- Haskell-Cafe - Haskell.org

Polymorphic type
by Sara Kenedy 23 Jun '06

23 Jun '06

Hello all, Now I am trying with the function of polymorphic type: This function returns the Nth element of list with type a. I try it as below. getNthElem :: Int -> [a] -> Maybe a getNthElemt _ [] = Nothing getNthElem 0 _ = Nothing getNthElem n s | n > length s = Nothing | otherwise = Just ((drop (n-1) (take n s))!!0) >getNthElem 2 ["a","b","c"] Just "b" However, I do not satisfy with this function because I want to return the Nth element of type a, not (Maybe a). For example, I want this function: getNthElem :: Int -> [a] -> a But, I do not know how to define the empty element of type a. getNthElemt _ [] = ???? getNthElem 0 _ = ???? If you have some ideas about this, please give me some clues. Thanks a lot. S.

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neural network code
by John Meacham 22 Jun '06

22 Jun '06

I was curious if there was any code floating around out there for dealing with neural networks in haskell? John -- John Meacham - ⑆repetae.net⑆john⑈

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Principal type in Haskell
by william kim 22 Jun '06

22 Jun '06

Hi All, I am confused by the notion of principal type in Haskell with type classes. Consider a simple example: f x y = [x] == [y] GHCi yields type f :: (Eq [a]) => a -> a -> Bool. But according to the paper "Type classes: an exploration of the design space", predicate Eq [a] should be reduced to Eq a. Is this reduction performed here? What should be the principal type of f? Thanks. william _________________________________________________________________ Get an advanced look at the new version of MSN Messenger. http://messenger.msn.com.sg/Beta/Default.aspx

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user type declarations in Haskell
by Vladimir Portnykh 22 Jun '06

22 Jun '06

I am trying to define the following types data MyStringType a = String deriving (Eq, Ord, Show) data QADouble a = Double deriving (Eq, Ord, Show) data HType a = QADouble a| DDTraceType a deriving (Eq, Ord, Show) So HType can represent strings or doubles. later I want to do something like the following: let a1 =QADouble 1 let a2 =QADouble 2 let a3 = a1 + a2 First, it is not working because Haskell complains about a3. it does not know how to calculate it. Is it a way to give him a hint? QADouble is Double...am I doing something absolutely wrong and silly? many thanks, vladimir _________________________________________________________________ Are you using the latest version of MSN Messenger? Download MSN Messenger 7.5 today! http://join.msn.com/messenger/overview

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Re: [Haskell-cafe] Re: Functional programming for processing oflargeraster images
by voigt.16734551＠bloglines.com 22 Jun '06

22 Jun '06

--- Matthias Fischmann <fis(a)wiwi.hu-berlin.de wrote: > On Thu, Jun 22, 2006 at 09:22:34AM +0100, Simon Peyton-Jones wrote: > > To: Brian Hulley <brianh(a)metamilk.com>, Joel Reymont <joelr1(a)gmail.com> > > Cc: haskell-cafe(a)haskell.org > > From: Simon Peyton-Jones <simonpj(a)microsoft.com> > > Date: Thu, 22 Jun 2006 09:22:34 +0100 > > Subject: RE: [Haskell-cafe] Re: Functional programming for processing > > oflargeraster images > > > > http://haskell.galois.com/cgi-bin/haskell-prime/trac.cgi/wiki/BangPatterns > > > > Bang patterns make it much more convenient to write a strict function. > > E.g > > f (x, !y) = ... > > is strict both in the pair (of course) but also in the second component > > of the pair, y. > > i am ecstatic to hear that :). Well, you shouldn't be too enthusiastic, but rather follow the above link ... > if it really means that 'y' will be fully evaluated (not top level > normal form, but whatsthenameforthis, in the way ocaml evaluates > expressions), it's something i have been missing so much that i was > thinking of switching back to a strict language again. ... to find out that that's exactly not what bang patterns will do for you. They are compiled into uses of seq, which means evaluation to weak head normal form. Ciao, Janis.

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ghc-pkg on Windows XP
by Jeff Polakow 22 Jun '06

22 Jun '06

Hello, I have installed ghc-6.4.2 on a windows XP machine. However, the machine refuses to execute ghc-pkg (thus preventing me from using Cabal) and complains that "C:\ghc\ghc-6.4.2\bin\ghc-pkg.exe is not a valid Win32 application." Is there something obvious I might have overlooked while installing? Also, I am new to using ghc on windows and was wondering what are the trade-offs of using cygwin or minsys versus just living in a emacs shell? thanks, Jeff -- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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Template haskell and scoping
by C Rodrigues 22 Jun '06

22 Jun '06

The (.&.) in the splice is out of scope according to GHC. If I use [||] then it works, but for my purposes it's easier to use the constructors. How should I refer to that variable? import Data.Bits import Language.Haskell.TH main = print $ $(return $ VarE $ mkName ".&.") 7 (14 :: Int)

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Re: [Haskell-cafe] Functional progr., images, laziness and all the rest
by Piotr Kalinowski 22 Jun '06

22 Jun '06

On 22/06/06, Brian Hulley <brianh(a)metamilk.com> wrote: > 1) Someone reading the code needs to do a lot of work to try to recover the > original equation That's what comments are for, I suppose. > 2) Wouldn't an imperative loop, using the original equation directly, have > made everything much simpler? > 3) Therefore laziness has lead to obfuscated code. In fact, according to your point 2, it is not the laziness that brought us here, but being functional. Just my 2 cents. Regards, Piotr Kalinowski -- Intelligence is like a river: the deeper it is, the less noise it makes

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haskell companies
by Andreas Kägi 21 Jun '06

21 Jun '06

hello i wonder if there are companies that use haskell in practice. i'm a computer science student from switzerland (europe) and need to do an internship. since i find haskell very interesting i thought these two things could maybe be combined.

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ACross
by Jenny678 20 Jun '06

20 Jun '06

Hello, Can somebody help me My Problem is to define a code for: >cross 7 -- means 7*7 >+++++++ >++ ++ >+ + + + >+ + + >+ + + + >++ ++ >+++++++ Another Example >cross 13 -- 13*13 >+++++++++++++ >++ ++ >+ + + + >+ + + + >+ + + + >+ + + + >+ + + >+ + + + >+ + + + >+ + + + >+ + + + >++ ++ >+++++++++++++ Can I use putString ??? Thanks for any help -- View this message in context: http://www.nabble.com/ACross-t1813549.html#a4943328 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

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Newbie question on iterating over IO list
by Taro Ikai 20 Jun '06

20 Jun '06

Here's a code fragment I have from working with Hal Daume's "Yet Another Haskell Tutorial". I cannot figure out how to iterate over the result from askForNumbers that returns "IO [Integer]", and print them. The "<-" operator seems to take IO off of the results from actions like getContents, which returns type "IO String", but I cannot seem to use it to take IO off of lists with it. What am I supposed to do? Also, as a side question, what should I be returning from the do function? >>>code>>>> module Main where import IO main = do hSetBuffering stdin LineBuffering s <- askForNumbers {-- This compiles --} map show s {-- But this doesn't. Why? --} {-- What expression should I be returning from do? --} askForNumbers = do putStrLn "Give me a number (or 0 to stop):" guess <- getLine let number = read guess if number == 0 then return [] else do rest <- askForNumbers return (number : rest) <<<code<<<< Thanks in advance. Taro Ikai

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Efficient way to edit a file
by Maurício 20 Jun '06

20 Jun '06

Hi, I need to edit big text files (5 to 500 Mb). But I just need to change one or two small lines, and save it. What is the best way to do that in Haskell, without creating copies of the whole files? Thanks, Maurício

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cabal-get and HXT?
by Greg Fitzgerald 20 Jun '06

20 Jun '06

I'd like to use a tool to automatically install Haskell XML Toolkit and its dependencies. What are my options? How's cabal-get coming? Thanks, Greg

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RE:[Haskell-cafe] A function using List and Tuple
by jotajota 19 Jun '06

19 Jun '06

Hi On 6/19/06, Sara Kenedy <sarakenedy(a)gmail.com> wrote: > Hello, > > I want to creat a function sumList as below examples: > sumList [("s1",2),("s2",4),("s3",3),("s4",2)] = ("#", 1/2 + 1/4 + 1/3 + 1/2) > sumList [("s1",2),("s2",4),("s3",3) = ("#", 1/2 + 1/4 + 1/3) > > I attempted it as following: > > sumList :: (Fractional a) => [(String,a)] -> (String, a) > sumList [] = ??? > sumList (x:xs) = ("#", 1/(snd x) + 1/snd(sumList xs)) > > I know this function cannot give the correct answer (even if I define > sumList []), but I did not find the right way. > If anyone can give me a suggestion, I really appereciate for that. Thanks. > Yo can use the foldr function: sumList a = ("#",foldr suma 0 a ) where suma (s,x) y = 1/x + y Juan Jose

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Parsec and destructuring HTML content
by Joel Reymont 19 Jun '06

19 Jun '06

Has anyone explored destructuring HTML with Parsec? Any other ideas on how to best do this? I'm looking to scrape bits of information from more or less unstructured HTML pages. I'm looking to structure, tag and classify the content afterwards. I think that developing HTML scrapers requires short tweak-compile- run cycles and is probably best done in Perl, Python, Ruby, i.e. dynamic languages but I wonder if someone has found otherwise. Thanks, Joel -- http://wagerlabs.com/

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A function using List and Tuple
by Sara Kenedy 19 Jun '06

19 Jun '06

Hello, I want to creat a function sumList as below examples: sumList [("s1",2),("s2",4),("s3",3),("s4",2)] = ("#", 1/2 + 1/4 + 1/3 + 1/2) sumList [("s1",2),("s2",4),("s3",3) = ("#", 1/2 + 1/4 + 1/3) I attempted it as following: sumList :: (Fractional a) => [(String,a)] -> (String, a) sumList [] = ??? sumList (x:xs) = ("#", 1/(snd x) + 1/snd(sumList xs)) I know this function cannot give the correct answer (even if I define sumList []), but I did not find the right way. If anyone can give me a suggestion, I really appereciate for that. Thanks. S.

2 1

Fibonacci numbers generator in Haskell
by Vladimir Portnykh 19 Jun '06

19 Jun '06

Fibonacci numbers implementations in Haskell one of the classical examples. An example I found is the following: fibs :: [Int] fibs = 0 : 1 : [ a + b | (a, b) <- zip fibs (tail fibs)] To get the k-th number you do the following: Result = fibs !! k It is elegant but creates a list of all Fibonacci numbers less than k-th, and the code is not very readable :). I wrote my own Fibonacci numbers generator: fib :: Int -> [Int] fib 0 = [0,0] fib 1 = [1,0] fib n = [sum prevFib, head prevFib] where a = fib (n - 1) To get the k-th number you do the following: result = head (fib k) It does not generate full list of Fibonacci numbers, but keeps only 2 previous numbers, and has only one recursive call. Because the list always has only 2 elements using the functions head and sum is a bit overkill. Can we do better? _________________________________________________________________ Are you using the latest version of MSN Messenger? Download MSN Messenger 7.5 today! http://join.msn.com/messenger/overview

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take the keywords from a string
by Sara Kenedy 18 Jun '06

18 Jun '06

Hi everybody, I have a function: takeKeyword :: String -> [String] This function will take the input as a string and return a list of keywords taken from the input string and they are elements of ListOfKeywords. The order of the result list is sequenced as: the last keyword found is set as the first element of the list, and so on. For example, ListOfKeywords = ["expand, "limit", "diff"] takeKeyword :: String -> [String] takeKeyword limit(x + expand((x+3)^2), x=3) = ["expand", "limit"] takeKeyword limit(x -7, x= 3) =["limit"] If you have any suggestions for that, please share with me. Thanks in advance. S.

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Sets symmetric difference
by Jenny678 18 Jun '06

18 Jun '06

Hello, Know Somebody the code for symmetric difference (Sets) examples >symmetric_difference [1..8][5..7] >[1,2,3,4,8] >symmetric_ difference [1..8][] >[1,2,3,4,5,6,7,8] >symmetric_ difference [1..12][10..20] >[1,2,3,4,5,6,7,8,9,13,14,15,16,17,18,19,20] Thanks for Help -- View this message in context: http://www.nabble.com/Sets-symmetric-difference-t1805165.html#a4919646 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

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SMTP, HTTP, Telnet
by Lyle Kopnicky 17 Jun '06

17 Jun '06

Hi all, Anybody know of some good Haskell libraries providing: * an SMTP client, * an HTTP client, * or a Telnet client? There's a significant amount to these protocols, over and above the socket layer. Thanks, Lyle Kopnicky

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RE: why isn't everything an instance of Data?
by Ralf Lammel 17 Jun '06

17 Jun '06

Hi Frederik, Cc Haskell-café as proposed by Frederik, > Why isn't everything an instance of Data? > > It seems natural to give every type which is defined via 'data' an > automatic Data instance. This would make implementing many things much > simpler. Certain types such as functions would only be able to have > "stub" instances, for instance not allowing traversal of constructor > arguments, but this would no more hurt the user than not having such > instances defined in the first place. To clarify, what I am suggesting > is that Data no longer be a class but an interface - one would be able > to call 'toConstr' on any type. [Ralf Lammel] I get your overall question but let me ask about one detail. When you say "an interface", what does this mean in the Haskell sense (where normally a type-class is like an OO interface *anyhow*)? > Is there a fear that this would lead to "sloppy programming"? [Ralf Lammel] I agree that it could be convenient to assume gfoldl and all that as true primitives. Personally, I wouldn't necessarily like that: Haskell's type system seems to be designed to (mostly) distinguish parametrically polymorphic functions from the rest; just as much as it (mostly) makes explicit any effects involved in a computation. Also, how would these primitives need to be implemented? We would need a whole lot of type information in the sense of reflection to be around at run-time. > Also, it would be nice if it were possible to have the compiler infer, > in a function such as > > cast :: a -> Maybe b > > that 'a' and 'b' are instances of the same classes. However, unless > I'm mistaken this would require 'cast' to have special status with the > compiler (or for there to be some new syntax allowing one to indicate > that two type variables implement the same classes). [Ralf Lammel] There are several problems here. First, I doubt that we want to establish "that 'a' and 'b' are instances of the same classes". That sounds like a too special case. I guess we would want to establish that a given type instantiates a given type class. Second, our new cast operation (which would indeed be very much like an OO down-cast) would need to be a special primitive indeed, including special syntax. If we had first-class type-classes, special syntax is not needed, but we would still need a primitive for the actual cast. Third, we need to come up with some fairly tricky means of getting the dictionary for a type "out of the blue" at run-time. This would be sort of possible (certainly for monomorphic types) if we had the type of the value/expression around at run-time, so that we can use it to look up a dictionary from a global pool. However, that type is not available without extra effort. To summarize, if this was the question, this expectation seems to lead pretty far away from what we have in Haskell and likely to get anywhere soon -- as far as I can tell. > But without such > inferences, the Generics stuff has limited usefulness - so much in > Haskell is based on classes, but the only class one can use in the > argument of say 'everywhere' is Data itself: > > everywhere :: (forall a . Data a => a -> a) -> forall a . Data a => a -> a [Ralf Lammel] There are two stories here: a) Any type-specific case can make any amount of type-class assumptions even in the original SYB1 style. This gets you pretty far in practice. b) If you want to write generic functions whose set of types is characterized by a designated type class, then you need to use SYB3 style. HTH, Ralf PS: Some related stuff ... - I uploaded a new SYB paper recently: http://www.cwi.nl/~ralf/syx/ - We also updated the SYB site to include other people's SYB papers: http://www.cs.vu.nl/boilerplate - See Pablo Nogueira's paper "Context-Parametric Polykinded Types" for some interesting GH-related issues on type classes; http://www.cs.nott.ac.uk/~pni/Papers/ - Finally, I plan to use my new blog for Haskell-related stuff as well: http://blogs.msdn.com/ralflammel/

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putStrLn
by Jenny678 16 Jun '06

16 Jun '06

Hello, Can somebody help me I want to work with putStrLn main n = putStrLn "*" for example How must I define the code that: >main 5 >***** Thanks for any help -- View this message in context: http://www.nabble.com/putStrLn-t1799896.html#a4905456 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

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Inferring types from functional dependencies
by Jeff.Harper＠handheld.com 16 Jun '06

16 Jun '06

Thank you. I followed your suggestion and implemented my default Divide as you've shown below. oleg(a)pobox.com wrote: > If we assume that we need Reciprocate only if we are going to use the > 'default' method, the solution becomes obvious. It does involve > overlapping and undecidable instances, sorry. These extensions are > really useful in practice. Here's the solution: > > > class Divide a b c | a b -> c where > > (/) :: a -> b -> c > > > Here's the most general instance. It applies when nothing more > specific does. It is in this case that we insist on being able to > take the reciprocal: > > > instance (Reciprocate b recip, Multiply a recip c) => > > Divide a b c where > > (/) x y = x * (recip y)

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Rotating matrices
by Thomas Sutton 16 Jun '06

16 Jun '06

Dear list, I'm currently engaged in an attempt to wrap my head around type arithmetic. To make sure that I've understood, I plan to write a few operations on matrices a la Oleg's Number Parameterised Types. Before I get down to it, I've been making sure I know how to implement the operations (so that all I need to think about later is the type arithmetic). Today I've been looking at rotating matrices, i.e: taking a column- wise matrix and making it row-wise and, in the process, swapping the dimensions (thus a 3*2 matrix becomes a 2*3 matrix). I've been working with lists of lists for simplicity and have come up with: > rotate' :: [[a]] -> [[a]] > rotate' [] = [] > rotate' xs = (map (head) xs ):(rotate' $ filter (not . null) $ map (tail) xs) which seems to work just fine. While this solution is adequate (it seems to work for infinite structures as well, which is good), I originally set out to solve this problem with a fold or a mapAccum of some sort (I don't really care if it doesn't work with infinite structures). Can anyone suggest a way to write this with a fold (or tell me why it isn't a good idea)? Cheers, Thomas Sutton

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what do you think of haskell ? (yes, it's a bit general ...:)
by minh thu 16 Jun '06

16 Jun '06

hi all folks, i'm diving into haskell for more than one year now. the reason for that is just that i like haskell. (i'm a computer science student) but i consider to move back to c/c++. here are my thoughts. i've no specific question but i'd like to have your opinion. here we go: haskell is really nice : short phrased, declarative, math-feeling, the type system ensure low bug amount (i surely forget other nice things). but haskell is quite ugly in some way : * array : if i want to write something involving array, i could use list, and a lot (too much!) of array types (io/st, mutable/immutable, c-friendly (storable). worst, code involving one type can need to be rewritten for another type. * laziness / array (again) (i am biased since i have no knowledge of haskell profiling / performance seeking) always with array code, i was forced (maybe it's because i dont know enough) to use iouarray : if not, performance|memory consumption were low|high. but with iouarray, i cant use an array of MyType; i have to use an array of (say) float (it's ok if i have only float in MyType). that kind of thing is what i think is *really* low-level. in c, i can have an array of what-i-want. * randomIO side-effect is nicely resolved with monad. and you have to thread your state. if you're writing your monad or use a transformer, things are quite explicitly (even if it's implicit in the do notation) threaded. but the threading of the randomIO argument is really not explicit for me : it just means that the underlying/threaded state in the IO monad can encapsulated a lot of things. it feels exactly like a c function. but usually, you cant design a c-like function in haskell (i.e. a function with state). * more things i dont remember... * generally my general feeling for haskell vs c is: in haskell i always have to learn new things to get my work done ; although haskell is really easy to learn in the first step, it's becoming increasingly hard to get what's the *trick* to do what i want. e.g. writing myfunction x1 .. xn | x1 `seq` ... False = undefined is not declarative (and i still have to learn to identify where it helps and where it doesn't) the c language take some more time to learn at the beginning but that's it! what you can learn by after is better c programming. there are memory management, pointers, side-effects, but you can do what you want, it behaves as expected and you dont have to learn (in a academic paper) how to use an array or how to do io. :) please don't tell me "you're stupid, go back to your c and leave this glorious mailing list" did you had the same feeling ? does it disappear ? how ? thanks a lot, vo minh thu

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Everything but the lazyness - idea for force/delay lists
by Brian Hulley 16 Jun '06

16 Jun '06

Hi - I've been thinking about how to get an extremely fast language with all the benefits of Haskell ie completely pure with no side effects, but with monads, higher order functions, type classes etc, but without the lazyness. I know this is controversial, but having started to write a serious program in Haskell, I find that almost all of it doesn't need any lazyness at all, yet I have to constantly mess up my code by using Tuple2 a b instead of (a,b), ! in all my data types, and millions of brackets to use $! (which has the wrong associativity for passing multiple arguments strictly) etc and I can't do anything about the slowness and possible space leaks introduced by library functions which are lazy, not to mention the fact that afaiu the lifted type system necessary for any language which supports lazyness, and general undecidability results means that it will probably be impossible to ever compile lazy code to be as fast as OCaml etc. (This is not to say Haskell is too slow - I find the app I'm writing at the moment runs fast enough (given all my strictness annotations) it's just that lazyness is making my life more difficult rather than easier and is probably also costing something in terms of speed.) The one place where I'm using lazyness is where I need to glue the output of one computation to another by using a lazy list. In effect I'm using a lazy list as the equivalent of an iterator in C++ - the elements are only created when needed, but once read, I only read them once, so I don't need the memoization properties of lazyness. When I first started to learn Haskell, I thought lazyness was essential for monads and hence an acceptable price to pay for using them. However I now think that monads would work perfectly without lazyness, since they are usually always defined in terms of >>= (not >> as I'd thought when I didn't know Haskell). Other uses of lazyness are of course infinite structures and fixpoints etc. I think the use of a lazy list as a read-once-on-demand stream could be achieved just as easily by a strict language with some syntactic sugar by redefining the meaning of []: data GlueList a = Empty | Cons a [a] type [a] = () -> GlueList a force :: (() -> a) -> a force f = f () Pattern matching could then be changed so that p [a] = exp is short for: p x = case force x of Cons a y -> case force y of Empty -> exp and p [h|t] = exp -- Prolog style list sugar p x = case force x of Cons h t -> exp and in an expression, [exp1,...] would be expanded into the appropriate delayed construction eg: x = [a] x = \() -> Cons a (\() -> Empty) (List comprehensions could just be written using do notation). There could also be extra syntactic support for force/delay eg !exp to mean exp () and ~exp to mean \()->exp. With this extra sugar, it might be relatively painless to still use infinite data structures. Lazyness seems to sometimes be used to accomplish things that could easily be implemented in other ways, perhaps even better, and with more control (and certainly more explicitness) over the exact aspect of lazyness that is being used in a certain situation eg memoization, desugared attribute grammars, list-as-glue, etc. The advantages would be that the resulting language would be simpler to understand and could execute like lightning so there would be no need to use C or ML ever again. Afaik the full speed advantage can't be achieved just by syntactic sugar for regular Haskell since regular Haskell, even with seq etc, still needs to use a lifted type system, but syntactic sugar could certainly be used to implement such a language on top of Haskell to investigate if the complete absence of lazyness would cause any problems in practice. If I get time I might try to do this but I've shared the idea here in the vague hope someone else might want to do it first :-) There certainly seems to be a "gap in the market" between the perfection of lazy Haskell's monads, typeclasses etc and strict ML's side effects and less expressive type system. Regards, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com

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do we have something like isDefined or isNull in Haskell?
by Vladimir Portnykh 16 Jun '06

16 Jun '06

Suppose there is a data definition in Haskell: data MyType = MyType { date :: Double, weight :: Double, height :: Double } deriving (Eq, Ord, Show) Is it possible to check if the field height, for example, is filled in(defined)? Can we give default values in Haskell? Many thanks and sorry fro so sily questions. Vladimir _________________________________________________________________ The new MSN Search Toolbar now includes Desktop search! http://join.msn.com/toolbar/overview

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Learning C after Haskell
by Chad Scherrer 15 Jun '06

15 Jun '06

Ok, so I'm doing things somewhat backward. I've been using Haskell for a while now, whenever I get a chance to. But in order to become more involved in high-performance computing projects at my work, I need to learn C. I've heard a lot of people say that experience in Haskell can improve one's abilities in other languages, but I also wonder how different the C "way of doing things" is different from Haskell's. My question is, as I learn C, are there any particular Haskell concepts I should keep in the back of my mind, or is it better to approach C from scratch? Thanks in advance! Preparing for a foot-shooting, Chad

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Re: [Haskell-cafe] do we have something like isDefined or isNull in Haskell?
by Bryan Burgers 15 Jun '06

15 Jun '06

Vladimir, I think you forgot to put Haskell-cafe as a recipient of this email, so first I'll repost what you wrote. On 6/15/06, Vladimir Portnykh <vportnykh(a)hotmail.com> wrote: > many thanks. i have the follwoing code: > > module MyType (DataContainer(..)) where > import Maybe > data DataContainer = > MyType { > date :: [Maybe Double], > weight ::[Maybe Double], > height ::[Maybe Double] > } > deriving (Eq, Ord, Show) > > myType = MyType { > date = > [38548,38562,38576,38590,38604,38618,38632,38646,38660,38674], > weight = [], > height = [] > } > > reportProblem = if isJust (height myType !! (length (date myType) - 1)) > then 0 else 1 > > > it failed to compiled saying "No instance for (Num (Maybe Double)) arising > from the literal 38674 . After reading your code for a while, I still don't know if I quite understand. Can you explain exactly what you are trying to do? My thought is that maybe you just want a list of objects, since you are indexing height at the same index as date. Then you just want a list of the structures that Duncan described. myObjects = [default {weight = 3.2}, default {weight = 7.9, date=38458},...] If not, hopefully somebody a little more knowledgable than I can help you. Bryan

1 0

Receiving multicasts
by Rich Neswold 15 Jun '06

15 Jun '06

Hello, Has anyone figured out a way to receive multicasts in a Haskell program? It doesn't appear that Network.Socket.setSocketOption provides enough information to join a multicast address. Any information would be appreciated. -- Rich AIM : rnezzy ICQ : 174908475

1 0

how to apply a function which returns IO() to a list?
by developer＠imaginando.net 14 Jun '06

14 Jun '06

Hi, I have this function which write a file: writeHtml b x y = do writeFile (b ++ ".html") (htmlCode b x y) i need to apply to each member of a given list: the way i always try to codify functions in haskell is allways the same, the most primitive one and i think i cant use it here. writeList [] = ??? writeList (x:xs) = (writeHtml x) ??? How should i do this? Many thx, Nuno SAntos

4 4

Closure trace?
by Michael Marte 14 Jun '06

14 Jun '06

Hello *, frequently we are seeing messages like "List.head: empty list" or "Maybe.fromJust: Nothing". It is clear what happened and where the messages were triggered yet the real cause is usually VERY hard to find unless the program is small and simple. I came to the conclusion that functions like head and fromJust are best to be avoided because their use may render large programs unmaintainable. Instead I use irrefutable pattern matching like (x : _) = l and Just bla = maybeBla whenever possible because when a pattern match fails the ghc runtime system gives me a nice error message naming the module, line and column. However, this procedure may become tedious when the error occurs in a call to a third-party library function. I wonder whether it is possible to print a "closure trace" similar to a stack trace in procedural programming. Say we have two modules A and B: module A where import B a = b module B b = c where c = error "go to hell" I would like to see something like this: sh$ ghci A <ghci startup messages> > a Runtime error "go to hell" in module B at line 2 Trace: B.b.c B.b A.a This way it would be easy to find the reason that actually caused, say, head to fail. I guess it would be sufficient to complement closures with information on the precise place of their definition. Michael

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Teaching Haskell -- any advice?
by David House 14 Jun '06

14 Jun '06

Hi all, Next year I will have the opportunity to teach Haskell at my school! I am very excited about this and hope to offer something beyond the VB6 and Pascal that is currently available with the school's Computing course. The time slots will be very tight -- one 25 minute session a week. However I will have a year's worth of weeks to teach it in, so I hope to cover most of the major topics with Haskell, including monads (which I appreciate some of the more pedestrian courses tend to skip :)). My question is whether anyone has any advice as to: 1) How to structure the course as a whole? 2) How to structure the lessons (theory/practical balance)? 3) How ambitious should I be? 4) Which pratical projects work well? Thanks in advance. -- -David House, dmhouse(a)gmail.com

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DBDirect trouble
by Ferenc Wagner 14 Jun '06

14 Jun '06

Hi! Let me repeat my question from IRC here, too... I installed HaskellDB and all the necessary stuff (I hope) as local (not system-wide) packages: $ ghc-pkg list /usr/lib/ghc-6.4.1/package.conf: rts-1.0, base-1.0, haskell98-1.0, template-haskell-1.0, unix-1.0, Cabal-1.0, parsec-1.0, haskell-src-1.0, network-1.0, QuickCheck-1.0, HUnit-1.1, mtl-1.0, fgl-5.2, X11-1.1, HGL-3.1, stm-1.0, readline-1.0, (lang-1.0), (concurrent-1.0), (posix-1.0), (util-1.0), (data-1.0), (text-1.0), (net-1.0), (hssource-1.0) /home/wferi/.ghc/i386-linux-6.4.1/package.conf: NewBinary-0.1, Crypto-3.0.3, (Cabal-1.0), Cabal-1.1.4, hsql-1.7, hsql-mysql-1.7, haskelldb-0.9, plugins-1.0, haskelldb-dynamic-0.9, haskelldb-hsql-0.9, haskelldb-hsql-mysql-0.9 Still, DBDirect doesn't run: $ ~/haskell/installed/bin/DBDirect Test mysql server=myserver,db=test,uid=wferi,pwd=secret DB/Direct: Daan Leijen (c) 1999, HWT (c) 2003-2004, Bjorn Bringert (c) 2005 Connecting to database... DBDirect: user error (Couldn't load Database.HaskellDB.HSQL.MySQL.driver from package haskelldb-hsql-mysql-0.9) strace -eopen shows it reads /usr/lib/ghc-6.4.1/package.conf only, not the local one... Any ideas how to persuade it to use the locally installed packages? -- Thanks, Feri.

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Bug in WinHugs?
by Lyle Kopnicky 13 Jun '06

13 Jun '06

Hi folks, I just installed the May 2006 release of Hugs. When I use the command-line version, the '$$' symbol to reference the last expression works fine. It does not work in WinHugs, but yields 'ERROR - Syntax error in expression (unexpected symbol "$$")'. I typed ':set' to verify the '-r$$' option was set properly. Is this a known bug, or am I doing something wrong? Thanks, Lyle

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elementsinlist
by Jenny678 13 Jun '06

13 Jun '06

Hallo I search a code for >elements_in_List([1,2],[1,2]). >True >elements_in_List([1,8],[1,2,3,4,8]). >True >elements_in_List([2,1],[1,2]). >True >elements_in_List([1,1],[1]). >False I have a code elements_in_List :: Eq a => [a] -> [a] -> Bool elements_in_List [] _ = True elements_in_List _ [] = False elements_in_List (x:xs) (y:ys) | x == y = elements_in_List xs ys | True = elements_in_List (x:xs) ys but it failed at >elements_in_List([2,1],[1,2]). >True I hope somebody can help me Please don't use built-in-Functions. Thanks for Help -- View this message in context: http://www.nabble.com/elementsinlist-t1782975.html#a4855615 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

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Re: [Haskell-cafe] elementsinlist
by Bryan Burgers 13 Jun '06

13 Jun '06

On 6/13/06, Jenny678 <mestor1(a)gmx.de> wrote: > > Hallo > I search a code for > > >elements_in_List([1,2],[1,2]). > >True > >elements_in_List([1,8],[1,2,3,4,8]). > >True > >elements_in_List([2,1],[1,2]). > >True > >elements_in_List([1,1],[1]). > >False > > I have a code > elements_in_List :: Eq a => [a] -> [a] -> Bool > elements_in_List [] _ = True > elements_in_List _ [] = False > elements_in_List (x:xs) (y:ys) > | x == y = elements_in_List xs ys > | True = elements_in_List (x:xs) ys > > but it failed at > >elements_in_List([2,1],[1,2]). > >True > > I hope somebody can help me > Please don't use built-in-Functions. > > Thanks for Help I haven't looked too far into this, but how about something like this: elements_in_list a b = and $ map (\x -> element_in_list x b) a where element_in_list = ... Maybe if you don't want to use built-in functions, define your own 'and' as well. Bryan

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Re: [Haskell-cafe] Separate a string into a list of strings
by Donn Cave 13 Jun '06

13 Jun '06

Quoth Clifford Beshers <clifford.beshers(a)linspire.com>: | Well, I couldn't resist the puzzle. Here are solutions using foldr and | unfoldr. Don't know if they are cunning or not, but they were kind of fun. ... | splitByElem1 e xs = | foldr f [[]] xs | where f a b = if a == e then [] : b else (a : head b) : (tail b) This does the right thing with trailing separators, which is not to be taken for granted among Haskell split implementations. The splits I have been seeing in this thread are conservative, so if the separator is ':', then "::a" splits to ["", "", "a"]. Frequently however the implementation fails to deal with the trailing separator, so "a:" is ["a"], where it should be ["a", ""]. It's not something you run into right away. In a liberal split, "a " should indeed be ["a"], but that's a different matter. Neither of the two I've looked at seems to be shooting for a liberal "words" white space split. Donn

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newbie type signature question
by Brock Peabody 13 Jun '06

13 Jun '06

Please excuse my newbiness, but in this snippet: data (Monad m) => DataType m = DataType { f :: Char -> m () } test_function :: (Monad m) => DataType m -> m () ^^^^^^^^^^^^ test_function d = f d 'C' Why is "(Monad m) =>" required, when the definition of DataType already implies it? Is there an easier way to do this or will I have to have it in all signatures containing DataType? Thanks, Brock

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Re: [Haskell-cafe] Learning C after Haskell
by Chad Scherrer 12 Jun '06

12 Jun '06

Thanks, Minh. So are things like recursion and memory sharing typically out the window? Also, I don't see how thinking about type classes will help, without the benefits of polymorphism. -Chad ---------- Forwarded message ---------- From: minh thu <noteed(a)gmail.com> Date: Jun 12, 2006 12:23 PM Subject: Re: [Haskell-cafe] Learning C after Haskell To: Chad Scherrer <chad.scherrer(a)gmail.com> hi, C is very different from Haskell. * you'lle have to manage explicitly memory de/allocation. * c programming is a bit like haskell io monad programming (but without the functionnal part) : you'lle use "=" in place of "<-" and the left hand side can be reassigned multiple times. * imperative programming (c is imperative) involve states (a lot !). (see how to write the equivalent of mapM print [1..10]); imperative programmers use loops, not much recursion * things you can keep in your mind : the way you organize things (module, type classes (but there is no polymorphism in c), have functions and not a huge amount of code lines...), the way you use self documenting names. well, not sure it helps, but here you are :) mt 2006/6/12, Chad Scherrer <chad.scherrer(a)gmail.com>: > Ok, so I'm doing things somewhat backward. I've been using Haskell for a > while now, whenever I get a chance to. But in order to become more involved > in high-performance computing projects at my work, I need to learn C. > > I've heard a lot of people say that experience in Haskell can improve one's > abilities in other languages, but I also wonder how different the C "way of > doing things" is different from Haskell's. > > My question is, as I learn C, are there any particular Haskell concepts I > should keep in the back of my mind, or is it better to approach C from > scratch? > > Thanks in advance! > > Preparing for a foot-shooting, > Chad > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Newbie: safe program termination in Windows
by Alberto G. Corona 12 Jun '06

12 Jun '06

Hi, I have a program that handle cached data. I don´t find the way to store the data when the program is killed by other program. There is nothing similar to signal in the Windows implementation and no exception appears to trigger when the program is killed by a service handler or by the Task manager.... Anyone knows how to deal with that?.

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HCAR
by Imam Tashdid ul Alam 12 Jun '06

12 Jun '06

does anyone know what happened to HCAR? or HWN? __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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RE: [Haskell-cafe] newbie type signature question
by Brock Peabody 12 Jun '06

12 Jun '06

> From: Brandon Moore > Getting them both is tricky, but you can do it if you use a GADT to > write a type that means "exists a such that a = m and a is a Monad": Is GADT a way to assemble types at compile-time? It looks really cool. > {-# OPTIONS -fglasgow-exts #-} > data TyEq (a :: * -> *) (b :: * -> *) where ^ ^ Compiling this fails here (the first '*') for me with "parse error on input '*'" (ghc 6.4.1), but I'll keep playing with it. Thanks, Brock

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Inferred type not most general?
by Greg Buchholz 10 Jun '06

10 Jun '06

I'm curious about a type inference oddity. In the code below, if I leave off the type signature for tmap, both GHC and Hugs infer that tmap has type... tmap :: (b -> a, b -> a) -> Twist b b -> Twist a a ...I'm wondering why they couldn't infer the more general... tmap :: (a -> b, c -> d) -> Twist a c -> Twist b d >data Twist a b = Nil | Cons a (Twist b a) deriving Show > >x = (Cons "foo" (Cons 1 (Cons "bar" (Cons 2 Nil)))) > >tmap :: (a->b,c->d) -> Twist a c -> Twist b d >tmap _ Nil = Nil >tmap (f,g) (Cons x rest) = Cons (f x) (tmap (g,f) rest) Thanks, Greg Buchholz

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Re: Inferring types from functional dependencies
by oleg＠pobox.com 10 Jun '06

10 Jun '06

Jeff Harper defined typeclasses > class Reciprocate a b | a -> b where > recip :: a -> b > class Multiply a b c | a b -> c where > (*) :: a -> b -> c and encountered the problem with > -- Here I define a default (/) operator that uses the > -- Reciprocate and Multiply class to perform division. > class (Reciprocate b recip, Multiply a recip c) => Divide a b c | a b -> c > where > (/) :: a -> b -> c > (/) x y = x * (recip y) which doesn't work, because the type variable 'recip' doesn't appear in the class head. He wrote > I also, wonder if it would be appropriate to include in future > versions of Haskell, the ability to infer functional dependences in a > new class definition, so that my first attempt at a definition of > class Divide works. I'm afraid there may be the problem with the example rather than with Haskell. Let us suppose that the above definition of Divide works. Jeff Harper writes elsewhere > However, taking the reciprocal and then multiplying may not always be > the best way of dividing. So, I'd like to put this into a divide > class, so (/) can be defined differently for different types. The key phrase is that we wish (/) defined differently for different types, because using reciprocals is not always the best way of dividing. Then why do we insist that every instance of Divide must be also an instance of Reciprocate (because that's what the definition of Divide says)? Why is that that whenever we divide something by the value of type 'b', it must be possible to reciprocate that value of type b? Even if we don't use reciprocals in the division? If we assume that we need Reciprocate only if we are going to use the 'default' method, the solution becomes obvious. It does involve overlapping and undecidable instances, sorry. These extensions are really useful in practice. Here's the solution: > class Divide a b c | a b -> c where > (/) :: a -> b -> c Here's the most general instance. It applies when nothing more specific does. It is in this case that we insist on being able to take the reciprocal: > instance (Reciprocate b recip, Multiply a recip c) => > Divide a b c where > (/) x y = x * (recip y) Now, a specific instance only for Integers, which involves no reciprocals: > instance Divide Integer Integer (Ratio Integer) where > (/) = (%) Another specific instance. It too involves no reciprocals. Incidentally, the posted code does not even define an instance of Reciprocate for Ints! > instance Divide Int Int (Ratio Integer) where > x / y = (fromIntegral x) % (fromIntegral y) A few tests: > test0 = (1::Int) / (2::Int) > test1 = (1::Integer) / (2::Integer) > test2 = (1::Double) /(2::Double) P.S. A remark for Haskell': the most general instance above (repeated below) > instance (Reciprocate b recip, Multiply a recip c) => > Divide a b c where > (/) x y = x * (recip y) shows why we can never be satisfied with only decidable instances -- why we need the undecidable instances extension. The instance makes it clear that we cannot decide if the instance selection process for Divide terminates just by looking at this instance. There is no information to decide on termination: we must examine instances of Reciprocate and Multiply (in particular, we should check if those instances refer back to Divide). Thus, no _local_ instance termination criterion would ever be able to handle above instance. OTH, it seems quite unreasonable to require a Haskell system to employ a global instance selection termination criterion (short of recursion stack limit or other such `dynamic' test).

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Features of Haskell
by Niels Van Och 10 Jun '06

10 Jun '06

Hi, I'm a Master student in Computer Science, and for my bachelor thesis, I've written 'an introduction to Haskell'. In this document, I've discussed syntax, evaluation, polymorphism, monads etc. Now, in a week I have to present this work formally, and I'm a bit stumped as to how I'm going to do this. I've got about 15-20 minutes, so I can only discuss the major features. Right now I'm thinking about: - Short introduction to functional programming, and Haskell in it - Basic syntax in a few minutes - Features like polymorphism, overloading, higher-order functions, comprehensions, .. - Lazy evaluation - Monads: why and how? However, I'd love to know what you think. Furthermore, do you think I should include an example on the usage of Haskell, and if so, which? Many thanks! Niels

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Inferring types from functional dependencies
by Jeff.Harper＠handheld.com 09 Jun '06

09 Jun '06

The following message is in a Haskell module. It will be easier to read in a fixed point font. {-# OPTIONS -fglasgow-exts #-} -- Hi, -- -- I ran into an issue while working with functional dependencies. -- Consider the following code. I'm rewriting many of the prelude -- operators using functional dependencies. The type of he return value -- is determined by the operators parameters. -- Use "P." in front of functions to access the preludes version of these -- functions. import qualified Prelude as P -- I override prelude operators with my own operators. import Prelude hiding ( (*), recip ) import Ratio -- recip returns the reciprocal of its parameter. I've given the -- Reciprocate class the ability to return a type that is different from -- its argument. class Reciprocate a b | a -> b where recip :: a -> b -- Here are some example instances of Reciprocate. In most cases, -- recip will return the same type as it's argument. However, taking -- the reciprocal of an Integer returns a (Ratio Integer). instance Reciprocate Double Double where recip = P.recip -- I call prelude's recip here. instance Reciprocate (Ratio Integer) (Ratio Integer) where recip = P.recip -- I call prelude's recip here. instance Reciprocate Integer (Ratio Integer) where recip x = (1::Integer) % x -- (*) multiplies its parameters. The resulting type is determined by -- the type of the arguments class Multiply a b c | a b -> c where (*) :: a -> b -> c -- Here are some example instances of Multiply. instance Multiply Double Double Double where (*) = (P.*) -- Multiplying Integer by Double returns a Double instance Multiply Integer Double Double where (*) x y = (P.*) (fromIntegral x) y instance Multiply Double Integer Double where (*) x y = (P.*) x (fromIntegral y) instance Multiply Integer (Ratio Integer) (Ratio Integer) where (*) x y = (P.*) (x%1) y -- Now, this is where I ran into some trouble I define a Divide class -- as follows. Here I define a default (/) operator that uses the -- Reciprocate and Multiply class to perform division. However, this code -- produces error messages. So, I commented it out. Even if I don't want -- to implement (/) with recip and (*), requiring this relationship -- -- | -- ---------------------------------------------- -- | -- v is consistent with defining -- _____________________________________ the divide operation in -- | | terms of the multiplicative -- | | inverse {- class (Reciprocate b recip, Multiply a recip c) => Divide a b c | a b -> c where (/) :: a -> b -> c (/) x y = x * (recip y) -} -- This definition of (/) works. However, taking the reciprocal and then -- multiplying may not always be the best way of dividing. So, I'd like to -- put this into a divide class, so (/) can be defined differently for -- different types. {- (/) :: (Reciprocate b recip, Multiply a recip c) => a -> b -> c (/) a b = a * (recip b) -} -- I finally discovered that the following definition of a Divide -- class works class (Reciprocate b recip_of_b, Multiply a recip_of_b c) => Divide a b c recip_of_b | a b -> c recip_of_b where (/) :: a -> b -> c (/) a b = a * (recip b) -- Default definition can be overridden -- The thing I don't like is that when defining a new Divide class, I have -- to place the reciprocal of the "b" type into the class definition. -- Here are some examples of Divide: -- -- This type ---------------------------- -- must be the type that is | -- returned when this | -- type ------------------ | -- is passed to recip. | | -- | | -- v v instance Divide Double Double Double Double where (/) x y = (P./) x y -- For Doubles -- Another example: -- -- This type ------------------------------------------ -- must be the type that is | -- returned when this | -- type ------------------- | -- is passed to recip. | | -- | | -- v v instance Divide Integer Integer (Ratio Integer) (Ratio Integer) where (/) x y = x % y -- The reason I don't like it is there is enough information available to infer -- the type of recip_of_b in the following class. The Reciprocate class is -- defined with functional dependencies, so that recip_of_b can be determined -- by the type of b.-- -- | -- ______________________ -- | | -- -- class (Reciprocate b recip_of_b, Multiply a recip_of_b c) -- => Divide a b c recip_of_b | a b -> c recip_of_b where -- (/) :: a -> b -> c -- (/) a b = a * (recip b) -- -- Respecifying the recip_of_b when I declare an instance of Divide -- seem redundant. I'm wondering if there is a better way to -- define this. I also, wonder if it would be appropriate to include -- in future versions of Haskell, the ability to infer functional -- dependences in a new class definition, so that my first attempt -- at a definition of class Divide works.

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Re: Newbie request
by Chad Scherrer 09 Jun '06

09 Jun '06

Philip, you wrote: ------------------------------------------------------------------------------------- I am trying to learn Haskell. As an exercise, I wrote a function to create a binary tree in level-order. I am attaching the code. I am sure there are a number of places where the code could be improved. Could you please point these out? Thanks, Philip >countNodes :: Tree a -> Integer >countNodes Empty = 0 >countNodes (Tree rootNode left right) = 1 + countNodes left + countNodes right -------------------------------------------------------------------------------------- Stylistically, this is usually written using a fold. Try writing a function fold :: (a -> b -> b -> b) -> b -> Tree a -> b Then rewrite countNodes as countNodes = fold f 0 where f x left right = 1 + left + right A huge number of other handy functions can then be written in terms of fold. -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Newbie request
by Geevarghese Philip 09 Jun '06

09 Jun '06

I am trying to learn Haskell. As an exercise, I wrote a function to create a binary tree in level-order. I am attaching the code. I am sure there are a number of places where the code could be improved. Could you please point these out? Thanks, Philip ------------------------------------------------------------------------------ BinTree.lhs : Implementation of a binary tree. createTree accepts a sequence and builds a binary tree in level-order. ------------------------------------------------------------------------------ >module BinTree where ------------------------------------------------------------------------------ A binary tree either 1. is empty, or 2. consists of three distinct binary trees : a root node, a left subtree, and a right subtree. ------------------------------------------------------------------------------ >data Tree a = Empty | Tree {rootNode::a, left::(Tree a), right::(Tree a)} deriving (Eq, Show) ------------------------------------------------------------------------------ Count the number of nodes in a binary tree, using the simple recursive definition of the count. ------------------------------------------------------------------------------ >countNodes :: Tree a -> Integer >countNodes Empty = 0 >countNodes (Tree rootNode left right) = 1 + countNodes left + countNodes right ------------------------------------------------------------------------------ Insert a single element into the proper place in the tree, as per level-order. ------------------------------------------------------------------------------ >insert :: Eq a => Tree a -> a -> Tree a >insert tree x = if tree == Empty > then Tree x Empty Empty > else if (left tree) == Empty > then Tree (rootNode tree) (Tree x Empty Empty) (right tree) > else if (right tree) == Empty > then Tree (rootNode tree) (left tree) (Tree x Empty Empty) > else if countNodes (left tree) <= countNodes (right tree) > then Tree (rootNode tree) (insert (left tree) x) (right tree) > else Tree (rootNode tree) (left tree) (insert (right tree) x) ------------------------------------------------------------------------------ Use insert to create a tree from a sequence. ------------------------------------------------------------------------------ >createTree :: Eq a => [a] -> Tree a >createTree [] = Empty >createTree (x:xs) = foldl insert (insert Empty x) xs

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Literate Haskell in Vim - a bug and a workaround (?)
by Piotr Kalinowski 09 Jun '06

09 Jun '06

Hi, I've been recently playing with literate haskell in latex mode in Vim. The versions are: Vim - 6.4.6 tex.vim - 30 lhaskell.vim - 1.01 I encountered a bug. If you put \section (or \subsection or \chapter or other similar command), haskell syntax highlighting is lost from this command onwards. I tried some googling and searching this list's archives but found nothing. After going through tex.vim and reading vim docs I found out, that adding 'containedin tex.*Zone' to the following line: syntax region lhsHaskellBeginEndBlock start="^\\begin{code}\s*$" matchgroup=NONE end="\%(^\\end{code}.*$\)\@=" contains=@haskellTop,@beginCode appears to solve the problem. Apparently the bug is caused by texSectionZone (and other zones) syntax region which extends from \section command until another \section command. Since lhsHaskellBeginEndBlock region was not allowed inside texSectionZone, it was not recognised. Adding containedin clause addresses this problem letting vim to recognise \begin{code} as beginning of the Haskell file part. Such change however breaks tex lexical checks. More specifically - texMatcher from tex.vim is unable to match braces around section titles (chapter, subsection, etc.). I tried tampering with lhaskell.vim for some time but found no way to fix it. (Which is not surprising, given the fact that I am far from being vim expert. In fact I know almost nothing about vim programming.) Therefore the patch I attach simply turns lexical checks off setting g:tex_no_error variable. The variable is then unset immediately after loading tex.vim in order not to affect other buffers where it can work correctly. I attach the patch with those two changes. It is diff -u output. Any comments are welcome. I would especially be happy to learn how to fix texMatcher issue without turning it off. Regards, Piotr Kalinowski -- Intelligence is like a river: the deeper it is, the less noise it makes

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