[GHC] #14855: Implementation of liftA2 for Const has high arity

[GHC]

#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:
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 {{{
 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.Const.html#line-69


 `(<*>)` 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>
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