November 2011 - Beginners

Trouble using MultiParamTypeClasses
by Amy de Buitléir 22 Nov '11

22 Nov '11

I would be very grateful if someone could tell me what I'm doing wrong. Here's my code: ----- {-# LANGUAGE MultiParamTypeClasses #-} class Eq a => Graph g a where nodes :: g a -> [a] neighbours :: g a -> a -> [a] data WeightedGraph a w = WeightedGraph [(a, a, w)] instance Eq a => Graph a (WeightedGraph w a) where nodes = [] --stub neighbours = [] --stub ----- And here's the error message: temp.hs:9:24: Kind mis-match The first argument of `Graph' should have kind `* -> *', but `a' has kind `*' In the instance declaration for `Graph a (WeightedGraph w a)'

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Exception Handling with Iteratees
by Michael Craig 22 Nov '11

22 Nov '11

I have some exception types defined ... data POSTOnlyException = POSTOnlyException deriving ( Show, Typeable ) instance Exception POSTOnlyException data BadPathException = BadPathException deriving ( Show, Typeable ) instance Exception BadPathException ... and I want to use Data.Enumerator.catchError ... catchError :: Monad m => Iteratee a m b -> (SomeException -> Iteratee a m b) -> Iteratee a m b ... so I define an error handler ... handleErrors :: SomeException -> Iteratee a m String handleErrors ex = case fromException ex of Just POSTOnlyException -> return "POSTs only!" Just BadPathException -> return "Bad path!" _ -> return "Unknown exception!" ... but of course this doesn't compile, because the types of the LHSs in the case statement are different. I can get around it with some ugliness ... handleErrors :: SomeException -> Iteratee a m String handleErrors ex = case fromException ex of Just POSTOnlyException -> return "POSTs only!" _ -> case fromException ex of Just BadPathException -> return "Bad path!" _ -> return "Unknown exception!" ... but there must be a better way. Enlighten me? Cheers, Mike S Craig

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User defined operators : setting precedence
by Hugo Ferreira 21 Nov '11

21 Nov '11

Hello, Can anyone point me to some documentation that describes how one may set precedence in a user defined operators? I have composition operators so that I can write: let ruleSuffixCapsFreq = ruleT |> suffixT .> capitalizeT .> freqT but I seem to be forced to write: let ruleSuffixCapsFreq = ruleT |> (suffixT .> capitalizeT .> freqT) and I would like to avoid this. TIA, Hugo F.

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Need advice on R vs Haskell
by haskell heath 21 Nov '11

21 Nov '11

I'm new to Haskell and I can't really call myself a decent programmer in any other language. Do you think it's wrong to think that I can contribute to the statistics library? I can't decide if I'm being pragmatic for me to learn Haskell and contribute to the statistics library in hopes of being as productive as I would be in learning R. I'm wanting to write a scraper and analyze that data, and I figure this is what the statistics module can be used for. The other alternative to Haskell and R, is to use something like node.js for scraping, js's regex for filtering, and analyzing the resulting data with RPy. Here's a brief overview of my programming exp: Python: - How to Think Like a Computer Scientist using Python - official tutorial - official reference material - a few project euler problems Javascript - all of the Mozilla docs Haskell - started with the RWH, switched to 98' Report, switched over to Yet Another Haskell Tutorial, switched to Gentle Intro, and finally switched to Learn You a Haskell and I'm on the seventh chapter now. I'm considering switching yet again to A Gentle Intro, because it's been months now, and I just want to finish one of these, and that seems to be quickest route. Other - I have briefly learned about C and C++, I start an intro to C++ programming next semester. Thank you for your help.

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data design for a questionnaire (retitled + update)
by Alia 21 Nov '11

21 Nov '11

I've retitled this post just in case (-: Having spent some more time on the prior post (questionnaire data design patterns), I've managed to produce a version that allows for heterogeneous lists but that seems to necessarily involve existential quantification. Three question remain: (1) figure out how to show 'questions' in version3.hs (I'm stumped so far) (2) figure out how to make Question and its subtypes polymorphic in the context of existential quantification: such that you can have a Choice a where a is an Int and another Choice Double, etc... yet still allow for heterogeneous lists so that questions = [open ...., test ...., choice ...] would still be possible. (3) translate version3.hs to use records (if possible) <version3.hs> {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE TypeSynonymInstances #-} module Main where import Text.Show.Functions -- dummy funcs for now class Question_ a where ask :: a -> String view :: a -> String parse :: a -> String data Question = forall a. Question_ a => Question a type Name = String type QString = String type AnswerType = String type CorrectAnswer = String type Option = (String, String) type Options = [Option] -- question types data Open = Open Name QString AnswerType data Test = Test Name QString AnswerType CorrectAnswer data Choice = Choice Name QString AnswerType CorrectAnswer Options instance Question_ Open where ask (Open n q a) = n view (Open n q a) = q parse (Open n q a) = a instance Question_ Test where ask (Test n q a c) = n ++ c view (Test n q a c) = q ++ c parse (Test n q a c) = a ++ c instance Question_ Choice where ask (Choice n q a c os) = n ++ c view (Choice n q a c os) = q ++ c parse (Choice n q a c os) = a ++ c instance Question_ Question where ask (Question q) = ask q view (Question q) = view q parse (Question q) = parse q -- -- Smart constructor -- open :: Name -> QString -> AnswerType -> Question open n q a = Question (Open n q a) test :: Name -> QString -> AnswerType -> CorrectAnswer -> Question test n q a c = Question (Test n q a c) choice :: Name -> QString -> AnswerType -> CorrectAnswer -> Options -> Question choice n q a c os = Question (Choice n q a c os) questions :: [Question] questions = [ open "q1" "what is your name" "str" , test "q2" "what is 1+1?" "int" "2" , choice "q3" "what is 2+2?" "int" "4" [("a", "4")] ] </version.hs> Alia ]

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Re: [Haskell-beginners] Typing/N00b question/My first haskell
by Bryan Hunt 21 Nov '11

21 Nov '11

Thank you for your helpful replies, program works correctly now: module Main where ageJudge age | age >= 40 = "You're too old!" | age <= 30 = "You're too young" | otherwise = "Not in the programming age range.." main = do putStrLn "What is your name?" name <- getLine putStrLn ("Nice to meet you, " ++ name ++ "!") putStrLn "What is your age?" age <- getLine let inpIntegral = (read age)::Int putStrLn ( "We decided:\n" ++ ageJudge inpIntegral )

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Typing/N00b question/My first haskell.
by bryan hunt 21 Nov '11

21 Nov '11

I'm trying to read in a value and convert it into a number for use in the ageJudge function. I've tried a number of different approaches but keep getting type conversion errors. This is literally my first Haskel program - so please don't laugh... module Main where ageJudge :: (Integral a) => a -> String ageJudge age | age >= 40 = "You're too old!" | age <= 30 = "Your too young" | otherwise = "Not in the programming age range.." main = do putStrLn "What is your name?" name <- getLine putStrLn ("Nice to meet you, " ++ name ++ "!") putStrLn "What is your age?" age <- getLine let inpIntegral = (read age)::Double -- let decission = ageJudge (read inpIntegral)::Integral putStrLn ("We decided!") -- putStrLn ("We decided:\n" ++ show(1) ++ "!") -- ageJudge inpIntegral

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Re: [Haskell-beginners] Typing/N00b question/My first haskell.
by Alia 21 Nov '11

21 Nov '11

The key function is to_int below: <snip> module Main where ageJudge :: (Integral a) => a -> String ageJudge age | age >= 40 = "You're too old!" | age <= 30 = "Your too young" | otherwise = "Not in the programming age range.." to_int s = read s :: Int eg = ageJudge (to_int "40") </snip>

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Compile single file within cabal-dev environment
by Petar Radosevic 20 Nov '11

20 Nov '11

It seems so trivial, but I haven't been able to do it. I'm trying to compile a single .hs file whose dependencies are available in a cabal-dev environment. For example, at my website repo [1] you can see that I have a ``wunki.hs`` which uses Hakyll [2] to create my website. I have installed Hakyll with cabal-dev. But now I want to ``ghc --make wunki.hs`` with that cabal-dev environment. Is this possible? [1]: https://github.com/wunki/www.wunki.org [2]: https://github.com/jaspervdj/hakyll -- Petar Radošević, Programmer wunki.org | @wunki

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cabal can't find stdc++ headers or library?
by Eric Lasian 20 Nov '11

20 Nov '11

Hi all, When trying to install double-conversion, I'm getting the following errors. $ cabal install double-conversion Resolving dependencies... Warning: double-conversion.cabal: Ignoring unknown section type: test-suite Configuring double-conversion-0.2.0.3... cabal: Missing dependency on a foreign library: * Missing C library: stdc++ This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: double-conversion-0.2.0.3 failed during the configure step. The exception was: ExitFailure 1 So it seems to me that everything is in the expected place: the headers are in /usr/include/c++/4.* and the library itself is in /usr/lib. Explicitly forcing cabal to search these directories (with --extra-include-dirs and --extra-lib-dirs as suggested) doesn't help. Any ideas? version info: cabal-install version 0.8.0, using version 1.8.0.2 of the Cabal library, The Glorious Glasgow Haskell Compilation System, version 7.2.1 Thanks, Eric

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