The difference is that foldl *must* produce the entire list of thunks, even if f is lazy in its first argument. There's no foldl that can perform better given a sufficiently-lazy f; given head = foldr go fail where go x y = x fail = error "head: empty list" head [a,b,c,d] = foldr go fail [a,b,c,d] = go a (foldr go fail [b,c,d]) = a you might think you can define last = foldl go fail where go x y = y fail = error "last: empty list" but this fails to be sufficiently lazy: last [a,b,c,d] = foldl go fail [a,b,c,d] = foldl go (go fail a) [b,c,d] = foldl go (go (go fail a) b) [c,d] = foldl go (go (go (go fail a) b) c) [d] = foldl go (go (go (go (go fail a) b) c) d) [] = go (go (go (go fail a) b) c) d = d which allocates lots of extra space for thunks, which may even take more memory than the original list. Whereas if last uses foldl': last [a,b,c,d] = foldl' go fail [a,b,c,d] = let z = go fail a in seq z $ foldl' go z [b,c,d] = foldl' go a [b,c,d] = let z = go a b in seq z $ foldl' go z [c,d] = foldl' go b [c,d] = ... = let z = go c d in seq z $ foldl' go z [] = foldl' go d [] = d -- ryan