On Oct 19, 2006, at 12:51 PM, David House 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 other words, the computation should go
something like this:
I wondered this too for a minute. I'm pretty sure that the answer is
that the 'length' function is the culprit, not (==).
Calling 'length' forces the spine of 'xs', which accounts for the
extra computation.
Just say 'no' to length (when you want laziness).
[snip]
--
-David House, [EMAIL PROTECTED]
Rob Dockins
Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
-- TMBG
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