May 2006 - Haskell-Cafe

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.

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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. **********

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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 23 May '06

23 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. **********

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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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