Re: [GHC] #14661: Cannot derive (newtype I a b = I (F a -> F b) deriving newtype Category) for type family F

11 Jan 2018

      #14661: Cannot derive (newtype I a b = I (F a -> F b) deriving newtype Category)
for type family F
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.4.1-alpha1
      Resolution:                    |             Keywords:
                                     |  DerivingStrategies, deriving,
                                     |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Iceland_jack):

 This can be derived (clunkily) with GND + `singletons`

 {{{#!hs
 import Data.Singletons

 type Cat ob = ob -> ob -> Type

 newtype WRAP :: (k' ~> k) -> (Cat k -> Cat k) where
   WRAP :: cat (f@@a) (f@@b)
        -> WRAP f cat a b

 instance Category cat => Category (WRAP f cat) where
   id :: forall a. WRAP f cat a a
   id = WRAP (id @cat @(f@@a))

   (.) :: forall b c a. WRAP f cat b c -> WRAP f cat a b -> WRAP f cat a c
   WRAP f . WRAP g = WRAP ((.) @cat @(f@@b) @(f@@c) @(f@@a) f g)

 data InterpSym0 :: TY ~> Type
 type instance InterpSym0 @@ ty = Interp ty
 }}}

 +

 {{{#!hs
 newtype Ixed a b = Ixed (WRAP InterpSym0 (->) a b)
   deriving newtype Category
 }}}

 or with `-XDerivingVia`

 {{{#!hs
 newtype Ixed a b = Ixed (Interp a -> Interp b)
   deriving Category
     via (WRAP InterpSym0 (->) a b)
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14661#comment:5
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler