -> Fkt a -> Bool tests if a function contains the variable. The solution works but is not very elegant. The complete module is appended to the mail. Does anyone have a more elegant solution or is there a package that provides derivatives in a similar way? Thank you, Gerhard ======================================== Gerhard Navratil Teaching- And Research-Assistant Technical University Vienna, Austria Institute of Geoinformation and Cartography Gusshausstr. 27-29, 1040 Vienna Tel.: ++43 (0) 1 / 58 801 - 12712 Fax.: ++43 (0) 1 / 58 801 - 12799 Cel.: ++43 (0) 699 / 197 44 761 http://www.geoinfo.tuwien.ac.at

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Re: [Haskell] how to 'accumulate' values efficiently (time and space) ?
by dons＠cse.unsw.edu.au 08 May '06

08 May '06

noteed: > Hi all, > > the problem is simple but i can't do it : > > I want to generate some values and 'accumulate' them. The meaning of > 'accumulating' is not so important. The only point is that to > 'accumulate' the values, I only have to know one value and the > accumulator (and not all the values). > > The most simple example is that of adding a list of numbers. > > I thought that I could use a haskell list to represent the values > so I could use foldr or mapM to do the accumulation (and not an > imperative-style loop). > > Below are four attempts to solve the problem. It's not solved : in > ghci, I have '*** Exception: stack overflow' with 'test1 5000000'. > > 5000000 is not so much for my application. > > How can I achieve what I want (and efficiently)? > ---------------------------------------------------------------------} > > import Control.Monad.State > import Control.Monad.ST > import Data.STRef > > acc1, acc2, acc3 :: [Int] -> Int > > ---------------------------------------------------------------------- > > acc1 ints = foldr (+) 0 ints Strictify the accumulator? Prelude List Data.List> foldl1 (+) (replicate 5000000 1) *** Exception: stack overflow versus Prelude List Data.List> foldl1' (+) (replicate 5000000 1) 5000000 (Redirected to haskell-cafe@) -- Don

1 0

throwing sugar into the void.
by Marc A. Ziegert 07 May '06

07 May '06

Hi@all! I'd like to hear some comments about the following ideas; maybe they are someway obsolete or even useless nonsense. A few days ago, I thought about abstracting the instance of an object away, like used in Foreign.Storable.sizeOf::(Storable a)=>a->Int, where only the type of an object is used. The problem with the function sizeOf is, that the result should be constant per definition(*), but how can we make the compiler know this? (*) <http://www.haskell.org/ghc/docs/latest/html/libraries/base/Foreign-Storable…> > sizeOf::a->Int > "Computes the storage requirements (in bytes) of the argument. The value of the argument is not used." So I thought we may need a new (abstract) datatype to wrap the type of an object, just to contain no dynamic data: data Type a typeOf :: a -> Type a typeOf = undefined #define TYPE(a) (undefined::Type (a)) ... sizeOf :: (Storable a) => Type a -> Int Maybe this is a little bit too ugly and cumbersomely, just to make sizeOf constant. This version of sizeOf is not really comfortable anymore; but it isn't comfortable to write "(undefined::a)", too. To be able to write "(::a)" instead, would be really nice, IMHO; but how about "(_::a)"? ---------- <trying to be equivocating> undefined is another way to implement ...nothing. </trying to be equivocating> More comfortable would it be to write a single symbol instead of the long word "undefined": _ :: a _ = undefined Using "_" as a function should not be such a problem: it cannot be mixed with the joker "_" as parameter. To define this function may be a problem: It needs a compiler-patch to allow this, ATM. ---------- By the way, this symbol is not used at type-level... How about praefix/postfix operators instead of only infix? -- praefix newtype BigLambda a = BigLambda a (/\) :: _ -> a -> BigLambda a /\ a = BigLambda a -- postfix newtype Lifted a = Lifted a (!^) :: a -> _ -> Lifted a a !^ = Lifted a May this be confusing? Or even impossible? Am I insane? Well, I don't know how hard it would be to implement those language features. I would not like to make anyone implement all this, if it is only a niceToHave-butIDontUseIt, like the implicit parameters feature. - marc

2 1

Haddock seems to generate wrong types in newtype deriving
by Brian Hulley 06 May '06

06 May '06

Hi - I have the following code: data MState = MState -- details omitted type MonadStateMState = MonadState MState -- necessary for Haddock newtype ManagerM a = ManagerM (StateT MState IO a) deriving (Monad, MonadIO, MonadStateMState) which means that ManagerM is an instance of Monad, MonadIO, and MonadState MState. However, the Haddock docs look like: data ManagerM a Instances ??? a => Monad (ManagerM a) ??? a => MonadIO (ManagerM a) ??? a => MonadStateMState (ManagerM a) which doesn't seem at all right to me. I'd have thought it should say: data ManagerM a Instances Monad ManagerM MonadIO ManagerM MonadStateMState ManagerM Is this just a bug in Haddock or am I misunderstanding something about Haskell? Thanks, Brian.

3 3

need help please [HOpenGL]
by enache alex 06 May '06

06 May '06

hello ; I am writing to ask you a thing ; I am writing a little game on Haskell's HOpenGL ; the game isn't much , but I want to make look a little better ; and for that I want to use bitmaps (or textures) ; I don't know very much about this subject ; I've tried to use de bitmap function from Graphics.Rendering.OpenGL.GL.Bitmap , but i gives me an error ; I've used the function like this ; bmp <- bitmap (Size (100,100)) (Vertex2 0 0 ) (Vector2 0 0 ) (Ptr "C:\1.bmp") end it tells me that he doesn't find the Ptr constructor ; if you can supply an full example on this matter it would great ; many thanks ; hope to hear from you soon ; thanks for your time in reading my letter ; PS : the example doesn't have to be big , it just has to contain all the things , in order for me not to miss something --------------------------------- Yahoo! Messenger with Voice. Make PC-to-Phone Calls to the US (and 30+ countries) for 2¢/min or less.

4 4

The case of the missing module
by ihope 06 May '06

06 May '06

In the Haddock source, there's the following line: import Paths_haddock ( getDataDir ) Paths_haddock doesn't seem to be anywhere inside the Haddock distribution, and a Google search doesn't turn up any useful results. Where can I find this thing?

4 3