Thomas Hartman wrote:
-- (myfoldl f q ) is a curried function that takes a list -- If I understand currectly, in this "lazy" fold, this curried function isn't applied immediately, because -- by default the value of q is still a thunk myfoldl f z [] = z myfoldl f z (x:xs) = ( myfoldl f q ) xs where q = z `f` x
Sorry to say "this curried function isn't applied immediately" is wrong. The curried function is applied immediately. This is independent of what happens to q. q remains a thunk. myfoldl f q xs moves on immediately. Next iteration's z is this iteration's q, and remains a thunk too. It is also noteworthy that a b c d = (a b c) d = ((a b) c) d They are syntactic sugar of each other.
-- here, because of the definition of seq, the curried function (myfoldl' f q) is applied immediately -- because the value of q is known already, so (myfoldl' f q ) is WHNF myfoldl' f z [] = z myfoldl' f z (x:xs) = seq q ( myfoldl' f q ) xs where q = z `f` x
The seq causes q to become WHNF. This is independent of what happens to (myfoldl' f q). In general in many compilers seq x y digresses to reduce x to WHNF, then "we now return you to the scheduled programming of whatever should happen to y".
--same as myfoldl' myfoldl'' f z [] = z myfoldl'' f !z (x:xs) = ( myfoldl'' f q ) xs where q = z `f` x
This is not the same as myfoldl'. This does not reduce q to WHNF - not now. Instead, z is reduced to WHNF now. As for q, this iteration's q becomes the next iteration's z, so this iteration's q will become WHNF next iteration. It is unusual to observe any difference because usually one experiments with number crunching only, where no one cares whether evaluation is one iteration early or late. However, this operator will show the difference. mydisj _ True = True mydisj True False = True mydisj False False = False myfoldl' mydisj (error "bottom") [True] --> True myfoldl'' mydisj (error "bottom") [True] --> exception: bottom
myfoldl''' f z [] = z myfoldl''' f z (x:xs) = (myfoldl''' f $! q) xs where q = z `f` x
This is the same as myfold'. myfold' and myfold''' are the same as what's on the wiki.