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

[Roelof Wobben]

Oke, 

 

I have now this as function definition.

 



halve (xs) | 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]
  

 

this one works except for empty lists.

So I thought this would work .

 

halve (xs) | length xs == 0 = []

           | 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 

 

 

but then I see this error : 

 

 

Error occurred
ERROR line 2 - Type error in guarded expression
*** Term           : (take n xs,drop n xs)
*** Type           : ([b],[b])
*** Does not match : [a]

 

So I assume that a function must always have the same output and can't have 1 or 2 lists as output.

 

Is this the right assumption.

 

Roelof


________________________________
> Date: Tue, 12 Jul 2011 10:34:25 +0100 
> Subject: Re: [Haskell-beginners] function defenition. Do I understand 
> it right? 
> From: edwards.benj at gmail.com 
> To: rwobben at hotmail.com 
> CC: beginners at haskell.org 
> 
> I don't even understand what you are trying to do :) 
> 
> if you want to pattern match on the empty list 
> 
> foo :: [a] -> [a] 
> foo [] = 0 
> foo (x:xs) = undefined 
> 
> if you want to use the guard syntax 
> 
> foo xs | null xs = 0 
> | otherwise = undefined 
> 
> 
> Ben 
> 
> On 12 July 2011 10:02, Roelof Wobben 
> <rwobben at hotmail.com<mailto:rwobben at hotmail.com>> wrote: 
> 
> 
> 
> hello 
> 
> 
> 
> Everyone thanks for the help. 
> 
> I'm now trying to make this work on a empty list. 
> 
> 
> 
> But my question is. 
> 
> When the definition is : 
> 
> 
> 
> [a] -> [a] [a] 
> 
> 
> 
> Is it correct that I don't can use. 
> 
> 
> 
> length xs = 0 | [] 
> 
> 
> 
> Roelof 
> 
> ---------------------------------------- 
> > Subject: Re: [Haskell-beginners] function defenition. Do I understand 
> it right? 
> > From: d at vidplace.com<mailto:d at vidplace.com> 
> > Date: Mon, 11 Jul 2011 18:56:42 -0400 
> > CC: beginners at haskell.org<mailto:beginners at haskell.org> 
> > To: rwobben at hotmail.com<mailto:rwobben at hotmail.com> 
> > 
> > On Jul 11, 2011, at 5:13 PM, Roelof Wobben wrote: 
> > 
> > > What I try to achieve is this: 
> > > 
> > > 
> > > 
> > > [1,2,3,4] will be [1,2] [3,4] 
> > > 
> > > [1,2,3,4,5] will be [1,2,3] [3,4,5] 
> > 
> > So, I think what you want is this http://codepad.org/kjpbtLfR 
> > 
> > Is that correct? 
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