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
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