[clean-list] Newbie Question

[Diederik van Arkel]

On Thursday, September 19, 2002, at 12:07 AM, Hal Daume III wrote:

> I think the general solution is to report and error if passed an empty
> list.  Otherwise, you would need to know the infimum of the type you 
> are
> talking about.  You could introduce a class to deal with this, but 
> that's
> probably too complex.
>
> --
> Hal Daume III
>
>  "Computer science is no more about computers    | hdaume@isi.edu
>   than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume
>
> On Wed, 18 Sep 2002, Wilfried Esken wrote:
>
>> Hello,
>> I tried to learn CLEAN and therefore I read the exercises in the 
>> Clean Book.
>> First I made a function MaxofList which should return the maximum 
>> value of a
>> list.
>>
>> I tried it this way:
>>
>> maxi :: a a -> a | <, == a
>> maxi a b
>> 	| a <= b = b
>> 	| otherwise = a
>>
>> MaxofList [] = 0
>> MaxofList [x] = x
>> MaxofList [x,y] = maxi x y
>> MaxofList [first:rest] =
>> 	maxi a b
>> 	where a = first
>> 	      b = MaxofList rest
>>
>>
>> This works fine for a list of integers, but I want to have a function 
>> like
>>
>> MaxofList :: [a] -> a
>>
>> This doesn't work because of
>> 	MaxofList [] = 0,
>> the compiler derives type Int for the function.
>> What is the solution for having a generell typed function in this 
>> case?
>>
>> Thank you,
>> Willi
>>
>>
Hal's suggestion is what is implemented in the StdEnv (see maxList in 
StdOrdList).
For your version replace the int 0 by the class zero as follows:

zmaxList::!.[a] -> a | Ord a & zero a
zmaxList [a:x] = max1 a x
where
	max1:: a !.[a] -> a | Ord a
	max1 m [hd:tl]
		| hd<m
			= max1 m tl
			= max1 hd tl
	max1 m []		= m
zmaxList []
	= zero

Regards,

Diederik