On Friday 07 January 2005 12:03, Ketil Malde wrote:
Naive use of foldl. I tend to think the default foldl should be strict (ie. replaced by foldl') -- are there important cases where it needs to be lazy?
Hi, One simple example would be,
reverse = foldl (flip (:)) []
A better example would be building some other "lazy structure" that is strict on it's elements... J.A. ----------------------------------------------- module Test where import Data.List data L = E | !Int :+: L deriving Show -- my head h (x:+:xs) = x h E = error "ops" -- rev1 = foldl (flip (:+:)) E rev2 = foldl' (flip (:+:)) E l = [error "", error "", 1::Int] ---------------------------------------------- *Test> h (rev1 l) 1 (0.00 secs, 264560 bytes) *Test> h (rev2 l) *** Exception: (0.01 secs, 264524 bytes)