November 2011 - Beginners

Haskell wants the type, but I only know the class.
by Amy de Buitléir 27 Mar '12

27 Mar '12

I'm trying to read something from a file, do something with it, and then write it out again. I don't know, or care about, the type of the object I'm reading, but I do know its class. Here's my code: ---------- import Data.Binary ( Binary, encode, decode ) import Data.ByteString.Lazy as B ( readFile, writeFile ) import Codec.Compression.GZip ( compress, decompress ) class ( Binary a, Show a, Eq a ) => Thing a where doSomething :: a -> IO a readThing :: ( Thing a ) => FilePath -> IO a readThing f = return . decode . decompress =<< B.readFile f writeThing :: ( Thing a ) => FilePath -> a -> IO () writeThing f = B.writeFile f . compress . encode main = do a <- readThing "file1.txt" a' <- doSomething a writeThing "file2.txt" a' ---------- And here's the compiler error I get. Amy.hs:18:3: Ambiguous type variable `a0' in the constraint: (Thing a0) arising from a use of `writeThing' Probable fix: add a type signature that fixes these type variable(s) In the expression: writeThing "file2.txt" a' In the expression: do { a <- readThing "file1.txt"; a' <- doSomething a; writeThing "file2.txt" a' } In an equation for `main': main = do { a <- readThing "file1.txt"; a' <- doSomething a; writeThing "file2.txt" a' } Is there a way to fix my code? I tried adding a type signature, e.g., a <- readThing "file1.txt" :: (Thing t) => IO t a' <- doSomething a :: (Thing t) => IO t but then it says... Couldn't match type `t0' with `t' because type variable `t' would escape its scope This (rigid, skolem) type variable is bound by an expression type signature: Thing t => IO t The following variables have types that mention t0 a :: t0 (bound at Amy.hs:16:3) In a stmt of a 'do' expression: a' <- doSomething a :: Thing t => IO t In the expression: do { a <- readThing "file1.txt" :: Thing t => IO t; a' <- doSomething a :: Thing t => IO t; writeThing "file2.txt" a' } In an equation for `main': main = do { a <- readThing "file1.txt" :: Thing t => IO t; a' <- doSomething a :: Thing t => IO t; writeThing "file2.txt" a' }

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Iterating a monadic action with memoization
by Tim Cowlishaw 29 Jan '12

29 Jan '12

Hi all, I was wondering if any of you knew whether the following was possible, (I discussed this a little on #haskell at the weekend but didn't quite get to the bottom of it): Say I have a monadic action: > f :: a -> m a and an initial value of type a. I'd like to iterate this action and collect the results of each iteration in a list, as with 'iterate' on normal functions: > iterate (>>= f) . return $ a However, I would also like to memoize the intermediate results of the iteration, such that the entire iteration runs in O(n) time rather than O(n^2). This also implies a slight semantic difference, as side-effecting or non-deterministic actions will not be repeated in each iteration, while maintaining the laziness of the iteration function. Is it possible to do this? Many thanks, Tim

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Diagnosing : Large memory usage + low CPU
by Hugo Ferreira 05 Dec '11

05 Dec '11

Hello, I am testing a simple algorithm and find that during part of the execution this is fast and uses acceptable memory. However, when it gets to a certain point the memory climbs to 75-80 % of the OS's memory and CPU plummets to a mere 5% at most. In the part of the application that executes ok I have calls as follows: let r9' = evalTagger'' (tagFun suffixCapsFreq) $ test let a9' = tagginAccuracy r9' putStrLn ("Suffix(3) + Caps + Freq tag result done = " ++ (show a9')) The slow one has: let r12' = evalTagger'' ruleSuffixCapsFreq $ test let a12' = tagginAccuracy r12' putStrLn ("Rules + Suffix(3) + Caps + Freq tag result done = " ++ (show a12')) The difference lies in the following functions: ruleApplication :: TransformationRule -> POSTags -> Maybe Tag ruleApplication (NextTagRule (Replacement old new) next) z = do (_, _, proposed) <- Z.safeCursor z (_, _, nextProposed) <- rightCursor z if proposed == old && nextProposed == next then Just new else Nothing .... updateState :: (TransformationRule,Int) -> POSTags -> POSTags updateState r = Z.fromList . reverse . Z.foldlz' (update r) [] where update (r,_) !xs z = case ruleApplication r z of Just tag -> (token, correct, tag):xs Nothing -> e:xs where e@(token, correct, _proposed) = Z.cursor z rulesT :: [(TransformationRule, Int)] -> POSTags -> POSTags rulesT rs state = L.foldl' tag state rs where tag !s rule = updateState rule s (.>) :: (POSTags -> Tag) -> (POSTags -> Tag) -> POSTags -> Tag f .> g = \ x -> case (g x) of "" -> f x t -> t (|>) :: (POSTags -> POSTags) -> (POSTags -> Tag) -> POSTags -> POSTags f |> g = \ x -> f $ tagFun g x let ruleT = rulesT top10Rules let suffixCapsFreq = suffixT .> capitalizeT .> freqT let ruleSuffixCapsFreq = ruleT |> suffixCapsFreq where top10Rules is a list. Note that suffixCapsFreq and tagFun are common to both calls. I have placed bangs in many places and used eager evaluation in the folds. However nothing seems to help. Can anyone tell me how I should diagnose this problem. Any suggestions are welcome. TIA, Hugo F.

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Cabal on mac
by Tim Pizey 29 Nov '11

29 Nov '11

Hi, I have installed cabal and HaxMl cabal install HaXml but my .hs file cannot find import Text.XML.HaXml.Xml2Haskell (readXml) giving *** Chasing dependencies: Chasing modules from: *ch.hs ch.hs:3:8: Could not find module `Text.XML.HaXml.Xml2Haskell': locations searched: Text/XML/HaXml/Xml2Haskell.hs Text/XML/HaXml/Xml2Haskell.lhs It would seem only to be looking in the current directory. Hoping I have just missed something obvious. Tim -- Tim Pizey http://pizey.net/~timp

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Multi-parameter type classes
by David Beacham 29 Nov '11

29 Nov '11

Hi all, I've been reading through Okasaki's purely functional data structures and noticed that the source he provides for the Set implementations uses multi-parameter type classes, whereas those for the Heap structures does not. class Set s a where empty :: s a insert :: a -> s a -> s a ... class Heap h where empty :: Ord a => h a insert :: Ord a => a -> h a -> h a ... The only difference I can see is that the instance declarations for Set contain the Ord constraint, whereas they are defined on the class functions for Heap. instance (Ord a) => Set RedBlackSet a where ... I realise that the type of `a` needs to be constrained to Ord instances at some point in the code, but is there any particular reason for doing it differently in each case? If not, should one method be preferred over the other? Thanks for the help, David.

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help with a class definition
by Rustom Mody 29 Nov '11

29 Nov '11

The Data.Vector.Unboxed page on hackage has the following line class (Vector Vector a, MVector MVector a) => Unbox a Can someone explain whats the repeating Vector (and MVector) there?

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Problems linking code from the book Haskell School of Expression
by Philippe Sismondi 27 Nov '11

27 Nov '11

Greetings. Yesterday I decided to try some of the code in Paul Hudak's book The Haskell School of Expression. I am having problems when I do ghc --make of the code on page 40 of the book, which is the first graphics program therein. I am running OS X Snow Leopard, with the Haskell Platform (32 bit), ghc version 7.0.3. I am very green when it comes to looking under the covers at what gcc, ld, etc do at the behest of ghc. This is where things seem to go wrong, as I outline below. In setting up to try this code I discovered that I needed to install the package soegtk to get Graphics.SOE.Gtk. The cabal install of soegtk was not smooth: it appears to me that cabal does not always handle dependencies as one would hope. However, I eventually got soegtk and the packages upon which it depends installed. Here is my list of local user packages as reported by ghc-pkg: cairo-0.12.2 gio-0.12.2 glib-0.12.2 gtk-0.12.2 pango-0.12.2 soegtk-0.12.1 Oddly, in the course of running cabal install to get the above packages, at some point I got a flurry of messages similar to the one below. (This is not one of the actual messages from cabal install; I did not keep those. This warning message is from the ghc --make step, as described below.) ld: warning: ignoring file /opt/local/lib/libgtk-x11-2.0.dylib, file was built for unsupported file format which is not the architecture being linked (i386) This puzzled me, and the same thing came up at a later stage, as I shall describe shortly. But initially I ignored the messages and proceeded to try to make my sample program. Here is the code from Hudak's book, which I put in Main.hs: module Main where import Graphics.SOE.Gtk main = runGraphics ( do w <- openWindow "SOE First Try" (300,300) drawInWindow w (text (100,100) "Hello Graphics World") k <- getKey w closeWindow w ) When I do ghc --make Main, it grinds forever at the link stage. The link fails. It also gives ld warnings about ignoring libraries in /opt/local/lib, as in the example above. Then, the link eventually fails because of a huge number of unresolved symbols. Here is a sample link error from the verbose output: Undefined symbols for architecture i386: "_cairo_arc", referenced from: _sbnE_info in libHSsoegtk-0.12.1.a(Gtk.o) _se2n_info in libHSsoegtk-0.12.1.a(Gtk.o) _sf5B_info in libHSsoegtk-0.12.1.a(Gtk.o) _svSr_info in libHScairo-0.12.2.a(Paths.o) _swiH_info in libHScairo-0.12.2.a(Paths.o) "_cairo_arc_negative", referenced from: _seqb_info in libHSsoegtk-0.12.1.a(Gtk.o) _svPR_info in libHScairo-0.12.2.a(Paths.o) _swg7_info in libHScairo-0.12.2.a(Paths.o) and on and on. I have no idea what is happening here. First, I don't know whether the linker messages about ignoring libs in /opt/local/lib is connected with the unresolved symbols, but one would suspect that it is. Second, it is not clear to me why ghc would link against anything in /opt/local/lib, which it clearly does. Here are the first couple of lines of verbose output from ghc --make, showing the inclusion of the flag -L/opt/local/lib: /Developer/usr/bin/gcc -v -o Main -march=i686 -m32 -fno-stack-protector -DDONT_WANT_WIN32_DLL_SUPPORT Main.o -L/Users/Philippe/Library/Haskell/ghc-7.0.3/lib/soegtk-0.12.1/lib -L/Library/Haskell/ghc-7.0.3/lib/stm-2.2.0.1/lib -L/Users/Philippe/Library/Haskell/ghc-7.0.3/lib/gtk-0.12.2/lib -L/opt/local/lib -L/Users/Philippe/Library/Haskell/ghc-7.0.3/lib/pango-0.12.2/lib I have plenty of macports stuff installed in /opt/local. But I have not intentionally included /opt/local/lib in any path. That is, I do NOT have, for example, LD_LIBRARY_PATH or DYLD_LIBRARY_PATH set. So is it normal for ghc to link against /opt/local/lib libraries, or is it something in my environment that is triggering this? And, in any case, is that why the link fails? What about the references to "architecture i386"? Do they provide the necessary clue for the clueless (me)? Any help is appreciated. - Phil -

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Re: [Haskell-beginners] data design for a questionnaire (retitled + update)
by Alia 27 Nov '11

27 Nov '11

Hi folks, As the code (from my prior post) is not so relevant to the issue at hand, it was quite reasonably suggested that I should simplify my question somewhat. So here goes: In my model, I have a Question record type, which has a type parameter to take into account that answers can be evaluated to different types. For example, I can ask a question which requires a Double as an answer and another which requires an Int as an answer: data Question a = Question {text:: String, answer:: Maybe a} This question type is wrapped/boxed in another type to allow the different questions to referenced in a list. Hence, data Question' = QuestionS (Question String) | QuestionI (Question Int) | QuestionD (Question Double) and I can now store the questions as follows: questions = [ QuestionI {text="What's 1+1?", answer=Maybe 2} , QuestionD {text="What's 1.0+1.5?", answer=Maybe 2.5} ] Now to extract Question a from Question' I would have liked to write the following: extract :: Question' -> Question a extract q = case q of QuestionS x -> extractQString q QuestionI x -> extractQInt q QuestionD x -> extractQDouble q but instead, I had to produce the following instead: extractQString :: Question' -> Question String extractQString (QuestionS q) = q extractQInt :: Question' -> Question Int extractQInt (QuestionI q) = q extractQDouble :: Question' -> Question Double extractQDouble (QuestionD q) = q From what I understand so far, that specialized extraction or unboxing functions are the way to go unless one uses language extensions like GADTs which can be conceptually more complicated. Is this indeed the case, or are there other ways to achieve generalized type unboxing while remaining within the Haskell98 specification? Best, Alia

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How to get HLint working?
by Avery Robinson 25 Nov '11

25 Nov '11

Hello, I'm trying to install HLint but I can't figure out how to use it. It seems like the install works, but when I type "hlint file.hs" my terminal gives an error. Here's a copy of my terminal output: http://pastebin.com/n071J2mL It looks like the install works, but at the end you can see that it doesn't function as I am expecting. I'm having difficulty finding relevant documentation. Help please? Avery

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lhs to html
by Tim Pizey 25 Nov '11

25 Nov '11

Hi, I am a happy user of lhs2tex but I cannot get the resultant .tex file to html, with line breaks intact. Both tex4ht and htlatex lose line breaks. htlatex also gives ! Missing { inserted. <to be read again> } l.814 \end{pboxed} pdflatex gives the correctly formatted pdf. Any suggestions? My do.sh: lhs2TeX -o xhtml.tex xhtml.lhs latex xhtml.tex latex xhtml.tex tex4ht xhtml.tex #htlatex xhtml.tex pdflatex xhtml.tex ghc -fhpc -o xhtml main.lhs xhtml.lhs xhtml cheers Tim -- Tim Pizey http://pizey.net/~timp

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