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Date: Tue, 12 Jul 2011 09:15:45 -0400 From: byorgey@seas.upenn.edu To: beginners@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