[clean-list] Newbie Question

[Hal Daume III]

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
> 
> 
> _______________________________________________
> clean-list mailing list
> clean-list@cs.kun.nl
> http://www.cs.kun.nl/mailman/listinfo/clean-list
>