[Haskell-beginners] function defenition. Do I understand it right?

[Roelof Wobben]

----------------------------------------
> Date: Tue, 12 Jul 2011 09:15:45 -0400
> From: byorgey at seas.upenn.edu
> To: beginners at haskell.org
> Subject: Re: [Haskell-beginners] function defenition. Do I understand it right?
>
> On Tue, Jul 12, 2011 at 11:43:10AM +0000, Roelof Wobben wrote:
> >
> >
> > Oke,
> >
> >
> >
> > So I changed everything to this :
> >
> >
> >
> >
> > halve (xs) | null xs = ([],[])
>
> You do not need this case. ([], []) is what halve [] already would
> have returned even without this case: length [] == 0, so it would
> evaluate to (take 0 [], drop 0 []) which is ([], []).
>
> > | length xs `mod` 2 == 0 = (take n xs, drop n xs)
> > | otherwise = (take (n+1) xs, drop (n+1) xs)
> > where n= length xs `div` 2
> >
> >
> > main = do
> > putStrLn $ show $ halve [1,2,3,4]
> > putStrLn $ show $ halve [1,2,3]
> > putStrLn $ show $ halve []
> >
> >
> >
> > But now I see this message :
> >
> >
> >
> > Error occurred
> > ERROR line 7 - Unresolved top-level overloading
> > *** Binding : main
> > *** Outstanding context : Show b
>
> This message is just because it cannot figure out the type of [] in
> 'putStrLn $ show $ halve []'. You can write
>
> putStrLn $ show $ halve ([] :: [Int])
>
> to give it an explicit type. It's a bit annoying since we happen to
> know that the type of the list makes no difference, but the compiler
> can't figure that out.
>
> -Brent
>
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Hello, 

 

Problem solved. See this url: http://codepad.org/jMPCO1UE

 

I have tested the difference with  [int] and [Char].

With Int You get this output ([],[]) and with Char this one ["",""]

 

Everyone thanks for the help and patience.

 

Roelof