GHC doesn't optimize (strict) composition with id

Newtype constructors and selectors have no runtime overhead, but some uses of them do. For example, given newtype Identity a = Identity { runIdentity :: a }, Identity turns into id, but Identity . f turns into id . f, which is distinct from f, because it gets eta-expanded to \x -> f x.

It would be nice to be able to compose a newtype constructor with a function without any overhead. The obvious thing to try is strict composition:

(#) :: (b -> c) -> (a -> b) -> a -> c
(#) f g = f `seq` g `seq` \x -> f (g x)

In theory this should get rid of the eta-expansion. In practice, the generated Core looks like this:

foo :: (a -> b) -> [a] -> [b]
foo f = map (id # f)
-- becomes
foo = \f e -> map (case f of g { __DEFAULT -> \x -> g x }) e

Different variations of (#), and turning -fpedantic-bottoms on, don't seem to affect this. A simpler version, foo f = map (f seq \x -> f x), generates the same sort of Core.

In one library we resorted to defining a bunch of functions of the form identityDot :: (a -> b) -> a -> Identity b; identityDot = unsafeCoerce. It would be better to be able to rely on GHC to do the optimization directly, if we use strict composition anyway.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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