#14855: Implementation of liftA2 for Const has high arity -------------------------------------+------------------------------------- Reporter: lyxia | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: | Version: 8.2.2 libraries/base | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{ instance Monoid m => Applicative (Const m) where pure _ = Const mempty liftA2 _ (Const x) (Const y) = Const (x `mappend` y) (<*>) = coerce (mappend :: m -> m -> m) }}} https://hackage.haskell.org/package/base-4.10.1.0/docs/src/Data.Functor.Cons... `(<*>)` is implemented with a `coerce` but `liftA2` isn't. Would the following not have better inlining behavior? {{{ liftA2 _ = coerce (mappend :: m -> m -> m) }}} Going further, should the unused argument also be moved to the RHS? What about `pure`? What are the pros and cons compared to this other alternative: {{{ pure = \_ -> mempty liftA2 = \_ -> coerce (mappend :: m -> m -> m) }}} This came up while implementing `Applicative` for `K1` in phab:D4447. `K1` is essentially the same type as `Const` and thus their instances should be identical for the sake of consistency. Is `Const`'s `Applicative` instance already optimal? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14855 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler