[GHC] #13957: Allow deriving multiparameter type classes with representationally equal arguments

[GHC]

#13957: Allow deriving multiparameter type classes with representationally equal
arguments
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by Iceland_jack:

@@ -41,1 +41,1 @@
-     SIEVE I
+     (SIEVE I)

New description:

 This currently works

 {{{#!hs
 {-# Language DerivingStrategies, MultiParamTypeClasses,
 FunctionalDependencies, GeneralizedNewtypeDeriving #-}

 import Data.Kind
 import Data.Functor.Identity

 class SIEVE f p | p -> f where
   sIEVE :: p a b -> a -> f b

 instance SIEVE Identity (->) where
   sIEVE = (Identity .)

 newtype ARR a b = ARR (a -> b)
   deriving newtype
     (SIEVE Identity)
 }}}

 But what if I want a `Sieve I ARR` instance, for `newtype I a = I a`
 (which is representationally equal to `newtype Identity a = Identity a`)?

 {{{#!hs
 {-# Language DerivingStrategies, MultiParamTypeClasses,
 FunctionalDependencies, GeneralizedNewtypeDeriving, RankNTypes,
 TypeApplications, ScopedTypeVariables, InstanceSigs #-}

 import Data.Kind
 import Data.Functor.Identity
 import Data.Coerce

 class SIEVE f p | p -> f where
   sIEVE :: p a b -> a -> f b

 instance SIEVE Identity (->) where
   sIEVE = (Identity .)

 newtype ARR a b = ARR (a -> b)
   deriving newtype
     (SIEVE I)

 newtype I a = I a
 }}}

 generating the following code (this is basically to code generated before,
 except replacing some `Identity`s with `I`s

 {{{#!hs
 instance SIEVE I ARR where
   sIEVE :: forall a b. ARR a b -> (a -> I b)
   sIEVE
     = coerce
       @((a -> b) -> a -> Identity b)
       @(ARR a b  -> a -> I b)
       sIEVE
 }}}

 GHC should be able to recover `Identity` due to the functional dependency.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13957#comment:2>
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