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AngloHaskell 2007
by Philippa Cowderoy 22 May '07

22 May '07

I'd like to announce AngloHaskell 2007, or at least the start of the process of organising it! Last year there was a get-together organised around the interviews for the job of GHC maintainer. There were talks, with a largely pragmatic flavour, and there was plenty of socialising. It was fun, and doing it again seems a good idea. In the vein of last year's process, organising an event this year shall take place on the Haskell Wiki page at: http://www.haskell.org/haskellwiki/AngloHaskell and on IRC, in #anglohaskell on irc.freenode.net So far this is almost all the detail available, as neither date nor venue have been set - a summer date seems good though, and by the time we're organised I imagine late July or August looks most likely. Simon Peyton-Jones has offered to ask Microsoft Research for space to give talks in. This worked well last year, but if anyone else wants to offer a venue that's fine! -- flippa(a)flippac.org The task of the academic is not to scale great intellectual mountains, but to flatten them.

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Behavior of groupBy
by Hans van Thiel 21 May '07

21 May '07

Hello All, The standard function groupBy of List.hs doesn't work as I expect in this case: groupBy (\x y -> (last x) == (last y)) ["abc", "bd","cac"] results in: [["abc"],["bd"],["cac"]] where I want: [["abc","cac"], ["bd"]] Am I doing something wrong, or is this a bug? The function is defined (in List.hs in the hugs Data library) as : -- | The 'groupBy' function is the non-overloaded version of 'group'. groupBy :: (a -> a -> Bool) -> [a] -> [[a]] groupBy _ [] = [] groupBy eq (x:xs) = (x:ys) : groupBy eq zs where (ys,zs) = span (eq x) xs Thanks, Hans van Thiel

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how can I select all the 3-element-combination out of a list efficiently
by geniusfat 21 May '07

21 May '07

hi dear haskell lover ;) what I want to do is simply this: select3 :: [a] -> [(a, a, a)] and how can it be done efficiently? thanks in advance! -- View this message in context: http://www.nabble.com/how-can-I-select-all-the-3-element-combination-out-of… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Lazy HTML parsing with HXT, HaXML/polyparse, what else?
by Henning Thielemann 21 May '07

21 May '07

I want to parse and process HTML lazily. I use HXT because the HTML parser is very liberal. However it uses Parsec and is thus strict. HaXML has a so called lazy parser, but it is not what I consider lazy: *Text.XML.HaXml.Html.ParseLazy> Text.XML.HaXml.Pretty.document $ htmlParse "text" $ "<html><head></head><body>"++undefined++"</body></html>" *** Exception: Prelude.undefined *Text.XML.HaXml.Html.ParseLazy> Text.XML.HaXml.Pretty.document $ htmlParse "text" $ "<html><head></head><body>&</body></html>" *** Exception: Expected "</" but found & at file text at line 1 col 26 If it would be lazy, it would return some HTML code before the error. HaXML uses the Polyparse package for parsing which contains a so called lazy parser. However it has return type (Either String a). That is, for the decision whether the parse was successful, the document has to be parsed completely. *Text.ParserCombinators.PolyLazy> runParser (exactly 4 (satisfy Char.isAlpha)) ("abc104"++undefined) ("*** Exception: Parse.satisfy: failed If it would have return type (String, a) it could return both a partial value of type 'a' and the error message as String. It would be even better if it has some handling for incorrect input texts, and returns ([String], a), where [String] is the type of a list of warnings and error messages and 'a' is the type of a total value of parse output. Is there some parser of this type? Unfortunately http://www.haskell.org/haskellwiki/Applications_and_libraries/Compiler_tools does not compare the laziness of the mentioned parsers.

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Ann: Emping 0.2
by Hans van Thiel 21 May '07

21 May '07

Hello All, Version 0.2 of Emping, a utility to derive heuristic rules from a table of nominal data, is available. In addition to a reduced normal form in .csv format, which can be read by Open Office Calc, it now also displays observed implications and equivalences in the reduced rules (if present) and ambiguities in original rules (if present). A bug in the response to a user typing error has been fixed and some code has been simplified, compared to 0.1. An online user guide is at: http://j-van-thiel.speedlinq.nl/emp/empug.html and the Cabal package can be downloaded from there, or from the HackageDB. Thanks, Hans van Thiel

1 0

Unresolved overloading error
by Scott Brown 21 May '07

21 May '07

I have written this code in Haskell which gives an unresolved overloading error. The function bernoulli should give the probability of j successes occuring in n trials, if each trial has a probability of p. > fac 0 = 1 > fac n = n * fac(n - 1) > binom n j = (fac n)/((fac j)*(fac (n - j))) > bernoulli n p j = (binom n j)*(p ^ j) * ((1 - p)^(n - j)) However, bernoulli 6 0.5 3 gives the error: ERROR - Unresolved overloading *** Type : (Fractional a, Integral a) => a *** Expression : bernoulli 6 0.5 3 Why doesn't Haskell infer the types? What kind of type casting or type definition can I use to fix the error? Send instant messages to your online friends http://au.messenger.yahoo.com

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Follow up on 'GroupBy behavior'
by Hans van Thiel 21 May '07

21 May '07

Thanks again for the help, and, to follow up, this now does what I need.. -- partitions a list according to an equivalence relation partition1 :: (a -> a -> Bool) -> [a] -> ([a],[a]) partition1 eq ls = partition ((head ls) `eq`) ls -- partitionBy :: (a -> a -> Bool) -> [a] -> [[a]] partitionBy eq [] = [] partitionBy eq ls = (fst x):(partitionBy eq (snd x)) where x = partition1 eq ls Regards, Hans van Thiel

2 1

A wish for relaxed layout syntax
by Benjamin Franksen 21 May '07

21 May '07

Hi, I often run into the following issue: I want to write a list of lengthy items like this mylist = [ quite_lengthy_list_item_number_one, quite_lengthy_list_item_number_two, quite_lengthy_list_item_number_three ] With the current layout rules this is a parse error (at the closing bracket). Normally I avoid this by indenting everything one level more as in mylist = [ quite_lengthy_list_item_number_one, quite_lengthy_list_item_number_two, quite_lengthy_list_item_number_three ] but I think this is a little ugly. Same issue comes up with parenthesized do-blocks, I would like to write when (condition met) (do first thing second thing ) So my wish is for a revised layout rule that allows closing brackets (of all sorts: ']', ')', '}') to be on the same indent level as the start of the definition/expression that contains the corresponding opening bracket. Cheers Ben

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SWIG with Haskell ?
by Mark Wassell 21 May '07

21 May '07

Hello, Has anyone got any pointers on using SWIG with Haskell to integrate a C++ library? For the library there other language bindings, so I am in a position to leverage off these. In general what are the particular issues with C++ and Haskell. One obvious one is management of object lifecycle - making sure the object stays around long enough but doesn't out stay its welcome. Is using ForeignPtr sufficient? Cheers Mark

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writer monad help
by iskaldur 21 May '07

21 May '07

I'm trying to learn to use the Writer Monad, but I'm having trouble with the following very simple program: import Control.Monad.Writer foo :: Writer String Int foo = tell "hello" Basically, I was trying to figure out the 'tell' function, so I want foo to return ((), "hello"). But I get this error: Couldn't match `Int' against `()' Expected type: Writer String Int Inferred type: Writer String () In the application `tell "hello"' In the definition of `foo': foo = tell "hello" What am I supposed to do? Write my own version of tell for Writer String Int? How exactly do I do this? -- View this message in context: http://www.nabble.com/writer-monad-help-tf3777133.html#a10680445 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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HaskellWiki XHTML not well-formed or valid
by Isaac Dupree 21 May '07

21 May '07

I mentioned this a few months ago in a place no-one noticed, it seems... http://www.haskell.org/haskellwiki/Talk:Haskell#Well-formedness_.2F_validity Please fix, or tell me who to contact (and how), or how to fix it myself? :-) Isaac

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Random idea
by Andrew Coppin 21 May '07

21 May '07

Greetings. I was thinking... we already have Lambdabot sitting in an IRC channel. How hard would it be to mangle Lambdabot to the point where it works over HTTP? You know - so you could type some Haskell into a form on a web page, hit [submit], and get the result sent back to you? (Again, assuming it can be computed in a sane amount of time/space, and truncated to some reasonable textual length.) I think it might be kinda neat to have such a thing linked from the Haskell.org homepage. You know, sort of "hey, wanna try Haskell without installing any stuff? Click here!" ...on the other hand, maybe I'm just strange... :-$

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Debunking tail recursion
by Jules Bean 21 May '07

21 May '07

A conversation on #haskell just showed that it's quite hard to explain (at least it is for me) to people attached to tail recursion why that is a red herring in Haskell. I had a poke around the wiki and couldn't see a page which explains it clearly. In fact http://www.haskell.org/haskellwiki/Performance/Accumulating_parameter is actuall slightly misleading since it suggests that the performance gain is from the tail recursion whereas really it's from the change from a function which accumulates an enormous thunk to an function which can do the work as it goes along. If someone has the energy and the clarity to explain this clearly and make a wiki page about it, that would be great. If someone finds an existing wiki page I'm too unperceptive to notice, even better! Jules

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Profiling, measuring time
by Steffen Mazanek 20 May '07

20 May '07

Hello, I have written a function f, that performs a quite complex operation on its argument. Furthermore I have another function genInput that takes a number and constructs an argument for f of this size. What I want now is a list [(n,time)] that gives me for every size of input n the time, f needs to compute this input. How can I do this? I have found the module Time and tried diffClockTimes, but I ever got zero as a delta. Help is greatly appreciated. There is probably a way to do this via ghcs profiling but I hope that there is also a "normal" Haskell way of doing this. Best regards, Steffen -- Dipl.-Inform. Steffen Mazanek Institut für Softwaretechnologie Fakultät Informatik Universität der Bundeswehr München 85577 Neubiberg Tel: +49 (0)89 6004-2505 Fax: +49 (0)89 6004-4447 E-Mail: steffen.mazanek(a)unibw.de

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Multiple DB drivers in HDBC
by kpierre 20 May '07

20 May '07

Hello. I have a problem with migration to HDBC 1.1. My application had transparent support for multiple database types, because of HDBC's Connection object. How can I preserve this feature with minimal code changes? I tried to use ConnWrapper, as migration guide suggest, but it seems there's no way to convert arbitrary IConnection object into ConnWrapper. Thanks in advance, pierre -- View this message in context: http://www.nabble.com/Multiple-DB-drivers-in-HDBC-tf3784996.html#a10703798 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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New YAHT maintainer
by Eric Y. Kow 20 May '07

20 May '07

Hi, I just wanted to announce that Sam Hughes will be taking over as Keeper of the YAHT. YAHT is the venerable Yet Another Haskell Tutorial. It is available both as a wikibook and as a PDF (*). The Keeper of the YAHT has the grave duty of synchronising the YAHT wikibook and tex source, and releasing an updated PDF from time to time. You can send modifications to YAHT either by (a) updating the wikibook or (b) darcs send'ing a patch to the tex source (patches will go to the Haskell Café mailing list for community review). Many thanks to Sam for stepping up! * http://darcs.haskell.org/yaht/yaht.pdf -- Eric Kow http://www.loria.fr/~kow PGP Key ID: 08AC04F9 Merci de corriger mon français.

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idlelog
by Leandro Lisboa Penz 20 May '07

20 May '07

Hi I made a program that detects user presence in a linux box by checking for keyboard interruptions in /proc/interrupts. Problem is, it is supposed to run for a long time, and it keeps about 40MB for itself. Yeah, this is one more "help me with this memory problem" mails... The code can be found in http://hpaste.org/1907, and in "darcs get http://lpenz.50webs.com/idlelog I guess I'm having some conceptual problem. Cheers Leandro Penz

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Re: [Haskell-cafe] Pesky monads...
by Matthew Cox 20 May '07

20 May '07

It occurred to me that the predicate will generally be a monadic function itself, so here's a refined version: :: Monad m => (a -> m Bool) -> (a -> m a) -> a -> m a untilM pred f x = do c <- pred x if c then return x else f x >>= untilM pred f Think of a computation in the State monad, a predicate will probably want to consult that state. Matthew Cox ----- Original Message ----- From: Matthew Cox To: <haskell-cafe(a)haskell.org> Sent: Sat, 19 May 2007 13:10:40 -0600 Subject: Re: [Haskell-cafe] Pesky monads... You can define one: untilM :: Monad m => (a -> Bool) -> (a -> m a) -> a -> m a untilM pred f x | pred x = return x | otherwise = f x >>= untilM pred f Matthew Cox ----- Original Message ----- From: Andrew Coppin To: haskell-cafe(a)haskell.org Sent: Sat, 19 May 2007 19:49:40 +0100 Subject: [Haskell-cafe] Pesky monads... I've been getting some pretty weird complaints from the type checker. And I just figured out why! Grr... Is there any function that does the same thing as "until", but in a monad? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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idlelog
by Leandro Lisboa Penz 20 May '07

20 May '07

Hi I made a program that detects user presence in a linux box by checking for keyboard interruptions in /proc/interrupts. Problem is, it is supposed to run for a long time, and it keeps about 40MB for itself. Yeah, this is one more "help me with this memory problem" mails... The code can be found in http://hpaste.org/1907, and in "darcs get http://lpenz.50webs.com/idlelog I guess I'm having some conceptual problem. Cheers Leandro Penz

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Who lives in a heap like this?
by Andrew Coppin 20 May '07

20 May '07

Greetings. I have just implemented a heap. But... um... I can't acutally figure out *which kind* of heap it is! LOL. Any ideas? (Seems to work really well, whatever it is. Oh, and I discovered that you can sort data just by shoving it all into a heap, and then taking it all out again. Apparently this is a standard algorithm, and it's known as a "heap sort", unsurprisingly. You learn something every day...) module Heap where import Data.List (intersperse, unfoldr) data Heap t = Node !t !(Heap t) !(Heap t) | Null instance (Show t) => Show (Heap t) where show = work "" where work p (Null) = p ++ "-" work p (Node v t0 t1) = concat $ intersperse "\n" $ [work (' ':p) t0, p ++ show v, work (' ':p) t1] empty = Null is_empty Null = True is_empty _ = False insert v (Null) = Node v Null Null insert v (Node v0 t0 t1) = let lo = min v v0 hi = max v v0 in Node hi (insert lo t1) t0 get_max (Null) = error "heap is empty" get_max (Node v _ _) = v delete_max (Null) = error "heap is empty" delete_max (Node _ Null Null) = Null delete_max (Node _ Null t1) = t1 delete_max (Node _ t0 Null) = t0 delete_max (Node _ t0 t1) | get_max t0 > get_max t1 = Node (get_max t0) (delete_max t0) t1 | otherwise = Node (get_max t1) t0 (delete_max t1) size (Null) = 0 size (Node _ t0 t1) = 1 + (size t0) + (size t1) from_list :: (Ord t) => [t] -> Heap t from_list = foldr insert empty to_list :: (Ord t) => Heap t -> [t] to_list = unfoldr (\h -> if is_empty h then Nothing else Just (get_max h, delete_max h)) heap_sort :: (Ord t) => [t] -> [t] heap_sort = to_list . from_list

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A special function
by Andrew Coppin 20 May '07

20 May '07

I'd like to share something with you. I call it "the evil ******* function from hell": (Results xss) >>= f = Results $ map concat $ diagonalise $ map (foldr longZip [] . map (unpack . f)) xss If you attempt to comprehend how this function works, you WILL in fact loose your mind! However, it does in fact work completely correctly. It's just not possible to know why. :-D newtype Results t = Results [[t]] unpack (Results xss) = xss longZip = similar to zip (++) diagonalise = transform an infinite list of infinite lists into an infinite list of finite lists.

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Haskell on OS X
by Johan Tibell 20 May '07

20 May '07

I just switched to OS X and was wondering if someone would like to share their setup. Install binaries from haskell.org or Mac Ports? Which emacs build? etc Johan

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Re: [Haskell-cafe] Pesky monads...
by Matthew Cox 20 May '07

20 May '07

You can define one: untilM :: Monad m => (a -> Bool) -> (a -> m a) -> a -> m a untilM pred f x | pred x = return x | otherwise = f x >>= untilM pred f Matthew Cox ----- Original Message ----- From: Andrew Coppin To: haskell-cafe(a)haskell.org Sent: Sat, 19 May 2007 19:49:40 +0100 Subject: [Haskell-cafe] Pesky monads... I've been getting some pretty weird complaints from the type checker. And I just figured out why! Grr... Is there any function that does the same thing as "until", but in a monad? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Pesky monads...
by Andrew Coppin 20 May '07

20 May '07

I've been getting some pretty weird complaints from the type checker. And I just figured out why! Grr... Is there any function that does the same thing as "until", but in a monad?

1 0

A small error
by Andrew Coppin 19 May '07

19 May '07

Um... why doesn't this work? my_fn x = do if x < 0 then do let y = 5 return () else do let y = 8 return () print y (Error: "y" is not in scope.)

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Accepted by Hugs, not by GHC
by Hans van Thiel 19 May '07

19 May '07

Hello All, Defining a partially ordered set as: data Porder = HI | LW | NT deriving (Eq) class (Eq a) => Poset a where pcompare :: a -> a -> Porder and defining an instance for type synonym RuRe: type RuRe = ([Int],[Int]) instance Poset RuRe where pcompare (x1,y1) (x2,y2) | isSub x2 x1 = HI | isSub x1 x2 = LW | otherwise = NT causes no problems in Hugs (-98 flag) but compiling with GHC produces: [hvt@localhost Sandbox]$ ghc --make Main.hs -O [5 of 6] Compiling Abduce ( Abduce.hs, Abduce.o ) Abduce.hs:110:0: Illegal instance declaration for `Poset RuRe' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `Poset RuRe' I've tried several changes, like not using the type synonym, but can't get it right. (I'm using the ghc 6.6.1 Fedora Core 6 package). Many thanks in advance! Hans van Thiel

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What really happens
by Andrew Coppin 19 May '07

19 May '07

Hi everybody. Conceptually, all "ordinary" data structures in Haskell are immutable. For example, map (2*) takes a list and generates a completely separate, *new* list. And conceptually, SELECT * FROM customer, order WHERE order.cid = customer.cid; computes the extended Cartesian product of the two tables and then restricts the result according to the predicate. Which is, of course, *absurdly* inefficient. However, no sane database engine in Creation actually *does* it this way. ;-) It's just that conceptually, that's how the result is defined. Is there any circumstances under which an expression like map (2*) would perform an in-place update rather than generating a new list? (Obviously this depends on which compiler we're talking about. I'm implicitly assuming GHC.) Assuming the old list isn't needed again, an in-place update is obviously significantly cheaper. (Less RAM used, less time allocating RAM, less GC time, etc.) For something like an integer which can be "unboxed", you could write a very tight loop of machine code to perform a multiplication by 2 over a single-linked list. Similarly, according to the rules, something like length . filter odd will take a list, generate a new list containing only the odd elements, and then proceed to count the size of that list and throw the list itself away. I might hope that this would be "optimized" into a tight loop of machine code that actually just traverses the original linked list and increments a CPU register with the count of odd elements found. But is GHC actually that cleaver? I spend lots of time telling people that Haskell compilers can actually make big optimizations like this, and coding my own stuff as if this will happen, but is it actually so? Am I assuming the optimizer is all-powerful when in fact it's not? Similarly, I could ask lots of other, related questions to the above. (E.g., can all types be optimized like this, or is it only built-in lists?) And I know of no way of finding answers, other than pestering the people on this mailing list. (Certainly I don't know enough about IA32 assembly to take apart the actual compiled code and see what it does.) Anybody have anything useful to say on the matter?

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Short and sweet
by Andrew Coppin 19 May '07

19 May '07

Greetings. Can anybody tell me what complexity class "nub" belongs to? (The implementation in the Language Report appears to have n^2 complexity. Do the actual implementations out there follow?)

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Made me smile
by Andrew Coppin 19 May '07

19 May '07

OK, so I was hanging out on this newsgroup I often lurk. And we're having a discussion. And I'm all like "Haskell is the greatest!" And they're all like "nah, Haskell sux". And this one dude goes "hey, look at this C++ code. How do you do that in Haskell?" And three other people look at this C++ and can't figure out what the heck it even does. So eventually he tells us what it does, and so I post some Haskell in a reply. And the some other guy writes this... (OK, not much point to this email. But it seriously made me LOL IRL!) Invisible wrote: >>>> int main() >>>> { >>>> typedef std::istream_iterator<std::string> Is; >>>> typedef std::ostream_iterator<std::string> Os; >>>> std::set<std::string> t((Is(std::cin)), Is()); >>>> std::copy(t.begin(), t.end(), Os(std::cout, "\n")); >>>> } > OK then. > main = interact $ unlines . sort . nub . words Pwned! -- Darren New / San Diego, CA, USA (PST) His kernel fu is strong. He studied at the Shao Linux Temple.

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Picking an architecture for a Haskell web app
by Joel Reymont 19 May '07

19 May '07

Folks, I have finished an alpha version of my EasyLanguage [1] to C# compiler and need to deploy it on Amazon EC2/S3. The compiler web interface is very simple: paste EL code, get back C# code or the same EL code with the error highlighted. I view the site as more than just a compiler, though, and want a blog, forums, etc. Last but not least, I want to be able to update portions of the site without shutting it down. My choice seems to boil down to HAppS [2], HOPE [3], or a combination of Ruby/Rails with Lambdabot. At first glance, HAppS lacks publishing (my ultimate need) but promises an interface to S3 sometime in the future. My understanding is that it's ready but is not being released. HAppS is heavy on features that I may not make use of. I had a hard time understanding the HPaste [4] code written using HAppS and I don't fancy using XSLT as my HTML template language. HOPE has a publishing framework (blog, RSS, comments). I could write the forums or just use Google Groups. I like the module system and the FCGI interface should let me couple HOPE with Lighty [5]. The HOPE code base looks clean and simple and I'm guessing that adding Lambdabot-style dynamic loading of modules would be straightforward. Overall, HOPE has everything that I need, without unnecessary extras. It does not have an interface to Amazon S3, though, and I would need that to rewrite the storage bits to use S3 instead of a regular database. Lambdabot by itself doesn't offer much apart from robust dynamic loading of Haskell code. Its features have been heavily tested, though, which gives me a warm feeling. I'm thinking that the same hot- loading technology that Lambdabot uses could be coupled with either HAppS or HOPE. Lambdabot has a FCGI which should make mixing it with HOPE particularly easy. Going with Ruby and Rails I will have everything I need and more. The compiler could run in a separate server (think Lambdabot) but I would need to manage two different platforms. Is my assessment correct? Did I miss anything? What approach do you favor? Thanks, Joel P.S. I have the same compiler written in OCaml which is what I started with. Developing it has been a relative breeze but deploying on the web was a tale of frustration. I gave up and finished the Haskell version. [1] http://lambda-the-ultimate.org/node/2201 [2] http://happs.org/HAppS/README.html [3] http://hope.bringert.net/about [4] http://hpaste.org [5] http://www.lighttpd.net/ -- http://wagerlabs.com/

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where do I point the type annotations
by Alex Jacobson 19 May '07

19 May '07

I am playing with using SYB to make generic indexed collections. The current code is this: data Syb = Syb [Dynamic] -- list of [Map val (Set a)] empty item = Syb $ gmapQ (toDyn . emp item) item where emp::x->y->Map.Map y (Set.Set x) emp x y = Map.empty insert x (Syb indices) = Syb $ zipWith f indices (gmapQ toDyn x) where f dynIndex dynAttr = toDyn $ Map.insert attr (maybe (Set.singleton x) (Set.insert x) $ Map.lookup attr index) index where index = fromJust $ fromDynamic dynIndex attr = fromJust $ fromDynamic dynAttr e = empty i where i=i::Test t1 = Test "foo" 2 c1 = insert t1 e Which causes the following error (the line numbers are wrong because there was other code in the original): Ambiguous type variable `a' in the constraints: `Ord a' arising from use of `insert' at Main.hs:113:5-15 `Typeable a' arising from use of `insert' at Main.hs:113:5-15 Probable fix: add a type signature that fixes these type variable(s) What am I doing wrong? Is there a better way to define insert that does not have this problem? Note: I also tried doing this so that each attribute tried to find a matching index using fromDynamic, but that gave me an error involving gmapQ not have an Ord constraint. -Alex-

2 1

Help with Programming in Haskell example
by Andre Nathan 19 May '07

19 May '07

Hello I've been reading Programming in Haskell, and I'm trying to go through the parser examples in chapter. However, I'm getting a type error when using the "do" notation. Here's the code I'm trying to load in ghci, which is copied from the book: import Prelude hiding ((>>=), return) import Char type Parser a = String -> [(a, String)] return :: a -> Parser a return v = \inp -> [(v, inp)] failure :: Parser a failure = \inp -> [] item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x, xs)] parse :: Parser a -> String -> [(a, String)] parse p inp = p inp (>>=) :: Parser a -> (a -> Parser b) -> Parser b p >>= f = \inp -> case parse p inp of [] -> [] [(v, out)] -> parse (f v) out p :: Parser (Char, Char) p = do x <- item item y <- item return (x, y) The problem is in the definition of "p": Couldn't match expected type `Char' against inferred type `[(Char, String)]' In the expression: x In the first argument of `return', namely `(x, y)' In the expression: return (x, y) Now if I define p as p :: Parser (Char, Char) p = item >>= \x -> item >>= \_ -> item >>= \y -> return (x, y) it works fine, so I'm wondering what else I need to do for the "do" notation to work. Thanks in advance, Andre

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Module dependency graph generator
by Alfonso Acosta 19 May '07

19 May '07

Hi all, I'm searching for a tool which generates a module dependency graph (a graphviz backend would be more than enough). Does anybody know a tool with such functionality? Cheers, Fons

2 1

Quick Sort Algorithm
by PR Stanley 19 May '07

19 May '07

Hi folks qSort (x:xs) = qSort smaller ++ [x] ++ qSort larger where smaller = [a | a . xs, a x ] larger = [b | b . xs, b > x ] Any idea why I can't get this to work? Thanks, Paul

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reversing big list with constant heap space used
by Sergey Perminov 18 May '07

18 May '07

How to solve task of reversing big list with constant heap space used? Amount of heap space used grows exponentially in following examples: 1: main = putStrLn.show.head $reverse [1..10000000] 2 (GHC): import Data.List main = putStrLn.show.head $foldl' (flip (:)) [] [1..10000000] 3 (GHC): import Control.Monad main = foldM (\x y -> return $ y:x) [] [1..10000000] >>= putStrLn.show.head -- Best regards, Sergey Perminov

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Tail Recursion within the IO Monad
by Rob Hoelz 18 May '07

18 May '07

Hello everyone, You may have seen my message about how I'm writing a binding to a C library. This is another question related to that. So, let's say I have a linked list implemented in C. Here's what its definition looks like: struct __linked_list { void *data; struct __linked_list *next; }; typedef struct __linked_list linked_list_t; void *linked_list_getdata(linked_list_t *); linked_list_t *linked_list_next(linked_list_t *); Keep in mind, this is just a segment. So using the Haskell FFI, I import these into my .hsc file: data LinkedList = LL (Ptr linked_list_t) foreign import ccall unsafe "linked_list.h" linked_list_getdata :: Ptr LinkedList -> IO Ptr a foreign import ccall unsafe "linked_list.h" linked_list_next :: Ptr LinkedList -> IO Ptr LinkedList So now that that's done, I attempt to write a Ptr LinkedList -> [String] function (assuming the given LinkedList is holding c strings): linkedListToStringList :: Ptr LinkedList -> IO [String] linkedListToStringList listPtr = if listPtr == nullPtr then return [] else do item <- linked_list_getdata listPtr next <- linked_list_next listPtr cStr <- peek item hStr <- peekCString cStr t <- linkedListToStringList next return (hStr : t) This is just ugly...making the recursive call first, THEN consing the value on? However, this is the only way I could think of doing it. I figure there are three possibilities from here: 1) Leave this code alone, as GHC will optimize it because it's smart. 2) There's a way to more effectively write this code! Change it! 3) Roll my own optimization. I know how to do 3, but I'd rather avoid it. I guess I'm looking for an answer to 2, but if 1 is true, that'd be ok too. Could anyone give me a hand? And as long as I'm asking, is there some kind of monadic function composition operator? I'd like to clean up the above with something like peekCString . peek . linked_list_getdata... Many thanks, Rob Hoelz

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Safe/Unsafe calls with the GHC Foreign Function Interface
by Rob Hoelz 18 May '07

18 May '07

More questions from Rob regarding his mysterious bindings... So I've been reading the docs for the FFI, and it's my understanding that foreign functions imported as "unsafe" are faster, but they've got a problem with callbacks. From what I read, I believe that I should just make a foreign function safe if it sets up a callback; is this correct? Normally I wouldn't bother people with questions like this, but I'd rather be absolutely certain and avoid tracking down a strange bug later. Many thanks as always, Rob

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haskell-lang.org
by Dipen Javia 18 May '07

18 May '07

Hello, I just registered haskell-lang.org a week ago for 1 year. I was wondering if anybody wants it, I am selling it for regular price i got it which is $8 from godaddy. If you have an account with godaddy, tranferring is easy and free. If you dont have an account you can get one for free. Thanks. -- Dipen Javia

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Re: global variables
by Big Chris 18 May '07

18 May '07

On Thu, 17 May 2007, Jason Dagit wrote: > Well, it seems to me that Haskell modules are actually very similar to > singletons. Perhaps all these problems with modules having top level > mutable state could be solved if Haskell modules were parameterizable > at "instantiation"? I'm not saying we should turn the Haskell module > system into an OO system, just that maybe it would be wise to borrow > some ideas from that paradigm. Well, I'm a little unclear as to how the discussion shifted from global variables to modules, but as long as we're here: Rather than looking at OO languages, why not take a lesson from more powerful module systems already in functional programming languages. In particular, the ML module system supports "parameterization at instantiation" with functors. Of course, there is considerable evidence that ML modules and Haskell type classes really do the same thing, but they do seem to be useful in very different situations. --Chris

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IO Monad operation madness
by Rob Hoelz 18 May '07

18 May '07

Hello again, I'm still working on that binding, and I've come to a problem that I just can't figure out. I'm sure it's staring me right in the face, so I think another pair of eyes to take a look. So if you look at my previous message, I defined a linked list data type in C. I'm trying to convert it to a Haskell list of strings, so here's my function: linkedListToHaskellStringList :: LinkedList -> IO [String] linkedListToHaskellStringList listPtr = let convertList' ptr = if listIsNull ptr then [] else let str = peekCString =<< (linked_list_getdata ptr) next <- linked_list_next ptr str : (convertList' next) in sequence $ convertList' listPtr listIsNull :: LinkedList -> Bool listIsNull (LinkedList ptr) = ptr == nullPtr So here's the compile error: Option.hsc:63:14: Couldn't match expected type `[t]' against inferred type `IO LinkedList' In a 'do' expression: next <- linked_list_next ptr In the expression: if listIsNull ptr then [] else do let str = peekCString =<< linked_list_getdata ptr next <- linked_list_next ptr str : (convertList' next) In the definition of `convertList'': convertList' ptr = if listIsNull ptr then [] else do let str = ... next <- linked_list_next ptr str : (convertList' next) Could anyone tell me what I'm doing wrong? Thanks a lot! -Rob

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Darcs annotate denies all knowledge of files...
by Dougal Stanton 18 May '07

18 May '07

I just did a fresh pull of hpodder to see if there was still a bug in it that I just found in the feisty version. I ran > $ darcs get --partial http://darcs.changelog.org/hpodder > $ darcs annotate Commands/SetTitle.hs > > darcs failed: There is no file or directory named 'Commands/SetTitle.hs' There is some mention online that Darcs does this when there are special characters in the filename. That isn't the case here. To be certain I ran it on several other files, all with the same result. I ran it on some of my own repositories and it worked fine. What gives? Cheers, Dougal.

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Imagining a G-machine
by Albert Y. C. Lai 18 May '07

18 May '07

A native G-machine --- physical, or chemical, or biological, but not a repressed simulation over the imperative cpu-memory architecture --- is the dream of every lazy-functional programmer of great devotion. If only it became the dominant computing architecture! People would say, Haskell is high-level assembly because it is the closest to the bare metal (or bare protein) and still supports all sorts of abstractions. People would ask, why is my C program five times slower than Haskell, and people would answer, that's because C is compiled to a stack of seven monad transformers (one of which is ContT to support longjmp), and because you should use more linked lists and fewer arrays. We truely rule the world when C programmers have to consult us for performance tips. A necessary condition for dominance is existence. Here is my crazy imagination of a native G-machine. It is worded as biotech but may as well be molecular computing or nanotech. The heap is a soup of nutrients. A node is a molecular structure made from the nutrients. A thunk is a lot of nodes linked up. Nodes and thunks are suspended in the soup. Allocation means constructing nodes and links from nutrients, and deallocation means unlinking nodes and eventually dismantling them back into nutrients. As a side effect, memory upgrade is cheap and convenient: Just go buy a can of Campbell soup (my favourite is the alphabet soup) and pour into your heap. There are 24-hour convenient stores at every street corner --- pervasive computing has never been so pervasive before. A thunk is nodes linked together and suspended in the soup. A theorem in graph theory asserts that all finite graphs can be embedded without crossing edges in 3D space, and we take advantage of this to space out the nodes in a complicated thunk. Still, it is not easy, being NP-hard and all. There is a relaxation heuristic for this: It simply requires nodes to be a bit repulsive to each other and links to be elastic, and they will sort things out themselves. (When they don't, a bit of shaking up helps tremendously.) Evaluation is performed by a small organism crawling over a thunk and performing its thunk-rewriting duty as per the G-machine. It applies enzymes to construct and deconstruct nodes and links from nutrients. It performs primitive arithmetic on its own. It maintains its own stack, inside or outside its body (to be investigated). It is possible to unleash several such evaluators for parallel computing. It is possible to enable reproduction to boost parallelism. To add garbage collection, roots send out a periodic (or sustained) signal to all connected nodes. Nodes receiving the signal do not self-destruct. Nodes not receiving the signal invokes their built-in self-destruct mechanism to dissolve themselves back into nutrients. There may be better schemes. Interacting with the outside world is open, but Andrew Gordon's thesis contains translations from monadic I/O to other I/O schemes, e.g., continuations, streams, etc. Perhaps one of them can be adapted. Debugging can be done by making evaluators send radio signals concerning operations they perform; then a second computer can log these and you can review them. You can also use radio signals to instruct the evaluators to perform unusual operations on demand. The existence of a native G-machine is a necessary but not sufficient condition for world dominance. We still need to make it fast, compact, and cheap. There may also be better but totally different ways to realize a native G-machine.

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Re: Type class help please
by oleg＠pobox.com 18 May '07

18 May '07

Adrian Hey wrote: > -- Instances of GT are instances of Eq -- > instance (GT map key, Eq a) => Eq (map a) where > map1 == map2 = assocsAscending map1 == assocsAscending map2 > ... > Overlapping instances for Eq [(key, a)] > arising from use of `==' at Test.hs:10:16-59 > Matching instances: > instance (Eq a) => Eq [a] -- Defined in GHC.Base > instance (GT map key, Eq a) => Eq (map a) -- Defined at Test.hs:9:0 > > How can my new instance overlap with the old (ghc) instance unless > [] is also an instance of GT for some key type (which it isn't). > Could someone explain? You are right in your explanation of the GHC behavior. Your instance |Eq (map a)| indeed overlaps the `standard` instance |Eq [a]|. The overlap may be easier to see if we write [a] as ([] a), which is what it is, an application of the type constructor [] to the type variable a. So, the type [a] (aka [] a) is a particular instance of the type (map a), with `map' being []. This is the overlapping that GHC is complaining about. Regarding the need for -fallow-undecidable-instance: GHC is not a general purpose termination checker. Furthermore, it seems reasonable for GHC to employ simple termination heuristics, which can be decided quickly, syntactically and locally (that is, considering only the instance in question, rather than collection of all instances in the program). Thus GHC employs a set of heuristics that look if the instance constraints are `smaller' than the instance head. That will assure termination. That criterion is of course not complete, so there are (many) terminating, decidable typeclass program that fail simple GHC termination tests and thus require -fallow-undecidable-instance extension. One method of avoiding overlapping instances is to define a newtype wrapper: newtype MyMap map a = MyMap{unMyMap::map a} class Ord key => GT map key | map -> key where assocsAscending :: map a -> [(key,a)] -- Just 1 of many methods instance GT (MyMap Data.IntMap) Int where ... The drawback is writing MyMap and unMyMap in many places. That spoils the appearance, but rarely fatal. Another solution is replacing the general instance instance (GT map key, Eq a) => Eq (map a) where map1 == map2 = assocsAscending map1 == assocsAscending map2 with its instantiations, for all `maps' in question: instance Eq a => Eq (Data.IntMap a) where ... instance Eq a => Eq (Data.Map key a) where ... instance Eq a => Eq (Data.IntSet a) where ...

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Bad "let..do" syntax
by Chad Scherrer 18 May '07

18 May '07

I've gotten into a habit of preceding most "do"s in my code with a "$", and indenting the next line. I kind of like this, since it makes the indentation more uniform. But it seems to have bitten me now. I'd like to write something like this s = sum $ do x <- [1,2,3] let b = sum $ do y <- [0..x + 1] return y return (x + b) But GHC complains of "Empty 'do' construct". It likes the alternative s' = sum $ do x <- [1,2,3] let b = sum $ do y <- [0..x + 1] return y return (x + b) just fine, but that looks horrible to me (ok, "horrible" is a bit strong, but I don't like it as much). So I'm wondering, (1) Is this intended to give an error, or is it just a momentary hiccup, and (2) if others have run into this, is there a more aesthetic alternative that works? Thanks, Chad

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negate and -
by Maurício 18 May '07

18 May '07

Hi, Haskell language reference says the special - operator translates to negate. However, this: (+ negate 5) 5 works but this: (+ -5) 5 is not valid. Why? Thanks, Maurício

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haskell quiry
by ashutosh dimri 17 May '07

17 May '07

Thanks a lot Mr. jules now the code is working perfectly for higher values , the problem was with ord .

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haskell quiry
by ashutosh dimri 17 May '07

17 May '07

I have written a code to convert hexadecimal numbers into base 10 number but its working only for 32 bits and I need it to work upto 160 bits , code is given below. please help. --its a program to convert hexadecimal numbers into base 10 numbers import Char -- the function "func" takes a list of strings in hexadecimal and convert -- it to a list of corresponding numbers in decimal format -- func "eefba12" = [14,14,15,11,10,1,2] func [] = [] func (x:xs) |(ord x > 57) = ((ord x)-87):func xs |otherwise = ((ord x)-48):func xs -- (mul.func) function takes a hexadecimal number and convert it to base 10 but its working only for 32 bits -- whereas haskell supports calculations much higher than 32 bits..... -- mul.func $ "ee" = 238 -- mul.func $ "e23e" = 57918 mul xs = mul1 xs ((length xs)-1) mul1 (x:xs) 0 = x mul1 (x:xs) n = (x*16^n)+ mul1 xs (n-1)

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ANNOUNCE: Dimensional 0.4 -- Statically checked physical dimensions
by Björn Buckwalter 17 May '07

17 May '07

Dear all, I am pleased to announce version 0.4 of Dimensional (working name). Dimensional is a library providing data types for performing arithmetic with physical quantities and units. Information about the physical dimensions of the quantities/units is embedded in their types and the validity of operations is verified by the type checker at compile time. The boxing and unboxing of numerical values as quantities is done by multiplication and division with units. The library is designed to, as far as is practical, enforce/encourage best practices [1] of unit usage. Since the previous formal announcement [2] (version 0.1) Dimensional has gone through several structural and stylistic improvements but usage remains fundamentally unchanged. Noteworthy changes are a vastly solidified 'NumType' module, a complete(?) set of elementary functions and a 'Prelude'-replacement for users' convenience. There is also an experimental 'Extensible' module supporting user-defined dimensions. Additional structural changes and additions (primarily of units) are planned and the library will continue to have an unstable API until the 1.0 release. However, apart from module reshuffling no changes are anticipated that would break user code. Additional information and code is available from the project web site [3]. Thank you, Bjorn Buckwalter [1] http://physics.nist.gov/Pubs/SP811/ [2] http://www.haskell.org/pipermail/haskell/2006-December/018993.html [3] http://code.google.com/p/dimensional/

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CUFP website
by Cyril Schmidt 17 May '07

17 May '07

I noticed recently that the website of CUFP conference (Commercial Uses of Function Programming), which used to be at http://www.galois.com/cufp, is not accessible anymore. Does anybody know where it moved? Cyril

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quiry
by ashutosh dimri 17 May '07

17 May '07

how to convert a hexadecimal into base 10 integer using haskell . I have written a code but its not working for large values , please help

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