February 2014 - Beginners

Type Class Instance Question
by Philippe Sismondi 21 Feb '14

21 Feb '14

In the Haskell 98 and 2010 reports it states that one may declare something of the form (tyvar1 -> tyvar2) to be an instance of a type class. Is that a function or what is it? Can anyone point me to an example of this? TIA. Best, - Phil -

2 2

Print values stored in an "Either" in the main method
by Mathew Phillips 20 Feb '14

20 Feb '14

I have a module that looks like this runEval :: (Show a) => Env -> Eval a -> Either String a runEval = ... data Value = IntVal Integer main :: IO () main = do let env = Map.fromList [("x",IntVal 3)] result <- runEval env (eval (Plus (Var "x") (Lit 2))) case result of Left err -> putStrLn "Error: "++err Right (IntVal i) -> print i when I call runEval from ghci I get back a result just fine let env = Map.fromList [("x",IntVal 3)] runEval env (eval (Plus (Var "x") (Lit 2))) This outputs "Right (IntVal 5)" as I would expect. But when I try and compile my main method I get the following error. Couldn't match expected type `Value' with actual type `Either t1 Value' In the pattern: Right (IntVal i) In a case alternative: Right (IntVal i) -> print i In a stmt of a 'do' block: case result of { Left err -> print err Right (IntVal i) -> print i } I also tried using (putStrLn . either show show) result, but this gave me the following error. Couldn't match expected type `Either a0 b0' with actual type `Value' In the first argument of `putStrLn . either show show', namely `result' In a stmt of a 'do' block: (putStrLn . either show show) result In the expression: do { let env = Map.fromList ...; result <- runEval3 env (eval3 (Plus (Var "x") (Lit 2))); (putStrLn . either show show) result } Why is it when I try and pattern match on Either String Value it says result is of type "Value", but when I try and use either it says result is of type "Either String Value"? Matt P.

3 5

QuickCheck for testing a parsing function
by Thomas Bach 20 Feb '14

20 Feb '14

Hi there, I have the following snippet: data Volume = Volume { name :: String , mount :: String , size :: String , fs :: String , mbr :: Bool } deriving (Show, Eq) defaultVolume :: Volume defaultVolume = Volume { name = "" , mount = "" , size = "" , fs = "ext4" , mbr = False } parseOptVols :: String -> Volume parseOptVols s = parseChain (defaultVolume, s) where parseChain = parseMBR . parseFS . parseSize . parseMount . parseName parseName (vol, s) = (vol {name = getThis s}, getNext s) parseMount (vol, s) = (vol {mount = getThis s}, getNext s) parseSize (vol, s) = (vol {size = getThis s}, getNext s) -- fs and mbr are optional parseFS (vol, "") = (vol, "") parseFS (vol, ',':s) = (vol, s) parseFS (vol, s) = (vol {fs = getThis s}, getNext s) parseMBR (vol, "t") = vol {mbr = True} parseMBR (vol, _ ) = vol getThis = takeWhile (',' /=) getNext = saveTail . dropWhile (',' /=) saveTail :: [a] -> [a] saveTail [] = [] saveTail (_:xs) = xs What it basically does is parse a string (handed over from the command line) like "boot,/boot,256M,ext3,t" and populates a Volume instance with this. The last two fields are optional. I like testing and coming from an object-oriented background unit testing is the most comfortable to me. But I wonder if there is some smart way to make QuickCheck produce test cases. I would need some generator for this which produces a pattern like "name,mount,size,fs,mbr" eventually leaving "fs" and/or "mbr" empty and sometimes omitting both commas or just one of them. And I also need access to these fields in order to be able to compare them with the resulting Volume instance. I can see how to achieve the former somehow, but I don't have any clue how to achieve the latter. Any hints? Or is this simply not a nail for the QuickCheck-hammer? Regards, Thomas Bach. PS: Any advice on how to improve the code above is highly appreciated as well.

2 1

Is a ghc package the same as a cabal package?
by Philippe Sismondi 18 Feb '14

18 Feb '14

Is a ghc package the same as a cabal package? As far as I can tell they are not. I mean "package" as defined by the documentation for each. In the ghc documentation it states that "A package specification is a Haskell record". A Cabal package seems to be a collection of stuff including the .cabal file etc. Finally, does Hackage use the term "package" differently again? Rant: package and/or build systems are the bane of my existence in any non-commercial language that I use. Sigh. I have been using Cabal and cabal-install naively (read "blindly") for long enough; time to come to grips with it. Thanks, if anyone has time to help. - P -

3 4

combine concatMap and mapM
by Renah Scarowsky 17 Feb '14

17 Feb '14

Hi, How would I combine the functionalities of concatMap and mapM to define a function like: Monad m => (a -> m [b]) -> [a] -> m [b] I need to create a recursive function within the IO monad which results in a list of the original type. So something like: goX :: MonadIO m => x -> m [x] goX = do .... x includes within it a list of xs, let's call it (children x) I need to call goX on each of the children and get a modified list of children as result ys <- ? return $ x { children = ys } : zs Thanks, Renah Scarowsky Suite Solutions Create>Manage>Deploy http://www.suite-sol.com<http://www.suite-sol.com/>

2 1

type error in sub-function
by James Toll 16 Feb '14

16 Feb '14

Hi, I am trying to write a simple function to determine the divisors of an integer. In its simplest form the type signature should be something like: divisors :: Int -> [Int] divisors x = 1 : lower ++ upper ++ x : [] where lower = filter (\y -> mod x y == 0) [2..(ceiling . sqrt) x] upper = sort $ map (div x) lower Although, I think my type signature isn’t complete as it ignores a lot of what’s going on in the function. Regardless, the function throws an error when evaluated. I don’t know if I just need a more accurate type signature, or if there is a bigger problem. From what I can tell, my problem is in the lower function, but when I deconstruct it into its parts, they work individually and as a whole, but not as a stand-alone function. For example, using x = 36: Prelude> [2..(ceiling . sqrt) 36] [2,3,4,5,6] Prelude> filter (\y -> mod 36 y == 0) it [2,3,4,6] Or as a whole: Prelude> filter (\y -> mod 36 y == 0) [2..(ceiling . sqrt) 36] [2,3,4,6] But when I define this as a function, it throws an error when evaluated. Prelude> let lower x = filter (\y -> mod x y == 0) [2..(ceiling . sqrt) x] Prelude> lower 36 <interactive>:6:1: No instance for (RealFrac b0) arising from a use of `lower' The type variable `b0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance RealFrac Double -- Defined in `GHC.Float' instance RealFrac Float -- Defined in `GHC.Float' instance Integral a => RealFrac (GHC.Real.Ratio a) -- Defined in `GHC.Real' In the expression: lower 36 In an equation for `it': it = lower 36 <interactive>:6:7: No instance for (Num b0) arising from the literal `36' The type variable `b0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance Num Double -- Defined in `GHC.Float' instance Num Float -- Defined in `GHC.Float' instance Integral a => Num (GHC.Real.Ratio a) -- Defined in `GHC.Real' ...plus three others In the first argument of `lower', namely `36' In the expression: lower 36 In an equation for `it': it = lower 36 At this point, I’m not sure what I need to do to get his working properly. It appears to be a type error, but I’m apparently not understanding the error message enough to fix the problem. Any suggestions would be appreciated. Thanks, James

5 10

filter by max length a list of lists with equivalent values
by James Toll 11 Feb '14

11 Feb '14

Hi, I have what’s proving to be a tricky little problem for me to solve. I’m trying to take a list of a list of Int’s and return the longest list for each list with elements of equivalent value. That’s not a great explanation, so let me try to give an example: If I have a list of lists grouped by element value. ghci> group [1,1,1,1,2,2,2,2,3,3,2,2,2,5,6,2,2,7] [[1,1,1,1],[2,2,2,2],[3,3],[2,2,2],[5],[6],[2,2],[7]] I would like to take the subset of the outer list containing only the longest of the inner lists for any particular element. So for this particular example, the desired output would be: [[1,1,1,1],[2,2,2,2],[3,3],[5],[6],[7]] The lists [2,2,2] and [2,2] would be removed because they’re shorter than [2,2,2,2]. Any thoughts on how to do this would be appreciated. Thanks and best regards, James

3 4

problems loading zeromq
by fa-ml 09 Feb '14

09 Feb '14

Hello list, I am trying to cabal install hbro. After a bit of fiddling I stumbled on this: Resolving dependencies... Configuring hbro-1.1.2.1... Building hbro-1.1.2.1... Preprocessing library hbro-1.1.2.1... [ 1 of 19] Compiling Paths_hbro ( dist/build/autogen/Paths_hbro.hs, dist/build/Paths_hbro.o ) [ 2 of 19] Compiling Hbro.Keys ( Hbro/Keys.hs, dist/build/Hbro/Keys.o ) Loading package ghc-prim ... linking ... done. [omissis] Loading package semigroups-0.12.2 ... linking ... done. Loading package zeromq3-haskell-0.5.1 ... linking ... ghc: /home/user/.cabal/lib/zeromq3-haskell-0.5.1/ghc-7.4.1/HSzeromq3-haskell-0.5.1.o: unknown symbol `zmq_ctx_new' ghc: unable to load package `zeromq3-haskell-0.5.1' cabal: Error: some packages failed to install: hbro-1.1.2.1 failed during the building phase. The exception was: ExitFailure 1 I am puzzled because I cabal installed zeromq3-haskell *and* downloaded/installed 0mq3 (./configure; make; su; make install). 0mq version is 3.2.5 and I am running debian/stable(i386). Any idea on what is happening here? Thanks in advance -F

1 0

Problem with GADTs and Parsec
by Lorenzo Tabacchini 09 Feb '14

09 Feb '14

Hello, I am developing a very simple programming language for didactic purposes. Expressions can be composed of different types, like Int, Bool, String etc... The AST is compiled to JavaScript and the result is shown in the browser. The code is more or less like this: {-# LANGUAGE GADTs #-} data Expr a where Int :: Int -> Expr Int Bool :: Bool -> Expr Bool Text :: String -> Expr String -- Int operations Add :: Expr Int -> Expr Int -> Expr Int -- Bool operations And :: Expr Bool -> Expr Bool -> Expr Bool -- etc. etc. -- converts to JavaScript compile :: Expr a -> String compile (Int n) = show n compile (Text t) = show t compile (Add x y) = compile x ++ "+" ++ compile y -- etc. etc. The compilation works perfectly, but I am stuck with the parser. Since I don't know the types in advance, I would like to parse the whole expression at once. If I do: exprParser :: Parsec String u (Expr a) exprParser = parens exprParser <|> (reserved "true" >> return (Bool True)) <|> (reserved "false" >> return (Bool False)) <|> (stringLiteral >>= return . Text) The compiler says: "Couldn't match type `Bool' with `[Char]'" Where am I wrong?

4 4

Getting GHC 7.8.1
by Mathew Phillips 08 Feb '14

08 Feb '14

Is there a recommended way to install a newer version of ghc when you already have the haskell platform installed? I've been reading the documentation and I can't seem to find anything that shows how to actually get it other than compiling from source, and I don't want to brick my current setup if both cant coexist. Matt P.

2 1