- Beginners - Haskell.org

Precedence of Infix Operators in Do Syntax
by Avery Robinson 23 Nov '11

23 Nov '11

Hello, I was reading this block of code from http://jasani.org/2008/02/18/command-line-haskell-and-error-handling-exampl… main = do m <- hGetContents stdin nums <- mapM readM . lines $ m print (sum nums) `catch` (\e -> hPutStrLn stderr ("couldn't sum lines: " ++ show e)) readM :: (Monad m, Read a) => String -> m a readM s | [x] <- parse = return x | otherwise = fail $ "Failed to parse \"" ++ s ++ "\" as a number." where parse = [x | (x,_) <- reads s] I don't understand how line 5 works. I thought that the do notation there would be the same as: main = hGetContents stdin >>= \m -> mapM readM . lines $ m >>= \nums -> print (sum nums) >> `catch` (\e -> hPutStrLn stderr ("couldn't sum lines: " ++ show e)) But I don't understand how that makes any sense with that infix `catch`. It looks like catch doesn't even have a first argument. Please enlighten me. Thanks.

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Re: [Haskell-beginners] Beginners Digest, Vol 41, Issue 30
by Lee Short 23 Nov '11

23 Nov '11

> On Tue, 22 Nov 2011 14:46:37 -0800 > Lee Short <blackcat(a)pro-ns.net> wrote: > >> hlint gives me a parse error on a clause using DoIfThenElse, even >> if >> I have the language pragma. I don't see any hlint options to get >> around that, are there any? >> >> Is it considered good style to write code like this? >> >> if "" == results >> then return True >> else return False >> >> The obvious way rewrite below just seems clunky to me (though I can >> see how others might prefer it to the code above). >> >> return $ if "" == results >> then True >> else False > > You've just pressed one of my language-independent style hot > buttons. Why on earth are you using an if/then/else here? What's > wrong > with the straightforward: > > return "" == results > > The expression results in a boolean, and it's even the one you want > to > return. So why not return it? > That's not my actual code, I was just using it as a simplified example and didn't put a lot of thought into it. My actual code is if delete /= "" then if exists then deleteFromTable req t key else return () else if exists then updateTable req t key else insertTable req t I'd love to hear comments on how to make that more idiomatic, if there's a way.

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hlint and DoIfThenElse
by Lee Short 22 Nov '11

22 Nov '11

hlint gives me a parse error on a clause using DoIfThenElse, even if I have the language pragma. I don't see any hlint options to get around that, are there any? Is it considered good style to write code like this? if "" == results then return True else return False The obvious way rewrite below just seems clunky to me (though I can see how others might prefer it to the code above). return $ if "" == results then True else False thanks Lee

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questionnaire data design patterns
by Alia 22 Nov '11

22 Nov '11

Hi folks, As a way to get my head wrapped around haskell data design idioms, I'm translating a questionnaire data model (in python) to haskell, I would appreciate some advice and criticism on whether I'm going about this the right way. Here's the initial translation: <version1.hs> module Main where data QuestionType = Open | Test | Choice deriving (Show, Eq) data Question = Question { questionName :: String , questionText :: String , questionType :: QuestionType , answerType :: String , correctAnswer :: Maybe String , options :: Maybe [(String, String)] } deriving (Show, Eq) data QuestionSet = QuestionSet { qsetTitle :: String , qsetQuestions :: [Question] } deriving (Show, Eq) data Questionnaire = Questionnaire { questionnaireTitle :: String , questionnaireQuestionSets :: [QuestionSet] } deriving (Show, Eq) q1 = Question { questionName = "q1" , questionText = "What is our name?" , questionType = Open , answerType = "str" , correctAnswer = Nothing , options = Nothing } q2 = Question { questionName = "q2" , questionText = "What is 1+1?" , questionType = Test , answerType = "int" , correctAnswer = Just "2" , options = Nothing } q3 = Question { questionName = "q2" , questionText = "What is 2+1?" , questionType = Choice , answerType = "int" , correctAnswer = Just "3" , options = Just [("1", "2"), ("2", "3"), ("3", "4")] } qset = QuestionSet { qsetTitle = "simple questions" , qsetQuestions = [q1, q2, q3] } questionnaire = Questionnaire { questionnaireTitle = "a questionnaire" , questionnaireQuestionSets = [qset] } </version1.hs> In this first version, the Question record basically holds all possible fields for any kind of question, and fields which are not used are assigned Nothing thanks to the Maybe monad. The final type of the answer is specified as a string in the record for later conversion. Now I wanted to take into account a scenario where I don't need to specify the answerType by creating a polymorphic Question type which is specialized to take different answertypes. So I came up with this version: <version2.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 QuestionSet a = QuestionSet { qsetTitle :: String , qsetQuestions :: [Question a] } 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 = [q1, q2, q3] } questionnaire = Questionnaire { questionnaireTitle = "a questionnaire" , questionnaireQuestionSets = [qset] } </version2.hs> The problem is that now all the questionsets, and questionnaires expect the same specialized type of question which is obviously not what I want and I get this error: Couldn't match expected type `[Char]' with actual type `Int' Expected type: Question String Actual type: Question Int In the expression: q2 In the `qsetQuestions' field of a record So what am I doing wrong here? Should I just revert back to version 1 which I sense is not the haskell way. Many thanks for any help or advice. Alia

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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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