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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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Re: [Haskell-beginners] Beginners Digest, Vol 41, Issue 33
by Google Wave 25 Nov '11

25 Nov '11

2011/11/24 <beginners-request(a)haskell.org> > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Basic "data" question (Costello, Roger L.) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Thu, 24 Nov 2011 10:43:58 +0000 > From: "Costello, Roger L." <costello(a)mitre.org> > Subject: [Haskell-beginners] Basic "data" question > To: "beginners(a)haskell.org" <beginners(a)haskell.org> > Message-ID: > <B5FEE00B53CF054AA8439027E8FE1775181911E9(a)IMCMBX04.MITRE.ORG> > Content-Type: text/plain; charset="us-ascii" > > Hi Folks, > > I am trying to define a data type for this: > > A Lambda term is one of these: > - Variable > - Application of a Lambda term to a Lambda term > - Abstraction of a Variable in a Lambda term > > Here's an example of a Lambda term: > > term = Abstraction (Variable X) (Application (Variable X) (Variable Y)) > > Is there a way to define Lambda term? > > Here's an attempt at defining it: > > data Var = V | W | X | Y | Z > > data Lambda = Variable Var | Application Lambda Lambda | Abstraction > (Variable Var) Lambda > > But that yields the following error: > > test.hs:5:71: Not in scope: type constructor or class 'Variable' > > Any suggestions would be much appreciated. > > /Roger > > > > ------------------------------ > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > > End of Beginners Digest, Vol 41, Issue 33 > ***************************************** > value constructor followed by types. And (Variable Var) belong to Lambda type. Could define like this: data Lambda var= Variable var | Application (Lambda var) (Lambda var) var be any types

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Re: [Haskell-beginners] Basic "data" question
by Brent Yorgey 24 Nov '11

24 Nov '11

On Thu, Nov 24, 2011 at 03:01:49PM +0000, Costello, Roger L. wrote: > Hi Brent, > > Thanks for the feedback. > > > data Var = V | W | X | Y | Z > > > > data Lambda = Variable Var | Application Lambda Lambda | Abstraction Var Lambda > > Unfortunately, that's not quite correct. The first argument of > Abstraction must be a Lambda term, specifically Variable Var. No, that isn't possible (and it also doesn't make sense). It isn't possible because there is no way to express "this field may only contain values built with such-and-such constructor".* It doesn't make sense because why must the first argument of Abstraction be a lambda term? You yourself said "Abstraction of a Variable in a Lambda term" -- it is not "abstraction of a lambda term in a lambda term", but abstraction of a *variable*. Variables have type Var. Hence, Abstraction Var Lambda (I can also use an argument from authority: I have coded many variants of the lambda-calculus in Haskell, and I can tell you that this is the way it is done. =) -Brent * Actually, it is sometimes possible to encode this using GADTs. But that wouldn't help you here.

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Basic "data" question
by Costello, Roger L. 24 Nov '11

24 Nov '11

Hi Folks, I am trying to define a data type for this: A Lambda term is one of these: - Variable - Application of a Lambda term to a Lambda term - Abstraction of a Variable in a Lambda term Here's an example of a Lambda term: term = Abstraction (Variable X) (Application (Variable X) (Variable Y)) Is there a way to define Lambda term? Here's an attempt at defining it: data Var = V | W | X | Y | Z data Lambda = Variable Var | Application Lambda Lambda | Abstraction (Variable Var) Lambda But that yields the following error: test.hs:5:71: Not in scope: type constructor or class 'Variable' Any suggestions would be much appreciated. /Roger

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help opengl object
by Song Zhang 24 Nov '11

24 Nov '11

Hi in Graphics.UI.GLUT.Objects there are some pre-defined object such as cube or cone I tried to draw objects in haskell but get black screen who can give me some examples to draw these objects and for drawing Cylinder in the documentation what is (*freeglut only) *means. do I need glut32.dll to render these object in windows? thanks -- the University of Nottingham, CS school the representative of SSFC CS school Staff of SIFEUNNC IT department I am Ice which is sleeping water. Sent via BlackBerry© by BerryMail

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Re: [Haskell-beginners] data design for a questionnaire (retitled + update)
by Alia 23 Nov '11

23 Nov '11

As a followup, I just received a simpler and more elegant solution due to Lorenzo Bolla which involves wrapping the question type in another type and achieves the objective nicely. I think this is much cleaner than using existential types as per my prior posting. <version4.hs> module Main where import Text.Show.Functions -- type converters str = id int s = read s :: Int float s = read s :: Double data QuestionType = Open | Test | Choice deriving (Show, Eq) data Question a = Question { questionName :: String , questionText :: String , questionType :: QuestionType , answerFunc :: (String -> a) , correctAnswer :: Maybe a , options :: Maybe [(String, a)] } deriving (Show) data Question' = QuestionS (Question String) | QuestionI (Question Int) deriving (Show) data QuestionSet a = QuestionSet { qsetTitle :: String , qsetQuestions :: [Question'] } deriving (Show) data Questionnaire a = Questionnaire { questionnaireTitle :: String , questionnaireQuestionSets :: [QuestionSet a] } deriving (Show) q1 = Question { questionName = "q1" , questionText = "What is our name?" , questionType = Open , answerFunc = id , correctAnswer = Nothing , options = Nothing } q2 = Question { questionName = "q2" , questionText = "What is 1+1?" , questionType = Test , answerFunc = int , correctAnswer = Just 2 , options = Nothing } q3 = Question { questionName = "q2" , questionText = "What is 2+1?" , questionType = Choice , answerFunc = int , correctAnswer = Just 3 , options = Just [("a", 2), ("b", 3), ("c", 4)] } qset = QuestionSet { qsetTitle = "simple questions" , qsetQuestions = [QuestionS q1, QuestionI q2, QuestionI q3] } questionnaire = Questionnaire { questionnaireTitle = "a questionnaire" , questionnaireQuestionSets = [qset] } </version4.hs> Alia

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List comprehension question
by Lee Short 23 Nov '11

23 Nov '11

So hlint suggests that I convert the line checked = if ival==input then [("checked","checked")] else [] to checked = [("checked","checked") | ival==input] I'm not really sure why this works. Is there some implicit list being filtered by ival==input? What's going on? thanks Lee

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