On Tue, 14 Oct 2003 17:05:03 +0100
Graham Klyne
I'm trying to use foldl or foldr to run a computation over a list, in such a way that only as much of the list as may be needed is actually examined.
I believe the 'and' function behaves like this (e.g. see testand below), but when I use a function that performs pairwise operations on the list elementsa, it seems that I always end up constructing the entire list, even if I don't need to examine the value of each element. I guess I could try and figure out my own 2-place folding function, but this seems as if there should be a common solution.
My test code is below. The tests are evaluated as: test allSame1 or test allSame2 to use a foldl- or foldr-versions of the function respectively
If I comment out the indefinitely long list case (test7) in the definition of 'test', the foldl version will pass all these test cases and foldr throws an error on test6. But if I include test7, using Hugs the foldr test case returns a C stack overflow, and the foldl case silently disappears.
foldl never works on infinite lists. foldr isn't tail recursive, so you don't use it with strict functions if you are expecting large input. With lazy (in the second argument) functions it's appropriate. You may also want to look at foldM. Instantiated for an exception-like monad (Maybe,Either,etc.), you can effectively "throw" a result (or lack thereof for Maybe).