- Haskell-Cafe - Haskell.org

Which type variables are allowed in a context?
by Vladimir Reshetnikov 31 May '09

31 May '09

Hi, Consider this (a bit pathological) Haskell code: -------------------------------------- class A a where foo :: A (b d) => a (c b) -------------------------------------- GHC compiles it successfully, but Hugs rejects it: Ambiguous type signature in class declaration *** ambiguous type : (A a, A (b c)) => a (d b) *** assigned to : foo What is the correct behavior, and which part of the haskell 98 report explains this? Thanks, Vladimir

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Question on kind inference
by Vladimir Reshetnikov 31 May '09

31 May '09

Hi, Consider this Haskell code: ----------------------------------------------- class A a where foo :: a b class B a class (A a, B a) => C a ----------------------------------------------- GHC compiles it without errors, but Hugs rejects it: "Illegal type in class constraint". What is the correct behavior, and which part of the haskell 98 report explains this? Thanks, Vladimir

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Re: [Haskell-cafe] Parsing command lines
by Kalman Noel 31 May '09

31 May '09

Patai Gergely schrieb: > is there a function that can safely split a command line into a FilePath > to the executable and its parameters? In the yi source code, in HConf.Utils, there's a function that does part of what you want, but maybe incorrectly (because I wrote it, and it traverses the string in a rather primtive way). -- | Break up a string the way a shell breaks up a command into arguments. -- Similar to 'words', but respects quotes and escaped spaces. TODO: Verify -- this function. shellWords :: String -> [String]

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Haskell and symbolic references
by Patrick LeBoutillier 31 May '09

31 May '09

Hi all, Is it possible with Haskell to call a function whose name is contained in a String? Something like: five = call_func "add" [2, 3] If not, perhaps this is acheivable using FFI? Thanks a lot, Patrick Leboutillier -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Re: [Haskell-cafe] Missing a "Deriving"?
by nowgate＠yahoo.com 31 May '09

31 May '09

Hi Ryan, Is there something missing or mislabeled in your post, because I don't see any definition of toDynamic. Michael --- On Sun, 5/31/09, Ryan Ingram <ryani.spam(a)gmail.com> wrote: From: Ryan Ingram <ryani.spam(a)gmail.com> Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: "David Menendez" <dave(a)zednenem.com> Cc: "michael rice" <nowgate(a)yahoo.com>, haskell-cafe(a)haskell.org, "Miguel Mitrofanov" <miguelimo38(a)yandex.ru> Date: Sunday, May 31, 2009, 12:45 AM On Sat, May 30, 2009 at 6:33 PM, David Menendez <dave(a)zednenem.com> wrote: > *Main> :t searchAll > searchAll :: (Computation c) => Graph t t1 -> Int -> Int -> c [Int] > > The way searchAll is written, the choice of which functions to use > depends on the type variable c. That's determined by the calling > context of searchAll, which is why you need to provide a type > signature when using it at the GHCi command line. This is actually one of the most interesting and important things to "get" about typeclasses; it's not *just* like an interface, because the instance type can appear in the result of a function and *not* in the arguments at all. In contrast, in Java/C++, the method to use is always chosen by the object being called; one could argue that all of COM is an attempt to get around this problem. Some examples: > fromInteger :: Num a => Integer -> a > fromDynamic :: Typeable a => Dynamic -> Maybe a In both of these cases, the choice of which instance to use is made by the caller, and often automatically by type inference: > test x = case fromDynamic x of > Just s -> s == "hello" > Nothing -> False Now (test $ toDynamic "hello") = True, but (test $ toDynamic 'a') = False. Notice that I never directly specified what type "fromDynamic" should return; but the case statement forces it to return Maybe String, since I compare (s == "hello") -- ryan

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Re: [Haskell-cafe] Missing a "Deriving"?
by michael rice 31 May '09

31 May '09

Belay that last question. I just realized that its the const function being used rather than a constant declaration in const Nothing and const []. MIchael --- On Sat, 5/30/09, David Menendez <dave(a)zednenem.com> wrote: From: David Menendez <dave(a)zednenem.com> Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: "michael rice" <nowgate(a)yahoo.com> Cc: "Miguel Mitrofanov" <miguelimo38(a)yandex.ru>, haskell-cafe(a)haskell.org Date: Saturday, May 30, 2009, 9:33 PM On Sat, May 30, 2009 at 9:00 PM, michael rice <nowgate(a)yahoo.com> wrote: > That works. but it gives just a single solution [1,2,3] when there are > supposed to be two [[1,2,3],[1,4,3]]. Of course the code in YAHT may be in > error. Works for me. *Main> searchAll g 1 3 :: [[Int]] [[1,2,3],[1,4,3]] *Main> searchAll g 1 3 :: Maybe [Int] Just [1,2,3] *Main> searchAll g 1 3 :: Failable [Int] Success [1,2,3] > Also, how the heck does Haskell decide which "success", "failure", > "augment", and "combine" to use in function "searchAll", since there are > five possibilities. *Main> :t searchAll searchAll :: (Computation c) => Graph t t1 -> Int -> Int -> c [Int] The way searchAll is written, the choice of which functions to use depends on the type variable c. That's determined by the calling context of searchAll, which is why you need to provide a type signature when using it at the GHCi command line. -- Dave Menendez <dave(a)zednenem.com> <http://www.eyrie.org/~zednenem/>

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Re: [Haskell-cafe] Missing a "Deriving"?
by michael rice 31 May '09

31 May '09

I figured out the [[Int]] case for myself, but hadn't considered the Failure case. Thanks. In function "searchAll", given a calling context Failable [Int], for the line where search' [] = failure "no path" "failure" would be "Fail", a constructor that takes a String. Right? But using either of the other two contexts, where failure equals either const Nothing or const [] it would seem like that same string argument "no path" would be passed to either Nothing or [], which doesn't make any sense. Explanation? Michael --- On Sat, 5/30/09, David Menendez <dave(a)zednenem.com> wrote: From: David Menendez <dave(a)zednenem.com> Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: "michael rice" <nowgate(a)yahoo.com> Cc: "Miguel Mitrofanov" <miguelimo38(a)yandex.ru>, haskell-cafe(a)haskell.org Date: Saturday, May 30, 2009, 9:33 PM On Sat, May 30, 2009 at 9:00 PM, michael rice <nowgate(a)yahoo.com> wrote: > That works. but it gives just a single solution [1,2,3] when there are > supposed to be two [[1,2,3],[1,4,3]]. Of course the code in YAHT may be in > error. Works for me. *Main> searchAll g 1 3 :: [[Int]] [[1,2,3],[1,4,3]] *Main> searchAll g 1 3 :: Maybe [Int] Just [1,2,3] *Main> searchAll g 1 3 :: Failable [Int] Success [1,2,3] > Also, how the heck does Haskell decide which "success", "failure", > "augment", and "combine" to use in function "searchAll", since there are > five possibilities. *Main> :t searchAll searchAll :: (Computation c) => Graph t t1 -> Int -> Int -> c [Int] The way searchAll is written, the choice of which functions to use depends on the type variable c. That's determined by the calling context of searchAll, which is why you need to provide a type signature when using it at the GHCi command line. -- Dave Menendez <dave(a)zednenem.com> <http://www.eyrie.org/~zednenem/>

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Re: [Haskell-cafe] Missing a "Deriving"?
by michael rice 31 May '09

31 May '09

Hi Miguel, That works. but it gives just a single solution [1,2,3] when there are supposed to be two [[1,2,3],[1,4,3]]. Of course the code in YAHT may be in error. Also, how the heck does Haskell decide which "success", "failure", "augment", and "combine" to use in function "searchAll", since there are five possibilities. MIchael --- On Sat, 5/30/09, Miguel Mitrofanov <miguelimo38(a)yandex.ru> wrote: From: Miguel Mitrofanov <miguelimo38(a)yandex.ru> Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Saturday, May 30, 2009, 5:36 PM It's trying to 'Show' the 'c [Int]' type, but doesn't know which 'c' to use; so it's trying to find a generic instance, which doesn't exist. You can't fix this with 'deriving' or anything like this; instead, provide the type annotation like this: *Main> searchAll g 1 3 :: Maybe [Int] On 31 May 2009, at 00:50, michael rice wrote: > The following code is from Section 8.4.2, pgs. 111-112 (PDF paging) of YAHT. > > It compiles fine, but upon trying it I get the following error message. > > It seems to be trying to 'Show' the Computation class but I'm not sure where to put the 'Deriving'. > > Michael > > > ============ > > Loading package ghc-prim ... linking ... done. > Loading package integer ... linking ... done. > Loading package base ... linking ... done. > [1 of 1] Compiling Main ( graph4.hs, interpreted ) > Ok, modules loaded: Main. > *Main> let g = Graph [(1,'a'),(2,'b'),(3,'c'),(4,'d')] [(1,2,'p'),(2,3,'q'),(1,4,'r'),(4,3,'s')] > *Main> searchAll g 1 3 > > <interactive>:1:0: > No instance for (Show (c [Int])) > arising from a use of `print' at <interactive>:1:0-14 > Possible fix: add an instance declaration for (Show (c [Int])) > In a stmt of a 'do' expression: print it > > ============================ > > data Failable a = Success a | Fail String deriving (Show) > > data Graph v e = Graph [(Int,v)] [(Int,Int,e)] > > class Computation c where > success :: a -> c a > failure :: String -> c a > augment :: c a -> (a -> c b) -> c b > combine :: c a -> c a -> c a > > instance Computation Maybe where > success = Just > failure = const Nothing > augment (Just x) f = f x > augment Nothing _ = Nothing > combine Nothing y = y > combine x _ = x > > instance Computation Failable where > success = Success > failure = Fail > augment (Success x) f = f x > augment (Fail s) _ = Fail s > combine (Fail _) y = y > combine x _ = x > > instance Computation [] where > success a = [a] > failure = const [] > augment l f = concat (map f l) > combine = (++) > > searchAll g@(Graph vl el) src dst > | src == dst = success [src] > | otherwise = search' el > where search' [] = failure "no path" > search' ((u,v,_):es) > | src == u = (searchAll g v dst `augment` > (success . (u:))) > `combine` search' es > | otherwise = search' es > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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HPC and derived instances
by Felipe Lessa 31 May '09

31 May '09

Hello! Why isn't there an option to control whether HPC, the Haskell Program Coverage, will consider derived instances "coverable". I'm using it and my top level coverage is 52% while my expression coverage is at 92%. Looking carefully we see that most non-tested top level definitions are derived Show, Eq and Ord instances that I always derive for most data types (as this is a library). I think it is nice that it recognizes class instances, but there should be a flag to ignore at least the derived ones. Am I missing something? Thanks! :) -- Felipe.

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[] == []
by Paul Keir 31 May '09

31 May '09

Hi all, GHC is not happy with this: f = [] == [] nor this: f' = ([]::(Eq a) => [a]) == ([]::(Eq a) => [a]) but this is OK: f'' = ([]::[Integer]) == ([]::[Integer]) GHCI is comfortable with [] == [], so why not GHC? 'Just curious. Cheers, Paul

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get Scale of window in HOpenGL?
by yu yang 31 May '09

31 May '09

Hi, I want to move one object to the border of window, then go back to the start point. Does anyone one have an idea to implement it ? Thank you! ___________________________________________________________ 好玩贺卡等你发，邮箱贺卡全新上线！ http://card.mail.cn.yahoo.com/

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Concurrent Haskell Actions with Timeout
by Cetin Sert 31 May '09

31 May '09

Hi how could one implement a function in concurrent haskell that either returns 'a' successfully or due timeout 'b'? timed :: Int → IO a → b → IO (Either a b) timed max act def = do Best Regards, Cetin Sert

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Missing a "Deriving"?
by michael rice 31 May '09

31 May '09

The following code is from Section 8.4.2, pgs. 111-112 (PDF paging) of YAHT. It compiles fine, but upon trying it I get the following error message. It seems to be trying to 'Show' the Computation class but I'm not sure where to put the 'Deriving'. Michael ============ Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( graph4.hs, interpreted ) Ok, modules loaded: Main. *Main> let g = Graph [(1,'a'),(2,'b'),(3,'c'),(4,'d')] [(1,2,'p'),(2,3,'q'),(1,4,'r'),(4,3,'s')] *Main> searchAll g 1 3 <interactive>:1:0: No instance for (Show (c [Int])) arising from a use of `print' at <interactive>:1:0-14 Possible fix: add an instance declaration for (Show (c [Int])) In a stmt of a 'do' expression: print it ============================ data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] class Computation c where success :: a -> c a failure :: String -> c a augment :: c a -> (a -> c b) -> c b combine :: c a -> c a -> c a instance Computation Maybe where success = Just failure = const Nothing augment (Just x) f = f x augment Nothing _ = Nothing combine Nothing y = y combine x _ = x instance Computation Failable where success = Success failure = Fail augment (Success x) f = f x augment (Fail s) _ = Fail s combine (Fail _) y = y combine x _ = x instance Computation [] where success a = [a] failure = const [] augment l f = concat (map f l) combine = (++) searchAll g@(Graph vl el) src dst | src == dst = success [src] | otherwise = search' el where search' [] = failure "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst `augment` (success . (u:))) `combine` search' es | otherwise = search' es

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can someone point me to more help about Database.HDBC.ODBC?
by Michael P Mossey 30 May '09

30 May '09

I'm trying to use Database.HDBC.ODBC to connect to a MySQL server, and I cannot figure out the docs. I want to establish a connection, and I know the server url, username, and password. According to these docs... http://software.complete.org/static/hdbc-odbc/doc//HDBC-odbc/Database-HDBC-… ...I am supposed to pass a string to connectODBC. These docs do not explain the meaning of the passed string, but refer me to a microsoft document: http://msdn2.microsoft.com/en-us/library/ms715433(VS.85).aspx I cannot make any sense of this microsoft document. I guess I don't know enough about database interfaces in general. Can someone explain how to make a connect to a specific URL, or point me to some clearer docs? Thanks, Mike

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Re: [Haskell-cafe] Parsec float
by Tillmann Vogt 30 May '09

30 May '09

Bartosz Wójcik wrote: > Hi Everybody (especially Parsec Creator), > > is there any reason why float parses only positive numbers? > > I find following defition: > > float = lexeme floating <?> "float" > > floating = do{ n <- decimal > ; fractExponent n > } > > If floating was defined like > > floating = do{ n <- integer ... > > or > > floating = do{ n <- int ... > > instead then it would parse negative ones as well. > > Hi Bartek, I had the same problem. Daan Leijen gave me a similar answer than Malcom Wallace just gave you: "Usually the minus sign is treated as an operator in the language and treated as a separate token" He also gave me a workaround which finally resulted in this: myfloat = try (do{ symbol "-"; n <- float; return (negate n) }) <|> try float <|> do { i<-integer; return(fromIntegral i) } -- 0 is not recognized as a float, so recognize it as an integer and then convert it to float

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Hugs vs. GHCi
by Vladimir Reshetnikov 30 May '09

30 May '09

Hi, The following expression evaluates to 1 in GHCi, but results in an error in Hugs: let f x = let g y = [x,y] in (g 1, g []) in 1 What is the correct behavior? Thanks Vladimir

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Ensuring Type Class instances follow the 'rules'
by Hemanth Kapila 30 May '09

30 May '09

Hi all, Recently, I participated in a coding competition. As part of it, I had to write a program wherein I had to make my data-type an instance of Ord. An error in my implementation of compare resulted in me losing quite a bit of valuable time. As I wrote it, I had tested it out on the ghci and it seemed to work fine but I noticed that there was trouble when the List.sort started giving weird output. I then noticed that for certain instances (say t1,t2), both compare t1 t2 AND compare t2 t1 returned LT. Once I spotted it, it was easy enough to fix but I did lose an hour or so thinking something went wrong before I fed the list to the sort function. I was the only haskeller in the competition and I believe am among the first to finish a correct application (by a large margin, and) and I did manage to raise a bit of haskell-awareness, but if I had managed to spot the error earlier, the results would have been dazzling. Does any one here have any advice on dealing with such maladroitness on the part of the programmer, especially while creating instances to type-classes? Thanks and regards, Hemanth

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Floating instance and pi
by Paul Keir 30 May '09

30 May '09

Hi, I'd like to make my ADT an instance of the Floating class, but I'm not sure what to put for pi, and GHC gives a warning without it: Warning: No explicit method nor default method for `GHC.Float.pi' I tried setting it to undefined, but that gives an error: `pi' is not a (visible) method of class `Floating' Any idea? Paul

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IFL 2009: Second Call for Papers
by IFL 2009 30 May '09

30 May '09

Call for Papers IFL 2009 Seton Hall University SOUTH ORANGE, NJ, USA http://tltc.shu.edu/blogs/projects/IFL2009/ ** NEW ** Accomodations information available: http://tltc.shu.edu/blogs/projects/IFL2009/accommodations.html Jane Street Capital has joined IFL 2009 as a sponsor ********* The 21st International Symposium on Implementation and Application of Functional Languages, IFL 2009, will be held for the first time in the USA. The hosting institution is Seton Hall University in South Orange, NJ, USA and the symposium dates are September 23-25, 2009. It is our goal to make IFL a regular event held in the USA and in Europe. The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2009 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Following the IFL tradition, IFL 2009 will use a post-symposium review process to produce a formal proceedings which will be published by Springer in the Lecture Notes in Computer Science series. All participants in IFL 2009 are invited to submit either a draft paper or an extended abstract describing work to be presented at the symposium. These submissions will be screened by the program committee chair to make sure they are within the scope of IFL and will appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. After the symposium, authors will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full arcticle for the formal review process. These revised submissions will be reviewed by the program committee using prevailing academic standards to select the best articles that will appear in the formal proceedings. TOPICS IFL welcomes submissions describing practical and theoretical work as well as submissions describing applications and tools. If you are not sure if your work is appropriate for IFL 2009, please contact the PC chair at ifl2009(a)shu.edu. Topics of interest include, but are not limited to: language concepts type checking contracts compilation techniques staged compilation runtime function specialization runtime code generation partial evaluation (abstract) interpretation generic programming techniques automatic program generation array processing concurrent/parallel programming concurrent/parallel program execution functional programming and embedded systems functional programming and web applications functional programming and security novel memory management techniques runtime profiling and performance measurements debugging and tracing virtual/abstract machine architectures validation and verification of functional programs tools and programming techniques FP in Education PAPER SUBMISSIONS Prospective authors are encouraged to submit papers or extended abstracts to be published in the draft proceedings and to present them at the symposium. All contributions must be written in English, conform to the Springer-Verlag LNCS series format and not exceed 16 pages. The draft proceedings will appear as a technical report of the Department of Mathematics and Computer Science of Seton Hall University. IMPORTANT DATES Registration deadline August 15, 2009 Presentation submission deadline August 15, 2009 IFL 2009 Symposium September 23-25, 2009 Submission for review process deadline November 1, 2009 Notification Accept/Reject December 22, 2009 Camera ready version February 1, 2010 PROGRAM COMMITTEE Peter Achten University of Nijmegen, The Netherlands Jost Berthold Philipps-Universität Marburg, Germany Andrew Butterfield University of Dublin, Ireland Robby Findler Northwestern University, USA Kathleen Fisher AT&T Research, USA Cormac Flanagan University of California at Santa Cruz, USA Matthew Flatt University of Utah, USA Matthew Fluet Toyota Technological Institute at Chicago, USA Daniel Friedman Indiana University, USA Andy Gill University of Kansas, USA Clemens Grelck University of Amsterdam/Hertfordshire, The Netherlands/UK Jurriaan Hage Utrecht University, The Netherlands Ralf Hinze Oxford University, UK Paul Hudak Yale University, USA John Hughes Chalmers University of Technology, Sweden Patricia Johann University of Strathclyde, UK Yukiyoshi Kameyama University of Tsukuba, Japan Marco T. Morazán (Chair) Seton Hall University, USA Rex Page University of Oklahoma, USA Fernando Rubio Universidad Complutense de Madrid, Spain Sven-Bodo Scholz University of Hertfordshire, UK Manuel Serrano INRIA Sophia-Antipolis, France Chung-chieh Shan Rutgers University, USA David Walker Princeton University, USA Viktória Zsók Eötvös Loránd University, Hungary PETER LANDIN PRIZE The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honored article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 euros.

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How to interprete profile output
by Sebastian Reese 29 May '09

29 May '09

Hi there, i am currently working on the mapreduce framework (http://darcs2.fh-wedel.de/repos/holumbus/) which can also be found in hackage. I was doing the simple WordFrequency example, which does nothing but counting word frequencies and runs a few seconds. When I run this program with ghc 6.10 and profiling option "+RTS -p -RTS" i get this (http://pastebin.com/f5163325) output. The interesting line is the one with "MAIN MAIN" in it. 1. Where does this MAIN come from? My main function is right under it. So is it some GHC profiling wrapper? 2. Why does it use over 90% of cpu time and 25% of allocated memory? So i know you can't tell me exactly why, but maybe someone can give me a hint. The mapreduce framework does a lot with network, IO, forkIO, MVars etc.. Is that maybe a reason for it? My guess is that there is some action, that can't be assinged to a specific cost centre and/or results from an empty MVar. thanks for your help Sebastian ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program.

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ghc - force library search order
by John Lask 29 May '09

29 May '09

I need to force a library to be searched for unresolved symbols after all other libraries have been searched, and I would rather not resort to constructing the linker command line directly. Is there a way to do this? i.e. I want for example -lfoo to appear after all other haskell libraries and before system libraries (for example -lmsvcrt) once ghc has constructed the link command. i.e. how is it possible to coerce ghc into respecting dependencies between the libraries. GHC does a good job in general with native libraries, but there are allways corner cases. I have a feeling this is not possible, but it dosn't hurt to ask. jvl

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Data.Binary and little endian encoding
by David Leimbach 29 May '09

29 May '09

I actually need little endian encoding... wondering if anyone else hit this with Data.Binary. (because I'm working with Bell Lab's 9P protocol which does encode things on the network in little-endian order). Anyone got some "tricks" for this? Dave

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attaching a ghci session to another process
by Kevin Smith 29 May '09

29 May '09

I am starting to get more involved with haskell programming and I'd like to create a program where I can use the interactive loop in ghci to run a haskell functions that create graphics in a separate openGL window. This would be a separate interactive window from the terminal i am running ghci in and have it's own event-loop/thread of execution. For a very simple example, I would have a haskel function called "w = window" , which when executed would create an openGL window (inside a seprate window manager shell) and return an identifier to it. A call to another fuinction might be "obj = cube [..] w" whch would draw an cube in the window. etc. The openGL would have it's own event loop running to handle mouse/keyboard and refresh events which means that I would be able to manipuolate the camera with the mouse (pan, zoom, rotate etc.). This would be separate fromt he ghci input loop running in the original terminal window. The graphics part of this is easy for me, I have this implemented in other langauges, but what i would like to do is create a functional programming environment for prototyping in 3D, so I would like to have a haskell session which I can "attach" to this 3D environment. Any comments/feedback on the architecture/design of such a program would be appreciated. thanks

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Re: [Haskell-cafe] What's the problem with iota's type signature?
by michael rice 29 May '09

29 May '09

And I thought everyone was puzzling these type signatures out all by themselves. ;-) The Haskell learning curve is about the steepest I've ever seen for a computer language, but I think I'm catching on. Thanks for the info. Michael --- On Thu, 5/28/09, wren ng thornton <wren(a)freegeek.org> wrote: From: wren ng thornton <wren(a)freegeek.org> Subject: Re: [Haskell-cafe] What's the problem with iota's type signature? To: "Haskell-cafe" <haskell-cafe(a)haskell.org> Date: Thursday, May 28, 2009, 11:14 PM michael rice wrote: > Yeah, I went back and tried double again, though I'd swear I got the dang thing to compile (and run) w/o errors. > > I guess I meant Num. So Num is a class and Int and Integer are types? What are the other classes? Docs? > > Unification, for me, is pattern matching ala Prolog. What's the meaning in Haskell? Docs? > > I've been digging into this stuff for months and it's still tripping me up. Very frustrating. As others have mentioned, using GHCi can give you a lot of information. GHCi commands begin with a colon followed by the command (kinda like vi), whereas anything not beginning with a colon is treated as a Haskell expression to be evaluated. The :? command will list all the GHCi commands, which is a good place to start. The :type command takes a Haskell expression and prints the type inferred for it. You can also use the command ":set +t" which will make GHCi print the type after evaluating each expression; you may want to use this until you get the hang of things. The :info command takes a whitespace separated list of identifiers and will print their type and other information (e.g. what module it comes from, what type class a function belongs to, all the functions that are members of a type class, fixity of operators,...) As far as type classes and the like, there are a bunch. Hoogle[1] is an excellent API search engine for looking up this kind of stuff. Just enter the name of a function, type class, data type, or enter a type signature. The results give links to the online Haddock documentation (either GHC official docs, or Hackage docs). Unification is used when doing type inference. It's the same unification as in Prolog, only at the type level. Because of the way inference works, it happens that the |a| and |b| type variables must be unified for both of the functions you gave. A whole lot more can be said here, though I'm not sure how much you care :) The Wikipedia page on Hindley--Milner[2] gives a good starting point, though Haskell's type inference has more bells and whistles than the basic HM algorithm. [1] http://haskell.org/hoogle/ [2] http://en.wikipedia.org/wiki/Hindley-Milner -- Live well, ~wren _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Problem w/YAHT code example
by michael rice 29 May '09

29 May '09

Thanks! I just copied the code from the PDF file. Whatever that character was it wasn't the correct one. Michael --- On Thu, 5/28/09, Andrew Wagner <wagner.andrew(a)gmail.com> wrote: From: Andrew Wagner <wagner.andrew(a)gmail.com> Subject: Re: [Haskell-cafe] Problem w/YAHT code example To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Thursday, May 28, 2009, 10:01 PM I'm not sure what you're using at the end of the identifier "search'", but it needs to be the single quote that's on the same key as the double quotes. I suspect that's where it's blowing up. On Thu, May 28, 2009 at 9:56 PM, michael rice <nowgate(a)yahoo.com> wrote: This code, from YAHT (Section 8.4.2, PDF pg. 119 of 192) seems to have a problem. Code below. Michael ================= [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> :l graph [1 of 1] Compiling Main ( graph.hs, interpreted ) graph.hs:6:24: lexical error at character '\8217' Failed, modules loaded: none. ============== Prelude> data Graph v e = Graph [(Int,v)] [(Int,Int,e)] search :: Graph v e -> Int -> Int -> Maybe [Int] search g@(Graph vl el) src dst | src == dst = Just [src] | otherwise = search’ el where search’ [] = Nothing search’ ((u,v,_):es) | src == u = case search g v dst of Just p -> Just (u:p) Nothing -> search’ es | otherwise = search’ es _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

We tried this functional, higher-order stuff with LISP and look what happened...
by Jason Dusek 29 May '09

29 May '09

What can we say to that? I'm well practiced in handling those who reject types outright (Python programmers), those who reject what is too different (C programmers), those who can not live without objects (Java programmers), those who insist we must move everything to message passing (Erlang programmers). It's not too often that I meet an embittered LISP programmer -- one who's well acquainted with a bold and well-supported community of functional programmers whose shooting star soon descended to dig a smoking hole in the ground. Who's to say Haskell (and the more typeful languages in general) do not find themselves in the same situation in just a few years' time? Is avoiding success at all costs really enough? -- Jason Dusek

3 4

Re: [Haskell-cafe] What's the problem with iota's type signature?
by michael rice 29 May '09

29 May '09

Yeah, I went back and tried double again, though I'd swear I got the dang thing to compile (and run) w/o errors. I guess I meant Num. So Num is a class and Int and Integer are types? What are the other classes? Docs? Unification, for me, is pattern matching ala Prolog. What's the meaning in Haskell? Docs? I've been digging into this stuff for months and it's still tripping me up. Very frustrating. Thanks. Michael --- On Thu, 5/28/09, wren ng thornton <wren(a)freegeek.org> wrote: From: wren ng thornton <wren(a)freegeek.org> Subject: Re: [Haskell-cafe] What's the problem with iota's type signature? To: "Haskell-cafe" <haskell-cafe(a)haskell.org> Date: Thursday, May 28, 2009, 12:40 AM michael rice wrote: > Still exploring monads. I don't understand why the type signature for double is OK, It isn't. The |a| and |b| variables must unify: Prelude> :t \x -> Just (x+x) \x -> Just (x+x) :: (Num a) => a -> Maybe a Prelude> :t (\x -> Just (x+x)) :: Num a => a -> Maybe b <interactive>:1:13: Couldn't match expected type `b' against inferred type `a' `b' is a rigid type variable bound by the polymorphic type `forall a b. (Num a) => a -> Maybe b' at <interactive>:1:0 `a' is a rigid type variable bound by the polymorphic type `forall a b. (Num a) => a -> Maybe b' at <interactive>:1:0 In the first argument of `Just', namely `(x + x)' In the expression: Just (x + x) > but not the one for iota. Again the |a| and |b| must unify. Also, Int isn't a type class (did you mean Num?) Also, the use of (..) requires the Enum class. Prelude> :t \n -> [1..n] \n -> [1..n] :: (Enum t, Num t) => t -> [t] Prelude> [3,4,5] >>= (\n -> [1..n]) [1,2,3,1,2,3,4,1,2,3,4,5] -- Live well, ~wren _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

4 3

Problem w/YAHT code example
by michael rice 29 May '09

29 May '09

This code, from YAHT (Section 8.4.2, PDF pg. 119 of 192) seems to have a problem. Code below. Michael ================= [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> :l graph [1 of 1] Compiling Main ( graph.hs, interpreted ) graph.hs:6:24: lexical error at character '\8217' Failed, modules loaded: none. ============== Prelude> data Graph v e = Graph [(Int,v)] [(Int,Int,e)] search :: Graph v e -> Int -> Int -> Maybe [Int] search g@(Graph vl el) src dst | src == dst = Just [src] | otherwise = search’ el where search’ [] = Nothing search’ ((u,v,_):es) | src == u = case search g v dst of Just p -> Just (u:p) Nothing -> search’ es | otherwise = search’ es

2 1

Who takes care of Haskell mailing lists?
by Maurício 29 May '09

29 May '09

Hi, I would like to create a mailing list for Portuguese speaking Haskell programmers. I tried checking haskell.org mailing lists page, but the only contact e-mail I see is 'mailman(a)haskell.org', and a message sent to that address is replied with an automatic message saying I'm not authorized to use it. Is there someone I could contact about that? Thanks, Maurício

3 2

Parsec float
by Bartosz Wójcik 29 May '09

29 May '09

Hi Everybody (especially Parsec Creator), is there any reason why float parses only positive numbers? I find following defition: float = lexeme floating <?> "float" floating = do{ n <- decimal ; fractExponent n } If floating was defined like floating = do{ n <- integer ... or floating = do{ n <- int ... instead then it would parse negative ones as well. Best regards, Bartek

1 0

ANNOUNCE: Hac φ: Haskell hackathon in Philadelphia, July 24-26
by Brent Yorgey 29 May '09

29 May '09

Greetings, I am very pleased to officially announce Hac phi, a Haskell hackathon/get-together to be held July 24-26 at the University of Pennsylvania in Philadelphia. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday (with breaks for sleep, of course). Thanks to everyone who has already expressed interest in attending, it's shaping up to be a fun event. I want to stress that everyone is welcome---you do not have to be a Haskell guru to attend! Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, please officially register [1], even if you already put your name on the wiki. Registration, travel, lodging and many other details can now be found on the Hac phi wiki [2]. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game---a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hope to see you in Philadelphia! -The Hac φ team [1] http://haskell.org/haskellwiki/Hac_%CF%86/Register [2] http://haskell.org/haskellwiki/Hac_%CF%86 [3] http://haskell.org/haskellwiki/Hac_%CF%86/Talks

1 0

Parsec float
by Malcolm Wallace 29 May '09

29 May '09

>> is there any reason why float parses only positive numbers? It is usual in parsing libraries to separate the recognition of a leading sign from recognition of the number itself: the sign-only parser can be reused in many contexts, e.g. in the Haskell'98 Numeric library, there is readSigned :: Real a => ReadS a -> ReadS a which takes a secondary parser as its argument. You can instantiate that argument to any of readDec :: Num a => ReadS a readHex :: Num a => ReadS a readFloat :: Num a => ReadS a etc. I do not know if the equivalent of "readSigned" exists in Parsec, but it in polyparse's Text.Parse module, it is called "parseSigned", rather unsurprisingly. Regards, Malcolm

1 0

Stack overflow
by Krzysztof Skrzętnicki 29 May '09

29 May '09

Hello Cafe I'm currently writing an app with heavy use of message passing. To see which messages takes most of the bandwidth I wrote the following code: -- data Counter = CNT !Int !Int !Int !Int cntMsg (CNT a b c d) (MoveOther _ _) = (CNT a+1 b c d) cntMsg (CNT a b c d) (MoveSelf _) = (CNT a b+1 c d) cntMsg (CNT a b c d) (NilMsg) = (CNT a b c+1 d) cntMsg (CNT a b c d) (RoundEnd) = (CNT a b c d+1) emptyCnt = CNT 0 0 0 0 showCnt (CNT a b c d) = printf "CNT MoveOther=%d MoveSelf=%d NilMsg=%d RoundEnd=%d" a b c d -- The code for modifying the counter: (\ msg -> atomicModifyIORef ioref (\ cnt -> (cntMsg cnt msg,()))) Running it without increased stack blows it. With 200M stack I get after a second or so: CNT MoveOther=2125764 MoveSelf=0 NilMsg=0 RoundEnd=2916 The datatype itself is strict. So where is the thunk actually accumulating? Best regards Christopher Skrzętnicki

3 6

About the lazy pattern
by 张旭 29 May '09

29 May '09

Hi, I am really new to haskell. I am reading "A gentle instruction to haskell" now. And I just cannot understand the chapter below. Is there anybody who can gives me some hints about why the pattern matching for "client" is so early? How does the pattern matching works here? Thank you so much for answering my questions! Sincerely, nemo 4.4 Lazy Patterns There is one other kind of pattern allowed in Haskell. It is called a lazy pattern, and has the form ~pat. Lazy patterns are irrefutable: matching a value v against ~pat always succeeds, regardless of pat. Operationally speaking, if an identifier in pat is later "used" on the right-hand-side, it will be bound to that portion of the value that would result if v were to successfully match pat, and _|_ otherwise. Lazy patterns are useful in contexts where infinite data structures are being defined recursively. For example, infinite lists are an excellent vehicle for writing simulation programs, and in this context the infinite lists are often called streams. Consider the simple case of simulating the interactions between a server process server and a client process client, where client sends a sequence of requests to server, and server replies to each request with some kind of response. This situation is shown pictorially in Figure 2. (Note that client also takes an initial message as argument.) Figure 2Using streams to simulate the message sequences, the Haskell code corresponding to this diagram is: reqs = client init resps resps = server reqs These recursive equations are a direct lexical transliteration of the diagram. Let us further assume that the structure of the server and client look something like this: client init (resp:resps) = init : client (next resp) resps server (req:reqs) = process req : server reqs where we assume that next is a function that, given a response from the server, determines the next request, and process is a function that processes a request from the client, returning an appropriate response. Unfortunately, this program has a serious problem: it will not produce any output! The problem is that client, as used in the recursive setting of reqs and resps, attempts a match on the response list before it has submitted its first request! In other words, the pattern matching is being done "too early." One way to fix this is to redefine client as follows: client init resps = init : client (next (head resps)) (tail resps) Although workable, this solution does not read as well as that given earlier. A better solution is to use a lazy pattern: client init ~(resp:resps) = init : client (next resp) resps Because lazy patterns are irrefutable, the match will immediately succeed, allowing the initial request to be "submitted", in turn allowing the first response to be generated; the engine is now "primed", and the recursion takes care of the rest. _________________________________________________________________ Messenger安全保护中心，免费修复系统漏洞，保护Messenger安全！ http://im.live.cn/safe/

6 6

Introducing Instances in GHC point releases
by John Goerzen 29 May '09

29 May '09

So this is annoying (CCing -cafe) I need NominalDiffTime and UTCTime to have Typeable instances. In 6.10.1, they didn't ship with them out of the box, so I added them. Apparently, in 6.10.3, they DO ship with those instances out of the box. Annoyingly, that means that my code breaks on 6.10.3. Even more annoyingly, __GLASGOW_HASKELL__ is still 610, so I can't even work around this via cpphs. There appears to be no way to make code that requires those Typeable instances work with both 6.10.1 and 6.10.3. Yet another reason to avoid API incompatibilities in point releases. Does anybody have an idea on the best way to handle this? -- John Don Stewart wrote: > convertible appears broken with 6.10.3. Any thoughts? > > Writing new package config file... done. > Downloading convertible-1.0.1... > Configuring convertible-1.0.1... > Preprocessing library convertible-1.0.1... > Preprocessing executables for convertible-1.0.1... > Building convertible-1.0.1... > [1 of 8] Compiling Data.Convertible.Base ( Data/Convertible/Base.hs, dist/build/Data/Convertible/Base.o ) > [2 of 8] Compiling Data.Convertible.Utils ( Data/Convertible/Utils.hs, dist/build/Data/Convertible/Utils.o ) > [3 of 8] Compiling Data.Convertible.Instances.Map ( Data/Convertible/Instances/Map.hs, dist/build/Data/Convertible/Instances/Map.o ) > [4 of 8] Compiling Data.Convertible.Instances.Num ( Data/Convertible/Instances/Num.hs, dist/build/Data/Convertible/Instances/Num.o ) > [5 of 8] Compiling Data.Convertible.Instances.C ( Data/Convertible/Instances/C.hs, dist/build/Data/Convertible/Instances/C.o ) > [6 of 8] Compiling Data.Convertible.Instances.Time ( Data/Convertible/Instances/Time.hs, dist/build/Data/Convertible/Instances/Time.o ) > > Data/Convertible/Instances/Time.hs:61:9: > Duplicate instance declarations: > instance Typeable NominalDiffTime > -- Defined at Data/Convertible/Instances/Time.hs:61:9-32 > instance Typeable NominalDiffTime > -- Defined in time-1.1.3:Data.Time.Clock.UTC > > Data/Convertible/Instances/Time.hs:64:9: > Duplicate instance declarations: > instance Typeable UTCTime > -- Defined at Data/Convertible/Instances/Time.hs:64:9-24 > instance Typeable UTCTime > -- Defined in time-1.1.3:Data.Time.Clock.UTC > cabal: Error: some packages failed to install: > HDBC-2.1.0 depends on convertible-1.0.1 which failed to install. > convertible-1.0.1 failed during the building phase. The exception was: > >

10 16

gtk2hs, TreeView and CellRenderer
by Zefirov Sergey 28 May '09

28 May '09

As far as I can tell, there is not way to create CellRenderer that could include icons with the text (I think it is because of incompatibilities of Haskell type classes and Gtk inheritance hierarchy). Is it a good way to use CellRendererPixbuf and render to pixbufs using Cairo? I think my TreeView hierarchies won't be very big, around 1000 items for a while.

1 0

Strange type error with associated type synonyms
by Peter Berry 28 May '09

28 May '09

> {-# LANGUAGE TypeFamilies, TypeSynonymInstances, ScopedTypeVariables #-} The following is a class of memo tries indexed by d: > class Fun d where > type Memo d :: * -> * > abst :: (d -> a) -> Memo d a > appl :: Memo d a -> (d -> a) > -- Law: abst . appl = id > -- Law: appl . abst = id (denotationally) Any such type Memo d is naturally a functor: > memo_fmap f x = abst (f . appl x) The type of memo_fmap (as given by ghci) is (Fun d) => (a -> c) -> Memo d a -> Memo d c. (Obviously this would also be the type of fmap for Memo d, so we could declare a Functor instance in principle.) If we add this signature: > memo_fmap' :: Fun d => (a -> b) -> Memo d a -> Memo d b > memo_fmap' f x = abst (f . appl x) it doesn't type check: TypeSynonymTest.hs:14:17: Couldn't match expected type `Memo d1 b' against inferred type `Memo d b' In the expression: abst (f . appl x) In the definition of `memo_fmap'': memo_fmap' f x = abst (f . appl x) TypeSynonymTest.hs:14:32: Couldn't match expected type `Memo d a' against inferred type `Memo d1 a' In the first argument of `appl', namely `x' In the second argument of `(.)', namely `appl x' In the first argument of `abst', namely `(f . appl x)' Failed, modules loaded: none. As I understand it, the type checker's thought process should be along these lines: 1) the type signature dictates that x has type Memo d a. 2) appl has type Memo d1 a -> d1 -> a for some d1. 3) we apply appl to x, so Memo d1 a = Memo d a. unify d = d1 But for some reason, step 3 fails. If we annotate appl with the correct type (using scoped type variables), it type checks: > -- thanks to mmorrow on #haskell for this > memo_fmap'' :: forall a b d. Fun d => (a -> b) -> Memo d a -> Memo d b > memo_fmap'' f x = abst (f . (appl :: Memo d a -> d -> a) x) My ghc is 6.8.2, but apparently this happens in 6.10 as well. -- Peter Berry <pwberry(a)gmail.com> Please avoid sending me Word or PowerPoint attachments. See http://www.gnu.org/philosophy/no-word-attachments.html

11 26

Type class context propagation investigation
by Paul Keir 28 May '09

28 May '09

Hi, How does the context of a class instance declaration affect its subclasses? The Num class instance outlined below has its requirement for Eq and Show satisfied on the preceding lines, and the code will compile. But if I, say, add an (Eq a) constraint to the Eq instance, in preparation for a simple (==) definition, I find that the Num instance declaration is left lacking. If I add the same (Eq a) constraint now to Num, calm is restored. data Foo a = F a instance Eq (Foo a) where (==) = undefined instance Show (Foo a) where show = undefined instance Num (Foo a) (+) = undefined ... etc. The thing that confuses me with this is that it seems like Num "knows" that an (Eq a) context has been applied, and so what it sees as a problem, is somehow also the solution. Any advice/rules of thumb? Does this situation occur elsewhere? How do these constraints propagate? Thanks, Paul

3 4

Re: [Haskell-cafe] Bool as type class to serve EDSLs.
by Sebastiaan Visser 28 May '09

28 May '09

Or overloading the `whitespace operator' for several kinds of application. On May 28, 2009, at 9:50 AM, Augustsson, Lennart wrote: > And explicit desugaring of list syntax. > > -----Original Message----- > From: Sittampalam, Ganesh [mailto:ganesh.sittampalam@credit- > suisse.com] > Sent: 28 May 2009 08:46 > To: Simon Peyton-Jones; Sebastiaan Visser; Haskell Cafe > Cc: Augustsson, Lennart > Subject: RE: [Haskell-cafe] Bool as type class to serve EDSLs. > > There are multiple possible classes that you might want under > different > circumstances (I think the most interesting issue is whether the class > (==), (>) etc is in has a fundep from the type of the thing being > compared to the type of the boolean), but if NoImplicitPrelude (or > some > other extension) just desugars if-then-else into ifthenelse (with a > default implementation of ifthenelse somewhere) I think that would be > enough. > > Of course once you've got ifthenelse you find yourself wanting > explicit > desugaring of pattern matching (could view patterns help here?), > recursion (into an explicit use of fix), etc... > > Cheers, > > Ganesh > > Simon Peyton-Jones wrote: >> You are absolutely right about the tantalising opportunity. >> ...

1 0

the problem of design by negation
by Michael Mossey 28 May '09

28 May '09

This is not directly related to Haskell, but it's a thought that occurred to me after exposure to the Haskell community. I've spent most of the past 15 years doing scientific programming. The lead software architect and software managers are using good software engineering practice, though (this is *scientific* programming, not *programming by scientists*, ha ha). But, there is a particular culture in my company that has become more obvious to me by contrast to the Haskell community. I call it "design by negation." When asked to justify his design, the lead software architect explains everything that *wouldn't* work. "We couldn't have a unique key for every entry because blah blah blah. We couldn't use a garbage collector because blah blah. We couldn't write a sugar layer because then you have to document it separately blah blah." So the chosen design seems to be the only thing left after eliminating everything you can't do. I want to aspire to "positive design." I want to list the goals, and think of design as making clever choices that meet all the goals. I don't mean to suggest that design is never constrained. It often is. But it's a mindset I'm talking about. I don't like this mindset of design by negation. -Mike

12 19

Template Haskell very wordy w/r/t Decs and Types
by Ross Mellgren 28 May '09

28 May '09

Hi all, I'm writing a small module that exposes a template haskell splice that takes a (very simplified) C struct definition and builds: - A data type definition, - an instance for Data.Binary.Binary, - and optionally a pretty print function for it However, it seems to do this I have to write a bunch of really ugly code that builds up the TH data structures "by hand" because quoting only works with splices for expressions, or so it seems. For example, to generate the binary instance I have this code: import qualified Language.Haskell.TH as TH -- tyname is the name of the data type I've already created, as a TH.Name -- tempnames is a list of temporary variable names that are used in lambda patterns -- fields is a list of tuples describing each field -- makeGetExp recursively builds a monadic computation consisting mostly of Binary.getWord32be >>= \ tempvar -> ... binaryInstDec <- liftM (TH.InstanceD [] (TH.AppT (TH.ConT $ TH.mkName "Data.Binary.Binary") (TH.ConT tyname))) [d| get = $(makeGetExp (reverse $ zip fields tempnames) returnExp) put = undefined |] I'd really rather write: binaryInstDec <- [d| instance Binary.Binary $(tyname) where get = $(makeGetExp (reverse $ zip fields tempnames) returnExp) put = undefined |] But GHC gives me a syntax error on the tyname splice. The docs seem to indicate this is the way it is -- that splices in type locations is plain not implemented. My question is whether or not this is just the way it is, and people writing TH declaration splices tend to have to start manually assembling a bunch of it, or is there some trick I've missed? Perhaps even better are there some tricks that people tend to use to make this less painful? I did try using some of the lowercased monadic constructors in Language.Haskell.TH.Lib but I didn't seem to get anything more succint out of it. -Ross P.S. This is for a one-off weekend project, and the code is fugly, so I'm not posting it in its entirety here. If you want the whole module and are willing not to laugh, I'd be glad to send it along ;-) Background: I'm doing this because I'm writing a tool to snoop around the filesystem structures of HFS+ to try and help a friend recover some data off a sufficiently dead drive that fsck doesn't want to touch it, and I don't want to pay money just to find out the drive is too toasty to pull out the data. In any case, the HFS+ docs have a bunch of C struct definitions that comprise the structures, and I got tired of hand-writing data definitions and binary instances, so I figured I'd make TH do it for me.

4 4

Design in Haskell?
by Dan 28 May '09

28 May '09

Hi, When I was learning to program in imperative languages like Java, there were plenty of resources to learn from about how to design large programs. Ideas like the GoF Design Patterns gave useful models that one could then scale up. Are there such resources for Haskell? As I'm learning the language, I'm finding it quite nice to write short programs (e.g. projecteuler, Haskell Road, etc.) But when I try to write something a bit larger, I find I'm tripping over myself and having a hard time organizing the code coherently. Are there any suggestions of wikis, books or particularly well-architected and readable projects I could look at to about learn larger-scale design in Haskell? Cheers, - Dan

6 6

Beginner SOS
by Manu Gupta 28 May '09

28 May '09

Dear anyone,I wish to learn HASKELL. However my institution does not teach it so plus I don't have a clue how to get around with it. Everything seems so unconventional and out of place Can you help me out in getting good tutorials that will help me to learn HASKELL by myself so that I can pursue it as a serious programming languages Till now I have referred Haskell wiki and have tried everywhere but does not seem to learn it PLZ, PLZ HELP ME OUT -- Regards MANU

7 7

What's the problem with iota's type signature?
by michael rice 28 May '09

28 May '09

Still exploring monads. I don't understand why the type signature for double is OK, but not the one for iota. Michael ================= --double :: (Int a) => a -> Maybe b --double x = Just (x + x) iota :: (Int a) => a -> [b] iota n = [1..n] --usage: [3,4,5] >>= iota --should produce [1,2,3,1,2,3,4,1,2,3,4,5]

5 4

DEFUN09: Final Call for Talks & Tutorials (co-located w/ ICFP09)
by Matthew Fluet (ICFP Publicity Chair) 28 May '09

28 May '09

Call for Talks and Tutorials ACM SIGPLAN 2009 Developer Tracks on Functional Programming http://www.defun2009.info/ Edinburgh, Scotland, September 3 and 5, 2009 The workshop will be held in conjunction with ICFP 2009 http://www.cs.nott.ac.uk/~gmh/icfp09.html Important dates Proposal Deadline: June 5, 2009 Notification: June 19, 2009 This is a second invitation to submit talk and tutorial proposals for DEFUN 2009, the ICFP 2009 Developer Tracks. The deadline for submissions is next Friday, June 5. We want to know about your favorite programming techniques, powerful libraries, and engineering approaches you've used that the world should know about and apply to other projects. We want to know how to be productive using functional programming, write better code, and avoid common pitfalls. DEFUN is contiguous with CUFP 2009, the goal of which is to build a community for users of functional programming languages and technology. DEFUN provides the more technical, teaching-oriented parts of functional programming, while CUFP focuses on the commercial, management, and software engineering aspects. For more details of the kinds of proposals we would like to see, and how to submit, please see our original call for proposals at http://www.defun2009.info/blog/call-for-talks-and-tutorials/

1 0

trying to understand space leaks....
by Daryoush Mehrtash 28 May '09

28 May '09

In Section 2.5 of "Generalizing Monads to Arrows" paper (link<http://www.cs.chalmers.se/%7Erjmh/Papers/arrows.ps>) John Huges talks about the space leak inherit in monadic implementation of backtracking parsers. newtype Parser s a = P( [s] => Maybe (a, [s])) instance MonadPlus Parser where P a mplus P b = P (\s -> case a s of Just (x, s') -> Just (x, s') Nothing -> b s) The problem (as I understand it) is that to implement the backtracking, the monad plus implementation needs to keep the state (the computation that returns the next token) around in case the parser fails (so that it can be passed to the other side of mplus). I am having hard time to understand... a)what exactly gets saved on the heap between the mplus calls? b)I am assuming the computation to get the next character for parsing to be an "IO Char" type computation, in that case, what would be the size of the heap buffer that is kept around in case the computation result needs to be reused? c) Assuming Pa in the above code reads n tokens from the input stream then fails, how does the run time returns the same token to the P b? Thanks, Daryoush

2 4

platform/cabal problems
by Johannes Waldmann 28 May '09

28 May '09

Hi all. I've written some Haskell program, and I wanted to give it to a friend, in source form, so he can run and modify it, and learn some Haskell while doing so. I was using some cabalized extra packages but hey, this looks like the typical use case for the haskell-platform. So: I told him to install the newest stable ghc and the current platform release and this happens: la:/home/b/appl.linux/haskell-platform-2009.2.0# ./configure ... checking for hsc2hs... /usr/local/bin/hsc2hs checking version of ghc... 6.10.3 checking version of ghc-pkg matches... yes checking ghc actually works... yes checking the ghc core packages are all installed... no configure: error: The core package editline-0.2.1.0 is missing. It should have been distributed with 6.10.2 ^^^^^^^^^^^^^^^^^^^^^^^^ Well, editline is available from hackage, but to access it, one needs cabal, which is included in the platform, which cannot be built, because, see above. Yes, I know a handfull of workarounds - but they all require some previous knowledge, experience, and/or extra time. Really, if the platform fails in this very simple setting (a fresh install with current stable releases) then it is not much help in advertising Haskell. What's the particular reason for the failure? editline was distributed with ghc-6.10.2, but not with 6.10.3? I wouldn't expect an incompatible change when (only) the patchlevel is bumped. The platform web site should probably replace "works with ghc-6.10.x" by "works with ghc-6.10.2 only". Best regards, Johannes.

2 1

cabal option to specify the ghc version?
by Johannes Waldmann 28 May '09

28 May '09

Hi. How can I tell cabal (= the executable from cabal-install) to use a specific ghc version (and not the one that's currently linked to "ghc")? - Thanks, J.W.

3 2

Facebook puzzles allow for GHC solutions
by David Leimbach 28 May '09

28 May '09

Interesting: http://www.facebook.com/careers/puzzles.php So they use Haskell at Facebook? Dave

5 5

Updating HsJudy to work with the latest Judy
by Robin Green 28 May '09

28 May '09

I would like to use the HsJudy bindings to the Judy high-performance trie library (on hackage), but unfortunately they have bitrotted. I can have a go at mending them but I have no experience with FFI. Any tips? -- Robin

3 2