- Haskell-Cafe - Haskell.org

Monad/Functor Book
by unknown＠example.com 30 Mar '07

30 Mar '07

Given the amount of material posted at haskell.org and elsewhere explaining IO, monads and functors, has anyone considered publishing a comprehensive book explaining those subjects? (I am trying to read all the material online, but books are easier to read and don't require sitting in front of a computer to do so. Plus I can write in books :-). ) Thanks, Dave Feustel

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Why the Prelude must die
by Andrzej Jaworski 30 Mar '07

30 Mar '07

>What I want to push is a 'mathematically sound' numeric prelude. A proper >numerical prelude should have bona fide mathematical obects like groups, >rings, and fields underlying common numerical classes. Why not Clifford algebra: to simplify mathematical physics, mechanics, computer vision, improve neural networks etc.? That would probably be easier with OCaml-style categorical machine architecture. But then why not go for combinatorial stuff like matroids or graphs? More mathematics in a programming language won't harm general applications written in it. On the contrary, almost any human endeavor could be mathematically structured and thus programmed. Present languages are office-oriented for lack of mathematical knowledge and not any hidden virtue. Cheers, -Andrzej

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Simple task for parsec.
by Paolo Veronelli 30 Mar '07

30 Mar '07

Hi, I had a bad time trying to parse the words of a text. I suspect I miss some parsec knowledge. In the end it seems working, though I haven't tested much and this example contains the main features I was looking. *Main> parseTest (parseLine eof) "paolo@gmail sara,mimmo! 9ab a9b ab9 cd\n" ["paolo@gmail","sara","mimmo","cd"] --------------------- manyTillT body terminator joiner = liftM2 joiner (manyTill body (lookAhead terminator)) terminator wordChar = letter <|> oneOf "_@" <?> "a valid word character" nonSeparator = wordChar <|> digit wordEnd = do x <- wordChar notFollowedBy nonSeparator return x word = manyTillT wordChar (try wordEnd) (\b t -> b ++ [t]) <?> "a word" wordStart = do (try nonSeparator >> unexpected "non separator") <|> anyChar lookAhead wordChar nextWord = manyTill anyChar (try wordStart) >> (try word <|> nextWord) parseLine end = do f <- option [] $ return `fmap` try word r <- many $ try nextWord manyTill anyChar end return (f ++ r) ----------- Any comment to simplify this code is welcome. Paolino.

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Parsing words with parsec
by paolino 30 Mar '07

30 Mar '07

Hi, I had a bad time trying to parse the words of a text. I suspect I miss some parsec knowledge. In the end it seems working, though I haven't tested much and this example contains the main features I was looking. *Main> parseTest (parseLine eof) "paolo@gmail sara,mimmo! 9ab a9b ab9 cd\n" ["paolo@gmail","sara","mimmo","cd"] --------------------- manyTillT body terminator joiner = liftM2 joiner (manyTill body (lookAhead terminator)) terminator wordChar = letter <|> oneOf "_@" <?> "a valid word character" nonSeparator = wordChar <|> digit wordEnd = do x <- wordChar notFollowedBy nonSeparator return x word = manyTillT wordChar (try wordEnd) (\b t -> b ++ [t]) <?> "a word" wordStart = do (try nonSeparator >> unexpected "non separator") <|> anyChar lookAhead wordChar nextWord = manyTill anyChar (try wordStart) >> (try word <|> nextWord) parseLine end = do f <- option [] $ return `fmap` try word r <- many $ try nextWord manyTill anyChar end return (f ++ r) ----------- Any comment to simplify this code is welcome. Paolino.

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A simple parsing task for parsec.
by paolino 30 Mar '07

30 Mar '07

Hi, I had a bad time trying to parse the words of a text. I suspect I miss some parsec knowledge. In the end it seems working, though I haven't tested much and this example contains the main features I was looking. *Main> parseTest (parseLine eof) "paolo@gmail sara,mimmo! 9ab a9b ab9 cd\n" ["paolo@gmail","sara","mimmo","cd"] --------------------- manyTillT body terminator joiner = liftM2 joiner (manyTill body (lookAhead terminator)) terminator wordChar = letter <|> oneOf "_@" <?> "a valid word character" nonSeparator = wordChar <|> digit wordEnd = do x <- wordChar notFollowedBy nonSeparator return x word = manyTillT wordChar (try wordEnd) (\b t -> b ++ [t]) <?> "a word" wordStart = do (try nonSeparator >> unexpected "non separator") <|> anyChar lookAhead wordChar nextWord = manyTill anyChar (try wordStart) >> (try word <|> nextWord) parseLine end = do f <- option [] $ return `fmap` try word r <- many $ try nextWord manyTill anyChar end return (f ++ r) ----------- Any comment to simplify this code is welcome. Paolino.

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A simple parsing task for parsec.
by paolino 30 Mar '07

30 Mar '07

Hi, I had a bad time trying to parse the words of a text. I suspect I miss some parsec knowledge. In the end it seems working, though I haven't tested much and this example contains the main features I was looking. *Main> parseTest (parseLine eof) "paolo@gmail sara,mimmo! 9ab a9b ab9 cd\n" ["paolo@gmail","sara","mimmo","cd"] --------------------- manyTillT body terminator joiner = liftM2 joiner (manyTill body (lookAhead terminator)) terminator wordChar = letter <|> oneOf "_@" <?> "a valid word character" nonSeparator = wordChar <|> digit wordEnd = do x <- wordChar notFollowedBy nonSeparator return x word = manyTillT wordChar (try wordEnd) (\b t -> b ++ [t]) <?> "a word" wordStart = do (try nonSeparator >> unexpected "non separator") <|> anyChar lookAhead wordChar nextWord = manyTill anyChar (try wordStart) >> (try word <|> nextWord) parseLine end = do f <- option [] $ return `fmap` try word r <- many $ try nextWord manyTill anyChar end return (f ++ r) ----------- Any comment to simplify this code is welcome. Paolino.

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A simple parsing task for parsec.
by paolino 30 Mar '07

30 Mar '07

Hi, I had a bad time trying to parse the words of a text. I suspect I miss some parsec knowledge. In the end it seems working, though I haven't tested much and this example contains the main features I was looking. *Main> parseTest (parseLine eof) "paolo@gmail sara,mimmo! 9ab a9b ab9 cd\n" ["paolo@gmail","sara","mimmo","cd"] --------------------- manyTillT body terminator joiner = liftM2 joiner (manyTill body (lookAhead terminator)) terminator wordChar = letter <|> oneOf "_@" <?> "a valid word character" nonSeparator = wordChar <|> digit wordEnd = do x <- wordChar notFollowedBy nonSeparator return x word = manyTillT wordChar (try wordEnd) (\b t -> b ++ [t]) <?> "a word" wordStart = do (try nonSeparator >> unexpected "non separator") <|> anyChar lookAhead wordChar nextWord = manyTill anyChar (try wordStart) >> (try word <|> nextWord) parseLine end = do f <- option [] $ return `fmap` try word r <- many $ try nextWord manyTill anyChar end return (f ++ r) ----------- Any comment to simplify this code is welcome. Paolino.

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Monad/Functor Book
by Dominic Steinitz 30 Mar '07

30 Mar '07

> On 3/27/07, Dave at haskell.org <Dave at haskell.org> wrote: > > Given the amount of material posted at haskell.org and elsewhere > > explaining IO, monads and functors, has anyone considered publishing > > a comprehensive book explaining those subjects? (I am trying to > > read all the material online, but books are easier to read and don't > > require sitting in front of a computer to do so. Plus I can write in > > books :-). ) > > I've thought about writing extended tutorials on the relationship > between Haskell programming and category theory but there's a problem > I run into. It's tempting to make the identifications: > types<->objects, Haskell function<->arrows, suitably polymorphic > functions<->natural transformations, and so on. But the fact is, this > doesn't really work in the obvious way even though it seems like it > should at first (eg. Haskell functions aren't always total functions > in the mathematical sense and if you allow partial functions you can > do weird stuff). So either: > > (1) we need some technical work to patch up the differences (and > unfortunately I don't know what the patch-up is), > (2) we restrict ourselves to certain types of Haskell function for > which the theory works or > (3) we deliberately leave things a little vague. > > I usually tend to go for option (3), but that wouldn't be satisfactory > for an extended treatment. > > Has anyone else given this subject much thought? > -- > Dan Dan, It's a long time since I looked at this so this might be a complete red herring but have you thought about using the category of \omega-CPOs. Types are then objects (\omega-cpos) and functions are then arrows (continuous functions in the \omega-cpo sense). These papers may be of use: 1. Meijer http://research.microsoft.com/~emeijer/Papers/CWIReport.pdf 2. Mitchell Wand: Fixed-Point Constructions in Order-Enriched Categories 3. Freyd: Recursive Types Reduced to Inductive Types. Dominic.

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Prologue junk? - compiling lambdabot:Plugin.Pl.Transform
by Iain Alexander 30 Mar '07

30 Mar '07

I'm trying to build lambdabot 4.0 with ghc-6.4.1 (gcc (GCC) 3.4.2 (mingw- special)) and Cabal-1.1.6.1 on WinXP SP2. I've got as far as Plugin.Pl.Transform.hs, and I get *** Mangler g:\ghc\ghc-6.4.1\perl g:\ghc\ghc-6.4.1\ghc-asm C:\DOCUME~1\IAINAL~1\LOCALS~1\Tem p\ghc2736.raw_s C:\DOCUME~1\IAINAL~1\LOCALS~1\Temp\ghc2736.s 4 Prologue junk?: _s7f7_ret: xorl %ecx, %ecx xorl %eax, %eax movl %ecx, 8(%esp) movl %eax, 12(%esp) /APP Transform.hs uses {-# OPTIONS -fvia-C -O2 -optc-O3 #-} I've tried and failed to make sense of why the mangler's failing. I don't know if this is a known problem with 6.4.1 on XP. I'm not sure what to try next. I could turn off the aggressive optimisation, but I don't know how much that would cost me. I've got 6.4.2 installed, so I could try that, although I'd need to rebuild all the prerequisites. Anything else? -- Iain Alexander ia(a)stryx.demon.co.uk

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cost of modules
by Fawzi Mohamed 29 Mar '07

29 Mar '07

ehm, never post from someone's else computer with a new mailer... sorry for the double posts (to glasgow-haskell-users(a)haskell.org and here). Simon Peyton-Jones wrote: > Generally speaking GHC will inline *across* modules just as much as > it does *within* modules, with a single large exception. > > If GHC sees that a function 'f' is called just once, it inlines it > regardless of how big 'f' is. But once 'f' is exported, GHC can > never see that it's called exactly once, even if that later turns out > to be the case. This inline-once optimisation is pretty important > in practice. > > So: do not export functions that are not used outside the module > (i.e. use an explicit export list, and keep it as small as possible). This is very interesting to know, as it explains a strange effect that I was seeing: I had an algorithm and I began to add variants of it as separate functions in the same module. Then I saw the original function (which I had not touched) get slower and slower. I even wondered if the compiler was not maybe taking common parts of my functions out of them to reduce the evaluations making my original function slower, but then it was that it wasn't inlining anymore some functions. This means the with haskell (with ghc) cleaning up old code and keeping the modules as clean as possible pays off even more than in other languages... good to know, and very important when benchmarking various algorithms: commenting out a variant is better as having it as separate function.... Fawzi

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Re: [Haskell-cafe] Newbie: a parser for a list of objects?
by Daniel Fischer 29 Mar '07

29 Mar '07

> -----Ursprüngliche Nachricht----- > Von: "Dmitri O.Kondratiev" <dokondr(a)gmail.com> > Gesendet: 26.03.07 16:44:12 > An: haskell-cafe(a)haskell.org > Betreff: [Haskell-cafe] Newbie: a parser for a list of objects? > Please see my questions inside comments {-- --} : > Thanks! > > --- > module Parser where > > import Data.Char > > type Parse a b = [a] -> [(b, [a])] > > {-- > Newbie: a parser for a list of objects? > > I am working with the section 17.5 "Case study: parsing expressions" of the book "Haskell The Craft of Functional Programming", where a parser for a list of objects is defined. > I called this function pList in order to avoid confusion with 'list' as a term for data structure. > > Please help me to understand how pList works (please, see the rest of the code at the end of this message): > --} > > pList :: Parse a b -> Parse a [b] > pList p = (succeed []) `alt` > ((p >*> pList p) `build` (uncurry (:))) > > > {-- > First of all, I don't quite understand why there must be a choice ('alt') between the function ('succeed') that always returns an empty list and the other part? This results in adding [] to the front, why? > Well, if the parser p doesn't succeed, we don't want the whole thing to fail. And p will (almost certainly) fail when the end of input is reached. So without the alternative 'succeed []', we'd get pL1 dig "12" = [(('1':y),rem) | (y,rem) <- pL1 dig "2"] = [(('1':y),rem) | (y,rem) <- [(('2':z),rem2) | (z,rem2) <- pL1 dig ""]] = [(('1':y),rem) | (y,rem) <- [(('2':z),rem2) | (z,rem2) <- []] = [(('1':y),rem) | (y,rem) <- []] = [] because dig "" = [] > I thought that 'simplified' version of pList should still work fine. Trying to prove this I wrote : > --} > > pL1 :: Parse a b -> Parse a [b] > pL1 p = (p >*> pL1 p) `build` (uncurry (:)) > > {-- > Which, as expected, does not work correctly - just gives an empty list [] - but I don't understand why: because the parser eventually fails when the end of input is reached. > > *Parser> t1 "12345" > [] > *Parser> > > Also, I don't understand why the textbook version of pList gives this result: > > *Parser> test "12345" > [("","12345"),("1","2345"),("12","345"),("123","45"),("1234","5"),("12345","")] That's because of the order of alt's arguments: (succeed [] `alt` p) inp = [([],inp)] ++ (p inp) with pList p = ((p >*> pList p) `build` (uncurry (:))) `alt` succeed [] the resulting list woulde be reversed. > > *Parser> > > In particular, I don't understand where the first element ("","12345") of the resulting list comes from? > > I am trying to figure out how pList recursively unfolds. To my mind operators in the expression: > > > (succeed []) `alt`((p >*> pList p) `build` (uncurry (:))) > > has the following execution order: > 1) >*> > 2) 'build' > 3) 'alt' > No, the first argument of alt gets evaluated first, because (p1 `alt` p2) inp = (p1 inp) ++ (p2 inp), thus we need p1 inp first. Then we see we haven't hit bottom, so we need the second argument of (++) (resp. alt). So next we need to evaluate p, then pList p, combine the results of those with the second argument of build, uncurry (:). > It seems that operation >*> should be done as many times as many elements the input list has. Right? > Unfortunately not. Let's stay with pList dig. Say your input starts with n digits. >From the example above you can conjecture that length (pList dig inp) == (n+1). Now in the outermost (dig >*> pList dig) branch, you apply (pList dig) to an input beginning with (n-1) digits, returning a list of length n, to each element of this list you adjoin the first digit, resulting in n + (n-1) + ... + 1 = n*(n+1)/2 applications of (>*>). (Lesson: you need an exclusive choice, using the second parser only if the first one fails and a maximal munch combinator in your library, too) > > Signature: > > (>*>) :: Parse a b -> Parse a c -> Parse a (b, c) > > implies that second argument of the expression: > > p >*> pList p > > should be of type 'Parse a c' but in this application it is of type 'Parse a b -> Parse a [b]' > c is [b], so p >*> pList p has type Parse a (b,[b]), then (p >*> pList p) `build` (uncurry (:)) has type Parse a [b] > How can that be? > How recursion termination conditinon is expressed in pList? recursion terminates when p fails. HTH, Daniel > --} > > none :: Parse a b > none inp = [] > > succeed :: b -> Parse a b > succeed val inp = [(val, inp)] > > suc:: b -> [a] -> [(b, [a])] > > suc val inp = [(val, inp)] > > spot :: (a -> Bool) -> Parse a a > spot p [] = [] > spot p (x:xs) > | p x = [(x, xs)] > | otherwise = [] > > alt :: Parse a b -> Parse a b -> Parse a b > alt p1 p2 inp = p1 inp ++ p2 inp > > bracket = spot (=='(') > dash = spot (== '-') > dig = spot isDigit > alpha = spot isAlpha > > infixr 5 >*> > > (>*>) :: Parse a b -> Parse a c -> Parse a (b, c) > > (>*>) p1 p2 inp = [((x,y), rem2) |(x, rem1) <- p1 inp, (y, rem2) <- p2 rem1] > > build :: Parse a b -> (b -> c) -> Parse a c > build p f inp = [ (f x, rem) | (x, rem) <- p inp] > > test = pList dig > t1 = pL1 dig > > > -----------------------------------------------------------------

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ANNOUNCE: binary 0.3 - bigger, better, faster
by Lennart Kolmodin 29 Mar '07

29 Mar '07

Greetings friends! I'm pleased to announce binary 0.3! The 'binary' package provides efficient serialization of Haskell values to and from lazy ByteStrings. ByteStrings constructed this way may then be written to disk, written to the network, or further processed (e.g. stored in memory directly, or compressed in memory with zlib or bzlib). In total 14 people have contributed code and many more given feedback and cheerleading on #haskell@freenode. Thanks to all of you! It's available through Hackage: tarball: http://hackage.haskell.org/packages/archive/binary/binary-0.3.tar.gz darcs: darcs get http://darcs.haskell.org/binary homepage: http://www.cse.unsw.edu.au/~dons/binary.html It's been a while since the last release of binary [1] the 25th of January earlier this year. Lets have a look of what's changed since then: API additions ------------- The first thing you're going to notice that differs is the polished API. Being used by more people now than it was in January, we've added features that where missing before. More functions has been added to the Get/Put monads giving information over bytes read, remaining bytes etc for whenever you need to do custom serialization. You can now also read and write words in host endian order. Instances has been added to handle Double, Float and Ratio. Performance ----------- Decoding speed has been tuned by a rewrite and is currently about half the speed of the lightning fast encoding. Don't hesitate to give feedback on #haskell or by mail, we've always got time for a chat :) The Binary Strike Force, Lennart Kolmodin Duncan Coutts Don Stewart Spencer Janssen David Himmelstrup Björn Bringert Ross Paterson Einar Karttunen John Meacham Ulf Norell Tomasz Zielonka Stefan Karrmann Bryan O'Sullivan Florian Weimer [1] http://article.gmane.org/gmane.comp.lang.haskell.general/14800 -- "The only thing that interferes with my learning is my education." -- Albert Einstein

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(no subject)
by Matthew Brecknell 29 Mar '07

29 Mar '07

I'm attempting to construct an abstract data type with a generalised (deferred) representation. For a simple motivating example, say I am building an abstract data type with this representation: > newtype Foo1 k e = Foo1 (Data.Map.Map k (Data.Set.Set e)) While this is a fine default representation, I would like to be able to substitute IntMap for Map or IntSet for Set in cases where k or e happen to be Int, or simple list-based map and set implementations for types lacking an Ord instance. Following is a prototype of the approach I've come up with so far. It's been quite an adventure just getting this to type-check (due to lack of experience with MPTCs and FDs), so I'd be grateful for comments on whether it is likely to withstand use in real programs, other ways to solve the problem, etc. > {-# LANGUAGE FunctionalDependencies #-} > > -- Map class, with generic newtype wrapper instance. > > class MapLike k v m | m -> k v where > emptyM :: m > insertWithM :: (v -> v -> v) -> k -> v -> m -> m > toListM :: m -> [(k,v)] > > newtype Map m k v = Map m > > instance MapLike k v m => MapLike k v (Map m k v) where > emptyM = Map emptyM > insertWithM f k v (Map m) = Map (insertWithM f k v m) > toListM (Map m) = toListM m > > -- Set class, with generic newtype wrapper instance. > > class SetLike e s | s -> e where > singletonS :: e -> s > unionWithS :: (e -> e -> e) -> s -> s -> s > toListS :: s -> [e] > > newtype Set s e = Set s > > instance SetLike e s => SetLike e (Set s e) where > singletonS e = Set (singletonS e) > unionWithS f (Set s1) (Set s2) = Set (unionWithS f s1 s2) > toListS (Set s) = toListS s > > -- Abstract datatype Foo, whose representation is deferred > -- through the Map and Set newtype wrappers. > > newtype Foo m k s e = Foo (Map m k (Set s e)) > > class FooLike k e s m | m -> k s, s -> e where > emptyF :: m > insertWithF :: (e -> e -> e) -> k -> e -> m -> m > toListF :: m -> [(k,e)] > > instance (MapLike k (Set s e) m, SetLike e s) => FooLike k e (Set s e) (Foo m k s e) where > emptyF = Foo emptyM > insertWithF f k e (Foo m) = Foo (insertWithM (unionWithS f) k (singletonS e) m) > toListF (Foo m) = [ (k,e) | (k,s) <- toListM m, e <- toListS s ] Note that the FooLike class is not strictly necessary to the approach, since its methods could be written as top-level functions, but I thought it couldn't hurt. It does have the advantages of consolidating class constraints in one place, making functional dependencies explicit, and supporting further composition using the same technique.

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Can we do better than duplicate APIs? [was: Data.CompactString 0.3]
by Benjamin Franksen 29 Mar '07

29 Mar '07

[sorry for the somewhat longer rant, you may want to skip to the more technical questions at the end of the post] Twan van Laarhoven wrote: > I would like to announce version 0.3 of my Data.CompactString library. > Data.CompactString is a wrapper around Data.ByteString that represents a > Unicode string. This new version supports different encodings, as can be > seen from the data type: > > [...] > > Homepage: http://twan.home.fmf.nl/compact-string/ > Haddock: http://twan.home.fmf.nl/compact-string/doc/html/ > Source: darcs get http://twan.home.fmf.nl/repos/compact-string After taking a look at the Haddock docs, I was impressed by the amount of repetition in the APIs. Not ony does Data.CompactString duplicate the whole Data.ByteString interface (~100 functions, adding some more for encoding and decoding), the whole interface is again repeated another four times, once for each supported encoding. Now, this is /not/ meant as a criticism of the compact-string package in particular. To the contrary, duplicating a fat interface for almost identical functionality is apparently state-of-the-art in Haskell library design, viz. the celebrated Data.Bytesting, whose API is similarly repetitive (see Data.List, Data.ByteString.Lazy, etc...), as well as Map/IntMap/SetIntSet etc. I greatly appreciate the effort that went into these libraries, and admire the elegance of the implementation as well as the stunning results wrt. efficiency gains etc.. However I fear that duplicating interfaces in this way will prove to be problematic in the long run. The problems I (for-)see are for maintenance and usability, both of which are of course two sides of the same coin. For the library implementer, maintenance will become more difficult, as ever more of such 'almost equal' interfaces must be maintained and kept in sync. One could use code generation or macro expansion to alleviate this, but IMO the necessity to use extra-language pre-processors points to a weakness in the language; it be much less complicated and more satisfying to use a language feature that avoids the repetition instead of generating code to facilitate it. On the other side of teh coin, usability suffers as one has to lookup the (almost) same function in more and more different (but 'almost equal') module interfaces, depending on whether the string in question is Char vs. Byte, strict vs. lazy, packed vs. unpacked, encoded in X or Y or Z..., especially since there is no guarantee that the function is /really/ spelled the same everywhere and also really does what the user expects.(*) I am certain that most, if not all, people involved with these new libraries are well aware of these infelicities. Of course, type classes come to mind as a possible solution. However, something seems to prevent developers from using them to capture e.g. a common String or ListLike interface. Whatever this 'something' is, I think it should be discussed and addressed, before the number of 'almost equal' APIs becomes unmanageable for users and maintainers. Here are some raw ideas: One reason why I think type classes have not (yet) been used to reduce the amount of API repetition is that Haskell doesn't (directly) support abstraction over type constraints nor over the number of type parameters (polykinded types?). Often such 'almost equal' module APIs differ in exactly these aspects, i.e. one has an additional type parameter, while yet another one needs slightly different or additional constraints on certain types. Oleg K. has shown that some if these limitations can be overcome w/o changing or adding features to the language, however these tricks are not easy to learn and apply. Another problem is the engineering question of how much to put into the class proper: there is a tension between keeping the class as simple as possible (few methods, many parametric functions) for maximum usability vs. making it large (many methods, less parametric functions) for maximum efficiency via specialized implementations. It is often hard to decide this question up front, i.e. before enough instances are available. (This has been stated as a cause for defering the decision for a common interface to list-like values or strings). Since the type of a function doesn't reveal whether it is a normal function with a class constraint or a real class method, I imagine a language feature that (somehow) enables me to specialize such a function for a particular instance even if it is not a proper class member. Or maybe we have come to the point where Haskell's lack of a 'real' module system, like e.g. in SML, actually starts to hurt? Can associated types come to the rescue? Cheers Ben -- (*) I know that strictly speaking a class doesn't guarantee any semantic conformance either, but at least there is a common place to document the expected laws that all implementations should obey. With duplicated module APIs there is no such single place.

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Re: Why the Prelude must die
by Lennart Augustsson 29 Mar '07

29 Mar '07

I agree, I think this is what we need. Plus a decision of what names the builtin syntax refers to, like the type of 'a'. -- Lennart On Mar 26, 2007, at 23:30 , Ashley Yakeley wrote: > Sebastian Sylvan wrote: >> The solution is simple: >> * If there is a "module M where" clause in the beginning of the file, >> then it's a "proper" module and shouldn't import the Prelude. >> * If there is no module declaration then it's a "quick'n dirty >> script" >> and should have the Prelude implicitly imported. >> * Interactive interpreters should probably import the Prelude. >> That should take of most issues. > > I think this is ideal. I've always thought writing "import Prelude" > was a good idea if one wants one's module to import the Prelude. > It's one less corner case. > > -- > Ashley Yakeley > > _______________________________________________ > Libraries mailing list > Libraries(a)haskell.org > http://www.haskell.org/mailman/listinfo/libraries

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Re: Why the Prelude must die
by David House 29 Mar '07

29 Mar '07

On 28/03/07, Jeffrey Yasskin <jyasskin(a)gmail.com> wrote: > I would vote for a Combinators module with the most important > combinators, and a somewhat larger Prelude to support quick scripts > and demonstrations. I'd hate to have to import a module just to get at ($). Like I've mentioned before, some combinators are so ubiqutos they're almost like syntax. Seeing as the actual syntax will always be in scope, these common combinators would also have to be in scope all the time, which means placing them in the narrowed Prelude. -- -David House, dmhouse(a)gmail.com

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Haskell and a new web developement mailinglist?
by Marc Weber 29 Mar '07

29 Mar '07

I've been talking to Chris Eidhof the last days and we'd like to suggest adding another specialized haskell mailinglist: Haskell and web developement. I think the global interest will increase in the feature, libraries are evolving and it would be nice to have a place to discuss them. Any comments? If there is no veto where can I request it? haskell-admin(a)haskell.org ? Marc

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cost of modules
by Fawzi Mohamed 29 Mar '07

29 Mar '07

I decided to cleanup my program by splitting it in different modules. As I was curious about the cost of splitting it, or dually the efficiency of the intermodule optimization I timed it before and after the split. These are the results (ghc-6.6.20070129 on Linux AMD64): Original: 3 Modules with the computational one with an export list real 0m0.385s user 0m0.378s sys 0m0.003s time:100% Variant 1: split computational module in 6 submodules without export list, kept computational module reexporting the stuff with an export list real 0m0.467s user 0m0.464s sys 0m0.003s time: 122% Variant 2: like Variant 1 but removing the old computational module (no reexport list) 134% real 0m0.513s user 0m0.506s sys 0m0.003s All the functions in my modules have type specialized signatures, so non-specialization should almost never be an issue. The module with the Main function was not the exported one. So 20% speed hit, I had hoped for 0, but it is not unbearable (even if my code has to be as fast as possible, this was a very short run), probably I will keep it split to have cleaner code. Not I will have to write an export list for each module a see it the things improve, and how much. If someone has an idea on how else I can improve timings please tell me. Fawzi

7 9

infinite lists
by Matthias Fischmann 28 Mar '07

28 Mar '07

hi, it just took me an eternity to undestand a bug in my code *after* i had narrowed it down to eight lines or so. and as these bugs go, i feel very good about having found it and have to share it. although it's probably not that exciting to anybody except me. (-: here is the bug, narrowed to four lines. a function that only sometimes terminates. f = do n <- randomRIO (0,5) let l = replicate n '*' i = join $ repeat l -- infinite extension print (take 12 i) how many seconds does it take you to shout "you idiot!"? :-) spoiler below. (what would have been your fix?) cheers, matthias =================================================================== spoiler. the problem is that l is sometimes empty, and taking an element out of an infinite concatenation of empty lists takes even longer than understanding that that takes quite long. just deal with the empty-list case separately: g = do n <- randomRIO (0,5) let l = replicate n '*' i | null l = [] | otherwise = join $ repeat l print (take 12 i)

3 3

(newbie) instance Enum MyType where, smarter way?
by Adrian Neumann 28 Mar '07

28 Mar '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: RIPEMD160 Hello, I defined an enumeration datatype like this data MyType = One | Two | Four | Eight and want to make it an instance of the class Enum. deriving Enum won't do what I want, as it labels the items 0,1,2,3. Is there a better way to do this than instance Enum MyType where fromEnum One = 1 fromEnum Two = 2 ... toEnum 8 = Eight Something like instance Enum MyType where fromEnum One = 1 fromEnum x = 2*pred x toEnum 1 = One toEnum x = succ (toEnum x `div` 2) Which doesn't work because succ and pred are not (properly?) defined. Is there a way to let deriving Enum do *some* of work (i.e. defining succ and pred) while manually defining the other functions? -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGCTRJ11V8mqIQMRsRA4GFAJwKaxQoKdEW91pHUskzJadDvh7lXgCeLg6P dmQKcjRZEg4fqoZfQ4jOuhg= =RWQ+ -----END PGP SIGNATURE-----

3 2

Re: [Haskell] Lift Info [moving to -cafe]
by Andrew Wagner 27 Mar '07

27 Mar '07

I'm no expert, but I will point you to 2 links that I think will be helpful on this topic on the wiki: http://www.haskell.org/haskellwiki/Lifting (especially if you're familiar with functors) http://www.haskell.org/haskellwiki/Simple_StateT_use (for a simple example with a monad transformer) I'm also re-directing this question to haskell-cafe, where it's more likely to get a better response. Best of luck! On 3/27/07, Dave(a)haskell.org <Dave(a)haskell.org> wrote: > I've been looking for an explanation of the Haskell lift function which > I don't yet get. There seem to be two kinds of lifting, one of which is > making normal functions usable inside monads. I don't understand either > type. I've googled for lift but what I've found has not helped me much. > Is there a tutorial on what problem using lift solves? > > Thanks, > > Dave Feustel > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

1 0

Lazy parser with parsec.
by David Brown 27 Mar '07

27 Mar '07

I'm trying to figure out how to write a simple parser in Parsec to tokenize a subset of RTF. The problem is that I haven't been able to come up with a way of writing the parser that doesn't try consuming all of the input just to return the first token. The 'many' primitive's implementation uses an accumulator, and obviously has to parse to the end. Trying to iterate myself causes stack overflows on large inputs. Does anyone know of any existing Parser parsers that don't consume their entire input, or am I probably best off making my own parser. Thanks, David

3 3

Re: Can we do better than duplicate APIs? [was: Data.CompactString 0.3]
by Benjamin Franksen 27 Mar '07

27 Mar '07

Jean-Philippe Bernardy wrote: > Please look at http://darcs.haskell.org/packages/collections/doc/html/Data-Collections.html > for an effort to make most common operation on bulk types fit in a > single framework. The last time I looked at this (shortly after you started the project) I wasn't sure if I would want to use it. Now it seems like an oasis in a desert to me. I am pretty much impressed, for instance, you managed to unify all the nine existing 'filter' types into a common type class. Cool. The only hair in the (otherwise very tasty) soup is "Portability: MPTC, FD, undecidable instances" which doesn't sound like it is going to replace the Prelude any time soon ;-) Never mind: I definitely consider using this instead of importing all these different Data.XYZ modules directly (and, heaven forbid, having to import them qualified whenever I need two of them in the same module). Do you forsee any particular obstacle to an integration (=providing the appropriate instances) of e.g. CompactStrings? I would even try to do this myself, as an exercise of sorts. How difficult is it in practice to work with 'undecidable instances'? Are there special traps one has to be careful to walk around? > Also, we expect indexed types to solve, or at least alleviate, some > problems you mention in your "rant". > http://haskell.org/haskellwiki/GHC/Indexed_types I have been hoping for that to resolve (some of) our troubles, but have been confused by the all the back and forth among the experts about whether they offer more, or less, or the same, as MPTCs+fundeps+whatever (and that they will probably not go into Haskell'). BTW, any reason I didn't find your collections library in the HackageDB (other than stupidity on my part)? (Just interested, I already found the darcs repo.) Cheers Ben PS: Since I read and post to the Haskell lists via gmane and a news client: Do mail clients usually respect the follow-up header, such as I insert when cross-posting, so as to restrict follow-ups to the intended list?

1 0

What ever happened to Haskell 98 as a "stablebranch"?
by daniel.is.fischer＠web.de 27 Mar '07

27 Mar '07

Accidentally sent to haskell(a)haskell.org instead of the cafe: > >Diversity is generated by mutations. > > This is hardly a revelation. It was, looong ago. > > My point was that you need two competing components in relative balance to > grow something meaningful. And I'd think the Haskell community could take the part of 'Selektionsdruck' (don't know the correct english term). But you have a good point below, might be too slow for our short lives :( > Cancer growth is based solely on mutation! > > Also I was not theological. Didn't think you seriously were, but you introduced the acronym, was too tempting. > It is the advice to multiply Prelude and use > time to verify them rather cosmic in scale. It implies the assumption that > the world will freeze and wait for the verdict. > > Cheers, Daniel

1 0

Re: [Haskell-cafe] What ever happened to Haskell 98 as a "stablebranch"?
by daniel.is.fischer＠web.de 27 Mar '07

27 Mar '07

> -----Ursprüngliche Nachricht----- > Von: "Andrzej Jaworski" <himself(a)poczta.nom.pl> > Gesendet: 26.03.07 18:34:00 > An: <daniel.is.fischer(a)web.de> > Betreff: Re: [Haskell-cafe] What ever happened to Haskell 98 as a "stablebranch"? > Hi, > > I apologize for mistakenly resending my answer to two lists. > Well, I can understand Simon, he has little choice but to press on, but you > have healthy distance and much wider intellectual angle to see that Haskell > is doomed if new features will come up without any incentive to prune the > tree. Agreed. (Though I'm not sure about "doomed", after all: Jeszcze Polska ... ) And having thought more about it, I'm less optimistic that better alternatives will let their predecessors wither fast enough. But perhaps we could keep the older tree until the new one bears fruit? And I also support the move for a smaller Prelude, only I don't know what to move to Data.Whatever (I strongly dislike the idea of abandoning the Prelude altogether, but I could live with[out] it). > > Cheers, > -Andrew > Cheers, Daniel

1 0

Re: [Haskell-cafe] What ever happened to Haskell 98 as a "stablebranch"?
by Andrzej Jaworski 27 Mar '07

27 Mar '07

Daniel Fischer has cared to inform me that: >Diversity is generated by mutations. With due respect, but this is hardly a revelation. My point was that you need two competing components in relative balance to grow something meaningful. Cancer growth is based solely on mutation! Also I was not theological. It is the advice to multiply Prelude and use time to verify them rather cosmic in scale. It implies the assumption that the world will freeze and wait for the verdict.

1 0

Newbie: a parser for a list of objects?
by Dmitri O.Kondratiev 26 Mar '07

26 Mar '07

Please see my questions inside comments {-- --} : Thanks! --- module Parser where import Data.Char type Parse a b = [a] -> [(b, [a])] {-- Newbie: a parser for a list of objects? I am working with the section 17.5 "Case study: parsing expressions" of the book "Haskell The Craft of Functional Programming", where a parser for a list of objects is defined. I called this function pList in order to avoid confusion with 'list' as a term for data structure. Please help me to understand how pList works (please, see the rest of the code at the end of this message): --} pList :: Parse a b -> Parse a [b] pList p = (succeed []) `alt` ((p >*> pList p) `build` (uncurry (:))) {-- First of all, I don't quite understand why there must be a choice ('alt') between the function ('succeed') that always returns an empty list and the other part? This results in adding [] to the front, why? I thought that 'simplified' version of pList should still work fine. Trying to prove this I wrote : --} pL1 :: Parse a b -> Parse a [b] pL1 p = (p >*> pL1 p) `build` (uncurry (:)) {-- Which, as expected, does not work correctly - just gives an empty list [] - but I don't understand why: *Parser> t1 "12345" [] *Parser> Also, I don't understand why the textbook version of pList gives this result: *Parser> test "12345" [("","12345"),("1","2345"),("12","345"),("123","45"),("1234","5"),("12345","")] *Parser> In particular, I don't understand where the first element ("","12345") of the resulting list comes from? I am trying to figure out how pList recursively unfolds. To my mind operators in the expression: (succeed []) `alt`((p >*> pList p) `build` (uncurry (:))) has the following execution order: 1) >*> 2) 'build' 3) 'alt' It seems that operation >*> should be done as many times as many elements the input list has. Right? Signature: (>*>) :: Parse a b -> Parse a c -> Parse a (b, c) implies that second argument of the expression: p >*> pList p should be of type 'Parse a c' but in this application it is of type 'Parse a b -> Parse a [b]' How can that be? How recursion termination conditinon is expressed in pList? --} none :: Parse a b none inp = [] succeed :: b -> Parse a b succeed val inp = [(val, inp)] suc:: b -> [a] -> [(b, [a])] suc val inp = [(val, inp)] spot :: (a -> Bool) -> Parse a a spot p [] = [] spot p (x:xs) | p x = [(x, xs)] | otherwise = [] alt :: Parse a b -> Parse a b -> Parse a b alt p1 p2 inp = p1 inp ++ p2 inp bracket = spot (=='(') dash = spot (== '-') dig = spot isDigit alpha = spot isAlpha infixr 5 >*> (>*>) :: Parse a b -> Parse a c -> Parse a (b, c) (>*>) p1 p2 inp = [((x,y), rem2) |(x, rem1) <- p1 inp, (y, rem2) <- p2 rem1] build :: Parse a b -> (b -> c) -> Parse a c build p f inp = [ (f x, rem) | (x, rem) <- p inp] test = pList dig t1 = pL1 dig

1 0

What ever happened to Haskell 98 as a "stable branch"?
by Stefan O'Rear 26 Mar '07

26 Mar '07

Upon more reflection... >From the Preface to the Haskell 98 Language and Libraries Report: > Haskell 98 was conceived as a relatively minor tidy-up of Haskell > 1.4, making some simplifications, and removing some pitfalls for the > unwary. It is intended to be a "stable" language in sense the > implementors are committed to supporting Haskell 98 exactly as > specified, for the foreseeable future. In other words, we already have a stable branch of Haskell. Why, then, are we so paranoid about introducing breaking changes in the development branch of haskell? Why are we stuck without the class system extension proposal? Why is Num so still so horribly mangled? Why can I not 'map' over a Set? Why must I use lists of characters if I desire standard sorting? Why can I not get a good error message from read? (If you answer "because H98 is obsolete", then file this away as a must-read after H' is released.) Stefan

4 4

A question about functional dependencies and existential quantification
by Jean-Marie Gaillourdet 26 Mar '07

26 Mar '07

Hi, I am trying to do something like the following: > {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} > module TestCase where > > data Any root = forall pos sel . T root pos sel => ANY pos > > class T root pos sel | pos -> root, root -> sel where > f :: pos -> sel -> Bool > > instance T root (Any root) sel where > f (ANY p) s = f p s There is a multi-parameter type class T with some functional dependencies. And I want do define an almost general type for T, Any root. I would like to have that type in T as well. But ghc is not able to type check f. It gives: > TestCase.hs:10:7: > Couldn't match expected type `sel1' (a rigid variable) > against inferred type `sel' (a rigid variable) > `sel1' is bound by the pattern for `ANY' at TestCase.hs:10:7-11 > `sel' is bound by the instance declaration at TestCase.hs:9:0 > When using functional dependencies to combine > T root pos sel, arising from use of `f' at TestCase.hs:10:18-22 > T root pos sel1, > arising from is bound by the pattern for `ANY' > at TestCase.hs:10:7-11 > at TestCase.hs:10:7-11 > In the pattern: ANY p > In the definition of `f': f (ANY p) s = f p s I think sel1 and sel are equal, because the root in the type of "(ANY p)" on the left side is the same as the root of the type of "f p s" on the right side. From that should follow with help of the functional dependency that sel1 and sel are the same types. Do I misunderstand the type system or is that a weakness of GHC? Or am I trying to do something silly? I have a solution that involves requiring sel to be in Typeable and using cast and I never came across a runtime error, so far. Is there a better way to express that? Any enlightenment is appreciated. Regards, Jean-Marie

2 3

Re: [Haskell-cafe] What ever happened to Haskell 98 as a "stablebranch"?
by Daniel Fischer 26 Mar '07

26 Mar '07

> -----Ursprüngliche Nachricht----- > Von: "Andrzej Jaworski" <himself(a)poczta.nom.pl> > Gesendet: 26.03.07 15:00:47 > An: "Simon Peyton-Jones" <simonpj(a)microsoft.com> > CC: Haskell-Cafe(a)haskell.org > Betreff: Re: [Haskell-cafe] What ever happened to Haskell 98 as a "stablebranch"? > >Haskell is rather a Darwinian sort of place. > > With whole respect. You need two components for evolution to work: the > survival of the fitness and Generator Of Diversity (GOD). Diversity is generated by mutations. Go ahead, be mutagenous. And don't worry about theology, Simon's not GOD's prophet, he's (both of them, actually) just a doctor venerabilis. > > Now, Haskell attracts originality and easily accommodates changes but nobody > burns tires in testing anything so that complexity and learning curve grow > while deficiencies remain dormant. > > Recent threads are a kind of healthy evolutionary pressure (survival of the > fitness), but you insist that Haskell should be committed to GOD;-) > > With great respect, > -Andrzej > With TIC, Daniel

1 0

AI Strike Force!
by Andrew Wagner 26 Mar '07

26 Mar '07

The time has come! Calling all Haskell programmers interested in AI! I've established a new home base at http://www.haskell.org/haskellwiki/AI . Functional programming has long been recognized as an excellent paradigm for Artificial Intelligence. It's time to make Haskell an important language in that area! The goal is to provide a toolkit of AI tools in Haskell that can easily be applied to a wide variety of applications: e.g., Neural Networks, genetic algorithms, natural language processing tools, etc.

3 3

Link error in ALUT "Hello, World"
by Nobuhito Mori 26 Mar '07

26 Mar '07

Hi, I installed OpenAL and ALUT bindings downloaded from http://hackage.haskell.org/packages/archive/pkg-list.html and tried compilation of "HelloWorld.hs" example which I got from http://darcs.haskell.org/packages/ALUT/ But it returns errors as follow and I can not get an '.exe' file. Though there are clearly "link errors", I can not understand why it happens. By option "-package ALUT", libalut.a (which made by pexports and dlltool because I do not know original "alut.lib" can be used by mingw) and other necessary libraries are automatically linked, I think. What is "@8" of "alutInit@8"? How can I avoid these errors? My environment is Windows XP SP2, GHC6.6, MinGW/MSYS (so some of mingw tools are duplicated. one is GHC's, the other is MinGW's) --------------- >ghc -package ALUT HelloWorld.hs C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x18): undefined reference to `alutInit@8' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0xdc): undefined reference to `alutInitWithoutContext@8' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x19d): undefined reference to `alutExit@0' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x1d9): undefined reference to `alutGetError@0' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x210): undefined reference to `alutCreateBufferFromFile@4' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x2b0): undefined reference to `alutCreateBufferFromFileImage@8' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x36d): undefined reference to `alutCreateBufferHelloWorld@0' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x3f7): undefined reference to `alutCreateBufferWaveform@16' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x584): undefined reference to `alutLoadMemoryFromFile@16' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x6b8): undefined reference to `alutLoadMemoryFromFileImage@20' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x7e8): undefined reference to `alutLoadMemoryHelloWorld@12' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0x92f): undefined reference to `alutLoadMemoryWaveform@28' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0xb2c): undefined reference to `alutGetMIMETypes@4' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0xbc9): undefined reference to `alutGetMajorVersion@0' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0xc01): undefined reference to `alutGetMinorVersion@0' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0xc55): undefined reference to `alutSleep@4' C:\Program Files\Haskell\ALUT-2.0\ghc-6.6/libHSALUT-2.0.a(Config.o):fake:(.text+ 0xd89): undefined reference to `alutGetErrorString@4' --------------------- -------------------- Nobuhito _________________________________________________________________ Express yourself instantly with MSN Messenger! Download today it's FREE! http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/

3 7

Re: [Haskell-cafe] What ever happened to Haskell 98 as a "stable branch"?
by Andrzej Jaworski 25 Mar '07

25 Mar '07

>If you answer "because H98 is obsolete", then file this away as a >must-read after H' is released Ideas always originate in a single mind. Good ideas are only footnotes to the best idea that determine them. Now: a team of people with different views on the same thing can achieve their best decorating a Christmas Tree. Unless, it is Haskell;-) Regards, -Andrzej

1 0

newbie "concatenating" monad question
by Leandro Penz 25 Mar '07

25 Mar '07

Hi I'm new here and this is my first mail to the list, so be gentle :) I have some functions that build big strings by calling other functions and appending the result, like buildStuff = func1 ++ func2 ++ func3 ++ func4 My idea is to have a monad with a concatenating >>, so that I can: bulidStuff = do func1 func2 func3 func4 I am thinking about using the writer monad, but I would still have to use "tell". The list monad is not an option either, as it does not concatenate. Is there a proper way of doing something like this (efficiently) ? Leandro Penz

6 6

Application of iterate
by Logesh Pillay 25 Mar '07

25 Mar '07

Ordinarily I would not post to a list when I've got a program running. But I've found learning Haskell so tough (this is my 4th try in the last two years) that I feel I have to bore everyone with my first non-trivial program. It is Newton's method adapted to give the square root of a number. And all in just one line which I think is an amazing demonstration of the power of Haskell. And it took no more than a couple of minutes to write (directly to the screen too as opposed to the usual many paper drafts with C). It works too, correct to 13 decimal points on my GHC system It is :- my_sqrt t = last (take 20 (iterate (\n -> n/2 + t/(2 * n)) t)) It is a bit crude though. 20 iterations is a bit arbitrary. I don't suppose there is a easy way to iterate until the results stop changing.

8 10

"runST $ return ()" /= "runST (return ())" ??
by Marc A. Ziegert 25 Mar '07

25 Mar '07

hi! i've just discovered this strange behaviour of existential quantifiers with runST: --- Prelude Control.Monad.ST> :t runST (return ()) runST (return ()) :: () Prelude Control.Monad.ST> :t runST $ (return ()) <interactive>:1:9: Couldn't match expected type `forall s. ST s a' against inferred type `m ()' In the second argument of `($)', namely `(return ())' Prelude Control.Monad.ST> --- the same with "id runST undefined". is this a bug or an unsolved problem? i'm not sure wheather it is a part of the wanted feature of runST's type definition. i did not find any discussions about this. can anyone enlighten me, please? - marc

2 2

OpenGL and GLUT in GHC
by Ruben Zilibowitz 24 Mar '07

24 Mar '07

Hi, I'm experimenting using OpenGL and GLUT in Haskell using GHC. There are modules Graphics.Rendering.OpenGL and Graphics.UI.GLUT. I am using these. I've encountered a strange bug which I'm having trouble with. If I render a sphere and a plane, then the plane is facing the wrong way and is shaded on the wrong side. If however I only render the plane then it appears to be facing the right way and is shaded on the correct side. I have made the source file available as a download here: http://www.cse.unsw.edu.au/~rubenz/stuff/test.hs It can be built by running: ghc --make test.hs I am using ghc 6.6 There is a comment in the source file saying: "Commenting out the line below here causes the plane to be rendered facing towards the camera..." The bug can be seen by commenting out the line of code that follows and recompiling. If anyone can help me by explaining why I am getting this bug or how to fix it that would be great. I'd be very appreciative. Cheers, Ruben

2 1

Performance Help
by Dominic Steinitz 24 Mar '07

24 Mar '07

I have re-written the sha1 code so that it is (hopefully) easy to see that it faithfully implements the algorithm (see http://www.itl.nist.gov/fipspubs/fip180-1.htm) Having got rid of the space leak, I have been trying to improve performance. Currently, the haskell code is 2 orders of magnitude slower than the sha1sum that ships with my linux. > dom@heisenberg:~/sha1/testdist/sha1> time ./perfTest perfTest > c7eae62ddabb653bb9ce4eb18fa8b94264f92a76 > Success > > real 0m2.152s > user 0m2.112s > sys 0m0.028s > dom@heisenberg:~/sha1/testdist/sha1> time sha1sum perfTest > c7eae62ddabb653bb9ce4eb18fa8b94264f92a76 perfTest > > real 0m0.057s > user 0m0.008s > sys 0m0.004s I've played around with profiling and doubled the performance of the haskell code but I'm nowhere near the C performance. > Sun Mar 11 19:32 2007 Time and Allocation Profiling Report (Final) > > perfTest +RTS -p -RTS eg > > total time = 6.75 secs (135 ticks @ 50 ms) > total alloc = 1,483,413,752 bytes (excludes profiling overheads) > > COST CENTRE MODULE %time %alloc > > oneBlock Data.Digest.SHA1 39.3 40.1 > $& Data.Digest.SHA1 20.7 21.6 > f Data.Digest.SHA1 13.3 6.2 > getWord32s Data.Digest.SHA1 7.4 6.6 > test2 Main 5.9 8.7 > blockWord8sIn32 Data.Digest.SHA1 5.2 5.3 > blockWord8sIn512 Data.Digest.SHA1 3.0 4.4 > pad Data.Digest.SHA1 1.5 3.5 > k Data.Digest.SHA1 1.5 0.0 > fromBytes Data.Digest.SHA1 0.0 3.5 Here's the code that is taking the majority of the time. > ($&) :: [Word32] -> [Word32] -> [Word32] > a $& b = zipWith (+) a b > > -- Word128 -> Word512 -> Word128 > oneBlock ss xs = Word128 (as!!80) (bs!!80) (cs!!80) (ds!!80) (es!!80) > where > ws = xs ++ map (rotL 1) (zipWith4 xxxor wm3s wm8s wm14s ws) > where > xxxor a b c d = a `xor` b `xor` c `xor` d > wm3s = drop (16-3) ws > wm8s = drop (16-8) ws > wm14s = drop (16-14) ws > as = ai:ts > bs = bi:as > cs = ci:(map (rotL 30) bs) > ds = di:cs > es = ei:ds > ts = (map (rotL 5) as) $& (zipWith4 f [0..] bs cs ds) $& es $& (map k > [0..]) $& ws > Word128 ai bi ci di ei = ss Any help would be appreciated. I've put a copy of a working system here if anyone wants to experiment (http://www.haskell.org/crypto/downloads/sha1.tar.gz) Thanks, Dominic.

7 10

Rank-2-polymorphism problem
by Martin Huschenbett 24 Mar '07

24 Mar '07

Hi, I'm writing some database code using HSQL and had to stop on a problem with rank-2-polymorphism that I can't solve. The essence of my code is: module Value where import Data.Maybe class SqlBind a where fromSqlValue :: String -> a data Field data Value emptyValue :: Field -> Value emptyValue _ = ... readValue :: Field -> (forall s. SqlBind s => s) -> Value readValue _ = ... That works just fine. But now I want a version of readValue that has a Maybe wrapped around the second parameter and that shall call readValue in the case of a Just and emptyValue in the case of Nothing. But I can't figure out how to write this function as I always get compiler errors. My trials were: -- The type I want to get. readValue' :: Field -> (forall s. SqlBind s => Maybe s) -> Value -- First trial: readValue' fld s = if isJust s then readValue fld (fromJust s) else emptyValue fld -- Second trial: readValue' fld s | isJust s = readValue fld (fromJust s) | otherwise = emptyValue fld -- Third trial: readValue' fld (Just s) = readValue fld s readValue' fld Nothing = emptyValue fld -- Fourth trial: readValue fld s = case s of Just s' -> readValue fld s' Nothing -> emptyValue fld But none of these trials worked. Is there any solution that works with GHC-6.6 for now? Thanks in advance, Martin.

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Re: Bouncing ball in Gtk2Hs
by Duncan Coutts 24 Mar '07

24 Mar '07

On Fri, 2007-03-23 at 20:24 +0300, Thach Si Lam wrote: > I'm trying to write Bouncing Ball demo in Gtk2hs. But i coudn't find > timer class (like in wxhaskell demo) in Gtk2hs. There is timeoutAdd: http://haskell.org/gtk2hs/docs/gtk2hs-docs-0.9.10/System-Glib-MainLoop.html… used like so: timeoutAdd action 1000 where action is an action with type IO Bool and the number is the time to wait in milliseconds, (ie 1000 is 1 second). If the action returns True then it will get called again at the next timeout, otherwise it will not be called again. > How to deal with it ? In fact i have 2 processes: p1 for computing > some variables and p2 for changing (applying those variables) in > canvas. How to let them communicate together and update to canvas ? > Please, help me. Thank you very much. I'm forwarding this to the Haskell Cafe mailing list where someone might be able to give you a pointer. Duncan

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Re: [Haskell-cafe] AI Strike Force!
by Andrzej Jaworski 24 Mar '07

24 Mar '07

Hi Andrew and all the involved, The idea is great. It will allow programming with a rewording feeling of depth to the exercise right from the start. However tackling some of the topics you mentioned, like GA, one first needs to develop new solid programming techniques that would circumvent Haskell's inbuilt reluctance to update variables distractively. I would myself wish to read a well documented study of using ST monad or finite maps to achieve this end. Perhaps then, your page should openly encourage the need to develop such "brute force" methods. Cheers, -Andrzej

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HDBC Windows binaries with SQLite 3 support
by Alex Queiroz 24 Mar '07

24 Mar '07

Hallo list, I guess the subject line says it all. Can anybody help me? -- -alex http://www.ventonegro.org/

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Last chance to join the Summer of Code!
by Malcolm Wallace 23 Mar '07

23 Mar '07

Haskell.org is currently looking rather short of student proposals for Summer of Code 2007 compared to last year. If you haven't yet thought about submitting an application, why not consider it now? You could be paid $4500 for three months work June-Aug! Apparently, the more proposals we get, the more places are likely to be funded. Also, Google have announced a relaxation of the deadline for student proposals. The new deadline is Mon 26th March at 1700 Pacific Standard Time (Tue 27th, 0000 UTC), so you have all weekend to get writing! Regards, Malcolm

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ghc warning "Var/Type length mismatch"
by Nicolas Frisby 23 Mar '07

23 Mar '07

When I load my program, GHC spits these messages at me, but doesn't fail Any idea what might be causing this or how to figure that out? Var/Type length mismatch: [] [a{tv aGIf} [tau]] ... Var/Type length mismatch: [] [a{tv aGN8} [tv]] ... I found the responsible code in GHC's darcs, but the context didn't lend any help my feeble non-GHC hacker brain. My program is kind of big and I'm using personal libraries that exercise the type system a lot, but I'll wait to see if anyone is interested or if this has been handled before I share the gory details. This tidbit might help: I have a couple of usages of undefined in my program b/c I'm just starting to code and if I replace a particular undefined of this with some typed dummy code, it removes the second set of mismatches, but not the first. I tried also replacing the other undefined, but the first set of mismatches remained. Glasgow Haskell Compiler, Version 6.7.20070214, for Haskell 98, compiled by GHC version 6.7.20070214 Also, is there a better list for this question? I half-heartedly tried haskell.org's list of mailing lists for an appropriate place for GHC bugs but didn't find one. Thanks for your time, Nick

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type refinement to a typeclass constrained type
by Daniil Elovkov 23 Mar '07

23 Mar '07

Hello folks A section on GADTs in the GHC user's guide gives a simple example: data Term a where Lit :: Int -> Term Int Succ :: Term Int -> Term Int IsZero :: Term Int -> Term Bool eval :: Term a -> a eval (Lit i) = i eval (Succ t) = 1 + eval t eval (IsZero t) = eval t == 0 let's add Sum :: Term Int -> Term Int -> Term Int eval (Sum l r) = eval l + eval r it's ok But when I do this Sum :: (Num a) => Term a -> Term a -> Term a eval (Sum l r) = eval l + eval r --- same it doesn't typecheck with the error "no isntance Num a" writing eval (Sum (l::Num b => Term b) (r::Num b => Term b)) doesn't help. I wonder, if there's a way to do what I'm trying to do. After all, I'm allowed to declare Term that way. Or is this a restriction of type refinement?

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Duplicate instance declaration
by Bas van Dijk 23 Mar '07

23 Mar '07

Hello, I'm making an assembly language DSEL in Haskell (just for fun) very similar to the one from Russel O' Conner in [1] I'm trying to specify a 'mov' instruction. A 'mov' instruction has two operands: a destination and a source. There are various constraints on the operands. They have to be of the same size (8-, 16- or 32-bit) and they have to be in a specific format: mov reg reg mov reg mem mov mem reg mov reg imm mov mem imm where reg, mem and imm are register, memmory and immediate values respectively. I would like the type system to check as many constraints as possible. I've managed to get the size constraints working. See the source below this mail. For example the following are all type-correct: valid1 = mov EAX EBX valid2 = mov BX DX valid3 = mov AH AL And the following doesn't pass the type checker: invalid1 = mov EAX BX -- Couldn't match expected type `Bit16' against inferred type `Bit32' I would also like to get the formatting constraints working. However the solution in the code below gives a "Duplicate instance declarations" error. If I -fallow-overlapping-instances than the type checker goes into an infinite loop. I would like to know why this is happening and if there's a way to fix it. Thanks in advance, Bas van Dijk [1] The Monad.Reader Issue 6, Russel O' Conner, Assembly: Circular Programming with Recursive do, http://haskell.org/sitewiki/images/1/14/TMR-Issue6.pdf \begin{code} {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-} module ASM where import Data.Word -- TODO: Not finished yet... -- This is the type of instructions. -- It's going to be a Monad like the one from Russel O'Conner: data AsmM = AsmM deriving Show -- Sizes of registers, memmory or immediate values data Bit32 data Bit16 data Bit8 -- A type-level function that determines the size of a value class Size x size | x -> size -- Types of values class Reg reg class Mem mem class Imm imm -- General Purpose Registers -- Accumulator data EAX = EAX; instance Reg EAX; instance Size EAX Bit32 data AX = AX; instance Reg AX; instance Size AX Bit16 data AH = AH; instance Reg AH; instance Size AH Bit8 data AL = AL; instance Reg AL; instance Size AL Bit8 -- Base data EBX = EBX; instance Reg EBX; instance Size EBX Bit32 data BX = BX; instance Reg BX; instance Size BX Bit16 data BH = BH; instance Reg BH; instance Size BH Bit8 data BL = BL; instance Reg BL; instance Size BL Bit8 -- Counter data ECX = ECX; instance Reg ECX; instance Size ECX Bit32 data CX = CX; instance Reg CX; instance Size CX Bit16 data CH = CH; instance Reg CH; instance Size CH Bit8 data CL = CL; instance Reg CL; instance Size CL Bit8 -- Data data EDX = EDX; instance Reg EDX; instance Size EDX Bit32 data DX = DX; instance Reg DX; instance Size DX Bit16 data DH = DH; instance Reg DH; instance Size DH Bit8 data DL = DL; instance Reg DL; instance Size DL Bit8 -- Memmory data Mem32 = Mem32 Word32; instance Mem Mem32; instance Size Mem32 Bit32 data Mem16 = Mem16 Word32; instance Mem Mem16; instance Size Mem16 Bit16 -- Instructions class Mov dest src where mov :: dest -> src -> AsmM instance ( Size dest size , Size src size , MovFormat dest src ) => Mov dest src where mov d s = AsmM -- TODO: Not finished yet... class MovFormat dest src -- If I have more than one MovFormat instance than I get a -- "Duplicate instance declaration" error: instance (Reg dest, Reg src) => MovFormat dest src instance (Reg dest, Mem src) => MovFormat dest src instance (Mem dest, Reg src) => MovFormat dest src instance (Mem dest, Imm src) => MovFormat dest src instance (Reg dest, Imm src) => MovFormat dest src -- Tests valid1 = mov EAX EBX valid2 = mov BX DX valid3 = mov AH AL -- invalid1 = mov EAX BX -- Couldn't match expected type `Bit16' against inferred type `Bit32' -- invalid2 = mov EAX (Mem32 0) -- No instance for (Reg Mem32) \end{code}

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problem in installing hat
by Vikrant 23 Mar '07

23 Mar '07

<hat(a)haskell.org>Hi, I wanted to try out hat debugger. I tried to install it according to instructions. I downloaded the source tgz and did following sequence of operations tar -xvzf hat-2.05.tar.gz cd hat-2.05 ./configure make it gives me following error! MkProg: hmake: the compiler 'ghc' is not known. Stop - hmake dependency error. I have following system configuration. ghc - Glasgow Haskell Compiler, Version 6.4.1, for Haskell 98, compiled by GHC version 6.4.1 hugs - Version: 20050308 hmake - version 3.10 (2005-03-11) gcc - gcc (GCC) 4.0.3 (Ubuntu 4.0.3-1ubuntu5) can anybody help me in this? Regards, Vikrant

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Re: [Haskell-cafe] XML Validation and Digestion in Haskell
by Chris Eidhof 23 Mar '07

23 Mar '07

Hey Aron, I think you intended to CC Haskell Café as well. HaXml has support for DtdToHaskell, but HXT supports validating with a Relax NG validator. I guess the best way to figure it out is thoroughly read the documentation. -chris On Mar 22, 2007, at 10:40 PM, Arun Suresh wrote: > thanks for the feedback you guyz.. > > First off.. I went through the HaXml documentation.. It looks > pretty promising.. > and as far as HXT is concerned.. I guess ill be spending the > weekend grappling with the concept of Arrows and monads in general :) > > I dont know if HXT or HaXml offers it.. but I was wondering if > either of them provides some sort of schema compilation facility.. > (a la XmlBeans.. for those from the Java world).. basically I was > wondering if given an XML schema, is there a utility that will > generate a Haskell type hierarchy from the same ?? > > regards > Arun > > > On 3/23/07, Chris Eidhof < chris(a)eidhof.nl> wrote:Don't spend too > much time on the various libraries though. I tried > some simple things with Haskell and XML, but I found it really hard > to actually parse a simple document. You really don't want to write > your own parser. > > The only tool that worked for me was HXT, which is based on arrows. > These are pretty hard to grasp right away, but really powerful. I > really like the way they are used in HXT, but it still feels a bit > weird sometimes. > > Good luck, and keep us posted. > > -chris > > On Mar 22, 2007, at 7:52 AM, Andrew Wagner wrote: > > > Hi Arun, > > Your problem description seems a little vague - which is > > understandable, considering how embedded in your business model > it is. > > As for general recommendations, I'm no guru, but I would suggest > > looking at the existing XML libraries in Haskell [1], and if that's > > not powerful enough, check out parsec[2] for building a custom > parser. > > I think to get more specific recommendations, you would need to give > > more details about the problem. Hope this helps somewhat. Andrew > > > > [1]: http://www.haskell.org/haskellwiki/Libraries_and_tools/ > > Web_programming#XML_2 > > [2]: http://www.cs.uu.nl/people/daan/download/parsec/parsec.html > > > > > > On 3/22/07, Arun Suresh < arun.suresh(a)gmail.com> wrote: > >> Hi.. > >> > >> I am currently working on a project where basically we do a lot of > >> XML > >> validation and digestion. For example : > >> We recieve an XML document A, if the document passes schema > >> validation, we > >> do some business level validations, then from A, we create XML > >> digests B, C > >> and D. > >> Our code-base is written completely in Java. We had earlier tried > >> using > >> XSLT, but it became too unwieldy to manage (we ended up making > >> calls to > >> other Java classes from the XSLT engine) > >> > >> The problem we are facing right now is that we have huge > >> hierarchies of > >> Validators, Digestors and Contexts. It just doesnt look right. I > >> just cant > >> help but wonder if theres a better way of doing this... > >> > >> I have been trying to get my hands dirty with Haskell for a couplo > >> weeks > >> now. all the while keeping the problem at hand in the back of my > >> mind.. I > >> just dont seem to be see how applying functional programming > >> concepts can > >> help in this scenario.. Was wondering if somebody could probably > >> give me > >> some pointers.. > >> > >> Regards > >> Arun > >> > >> > >> > >> > >> > >> > >> _______________________________________________ > >> Haskell-Cafe mailing list > >> Haskell-Cafe(a)haskell.org > >> http://www.haskell.org/mailman/listinfo/haskell-cafe > >> > >> > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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XML Validation and Digestion in Haskell
by Arun Suresh 23 Mar '07

23 Mar '07

Hi.. I am currently working on a project where basically we do a lot of XML validation and digestion. For example : We recieve an XML document A, if the document passes schema validation, we do some business level validations, then from A, we create XML digests B, C and D. Our code-base is written completely in Java. We had earlier tried using XSLT, but it became too unwieldy to manage (we ended up making calls to other Java classes from the XSLT engine) The problem we are facing right now is that we have huge hierarchies of Validators, Digestors and Contexts. It just doesnt look right. I just cant help but wonder if theres a better way of doing this... I have been trying to get my hands dirty with Haskell for a couplo weeks now. all the while keeping the problem at hand in the back of my mind.. I just dont seem to be see how applying functional programming concepts can help in this scenario.. Was wondering if somebody could probably give me some pointers.. Regards Arun

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random number generators
by david karapetyan 23 Mar '07

23 Mar '07

hi, i have been trying to learn haskell and i would like to translate the following object-oriented pseudo-code into working haskell code but i'm stumped on how to write the next function in the haskell code. any help is appreciated. class Random ... end ran = Random.new(300) ran.next() <- this call generates a random number ran.next() <- this one generates a different random number so far what i have in haskell is: data Random = Ran Int Int next :: Random->Int <- i have no idea how to write the next function that would give me the desired effect because the object in the object-oriented code can remember how many times 'next' was called and i would like to do the same for 'ran' in the haskell code. ran = Ran 300 0 next ran <- this generates a random number next ran <- this generates another random number but since haskell is purely functional it is going to be the same as from the previous call of 'next' which is not what i want

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