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

[Roelof Wobben]

Oke, 

 

So I changed everything to this : 

 


halve (xs) | null 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]
  putStrLn $ show $ halve []

 

But now I see this message :

 

Error occurred
ERROR line 7 - Unresolved top-level overloading
*** Binding             : main
*** Outstanding context : Show b


 

Roelof

 

________________________________
> Date: Tue, 12 Jul 2011 10:57:04 +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 
> 
> Yes, you are totally correct. 
> 
> your guard for length zero should be ([],[]). The type of the function is 
> 
> halve :: [a] -> ([a], [a]) 
> 
> so all the code paths have to finish with that type. 
> 
> Incidentally taking the length of a linked list forces you to walk the 
> entire list. You are better off checking as to whether it is empty or 
> not. 
> 
> Regards, 
> Ben 
> 
> On 12 July 2011 10:44, Roelof Wobben 
> <rwobben at hotmail.com<mailto:rwobben at hotmail.com>> wrote: 
> 
> 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<mailto:edwards.benj at gmail.com> 
> > To: rwobben at hotmail.com<mailto:rwobben at hotmail.com> 
> > CC: beginners at haskell.org<mailto: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><mailto: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><mailto: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><mailto:beginners at haskell.org<mailto:beginners at haskell.org>> 
> > > To: 
> rwobben at hotmail.com<mailto:rwobben at hotmail.com><mailto: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? 
> > _______________________________________________ 
> > Beginners mailing list 
> > 
> Beginners at haskell.org<mailto:Beginners at haskell.org><mailto:Beginners at haskell.org<mailto:Beginners at haskell.org>> 
> > http://www.haskell.org/mailman/listinfo/beginners 
> > 
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