On 19/10/06, David House <[EMAIL PROTECTED]> wrote:
On 19/10/06, Mikael Johansson <[EMAIL PROTECTED]> wrote:
> isIdentity xs = all (\(i,j) -> i==j) (zip [1..] xs)
> isIdentity' xs = xs == [1..(length xs)]
>
> Then
> isIdentity 1:3:2:[4..100000]
> finishes in an instant, whereas
> isIdentity' 1:3:2:[4..100000]
> takes noticable time before completing.
Why is this so? I'd have thought that the equality function for lists
only forces evaluation of as many elements from its arguments as to
determine the answer.
In order to determine if [1..length xs] has an element at all, you
have to evaluate length xs, which involves forcing the entire spine of
xs, because integers can't be partially evaluated. Computing lengths
of lists is a great way to introduce strictness.
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
