ちゃんと書くとこんな感じになる

primes = 2:(section [3..] composites)
　where
　　composites = union [multiples p |p<-primes]
　　multiples n = map (n*) [n..]

section (x:xs) (y:ys)
　| x < y = x : (section xs (y:ys))
　| x == y = section xs ys
　| x > y = section (x:xs) ys

union = foldr merge []
　where
　　merge (x:xs) ys = x:merge' xs ys
　　merge' (x:xs) (y:ys)
　　　| x < y = x : merge' xs (y:ys)
　　　| x == y = x : merge' xs ys
　　　| x > y = y : merge' (x:xs) ys