Hello, in reformulation of a code with no space leak, the leak reappeares. It takes near constant space to get at the n-th elt in the produced list here (http://ideone.com/fiifl): {-# OPTIONS_GHC -O2 -fno-cse #-} primes = 2 : ([3,5..] `minus` foldi (\(x:xs) -> (x:) . union xs) [[x*x, x*x+2*x..] | x<- ys]) where ys = 3 : ([5,7..] `minus` foldi (\(x:xs) -> (x:) . union xs) [[x*x, x*x+2*x..] | x<- ys]) foldi f (x:xs) = f x (foldi f (pairs f xs)) pairs f (x:y:t) = f x y : pairs f t Here it doesn't (http://ideone.com/m744a): {-# OPTIONS_GHC -O2 -fno-cse #-} primes = 2 : g (fix g) where g = (3:) . ([5,7..] `minus`) . foldi (\(x:xs) -> (x:) . union xs) . map (\x-> [x*x, x*x+2*x..]) I expected the 2nd to be equivalent to the 1st. In Hugs (Nov 2002) the reported stats and run times are very similar for the both versions. Is this a GHC thing, or a language thing? Thanks,