How to set strictness in list comprehension?

Question

I'm bit stuck how to rewrite following strict-evaluated list comprehension to use seq instead of bang pattern:

zipWith' f l1 l2 = [ f e1 e2 | (!e1, !e2) <- zip l1 l2 ]

Any idea ?

I've tried

zipWith' f l1 l2 = [ e1 `seq` e2 `seq` f e1 e2 | (e1, e2) <- zip l1 l2 ]

but this unfortunately does not force an evaluation to WHNF.

Answer 1

You can mechanically translate bang patterns into calls to seq following the GHC manual:

This:

zipWith' f l1 l2 = [ f e1 e2 | (!e1, !e2) <- zip l1 l2 ]

Becomes Too lazy:

zipWith' f l1 l2 =
    [ f e1 e2
    | e <- zip l1 l2
    , let t = case e of (x,y) -> x `seq` y `seq` (x,y)
    , let e1 = fst t
    , let e2 = snd t
    ]

Which is more concisely written as (too lazy):

zipWith' f l1 l2 =
    [ f e1 e2
    | e <- zip l1 l2
    , let (e1,e2) = case e of (x,y) -> x `seq` y `seq` (x,y)
    ]

Though I'd write it as (wrong, too lazy)

zipWith' f l1 l2 = zipWith (\x y -> uncurry f (k x y)) l1 l2
    where
        k x y = x `seq` y `seq` (x, y)

You could also move the strictness hint to a data structure:

data P = P !Integer !Integer

zipWith' f l1 l2 = [ f e1 e2 | P e1 e2 <- zipWith P l1 l2 ]

Or as:

zipWith' f l1 l2 = [ f e1 e2 | (e1, e2) <- zipWith k l1 l2 ]
    where
        k x y = x `seq` y `seq` (x,y)