Re: [GHC] #11732: Deriving Generic1 interacts poorly with TypeInType

29 Mar 2016

      #11732: Deriving Generic1 interacts poorly with TypeInType
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:  TypeInType,
                                     |  Generics
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 RyanGlScott):

 Replying to [comment:13 goldfire]:
...
This all means we can just do nothing about this particular issue. Which
 is nice.
That is nice!

 What then, should we do about this program? (from
 [https://mail.haskell.org/pipermail/ghc-devs/2016-March/011645.html here])

 {{{#!hs
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TypeInType #-}
 module Cat where

 import Data.Kind

 class Cat k (cat :: k -> k -> *) where
   catId   :: cat a a
   catComp :: cat b c -> cat a b -> cat a c

 instance Cat * (->) where
   catId   = id
   catComp = (.)

 newtype Fun a b = Fun (a -> b) deriving (Cat k)
 }}}

 Currently, this generates an instance of the form `instance Cat * Fun`
 (i.e., the same thing as if you had written `deriving (Cat *)`. Would this
 be acceptable?

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11732#comment:14
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler