zipWith does not inline

zipWith is currently defined

zipWith :: (a->b->c) -> [a]->[b]->[c]
zipWith _f []     _bs    = []
zipWith _f _as    []     = []
zipWith f  (a:as) (b:bs) = f a b : zipWith f as bs

For now (I gather that might change soon?), the fact that it's recursive means that it will never inline. But we really want it to inline when applied to a function, for the same reasons we want map to inline. If f is something like const, flip const, or a lazy constructor, it's wasteful to create a thunk to apply it. All three of those cases are common. So unless/until we start inlining recursive functions, we probably want to use

zipWith f = go where
  go [] _ = []
  go _ [] = []
  go (x:xs) (y:ys) = f x y : go xs ys

There will probably be some regressions, of course.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed milestone to %8.4.1

changed weight to 5

added Tbug Trac import core libraries labels

Loopification in the current planned form does not apply here (zipWith is not tail-recursive), so if that is what you referring to: It won’t help here.

The proposed form looks reasonable.

I agree, separating out the recursive bit as proposed sounds quite reasonable.

Would you care to offer a patch? It would be a good idea to mention this is the changelog of base while you are at it.

Trac metadata

Trac field	Value
Differential revisions	- → D3986

mentioned in commit 11d9615e

closed

Thanks sighingnow!

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added runtime perf label

added Pnormal label