[GHC] #10634: Type class with bijective type functions

[GHC]

#10634: Type class with bijective type functions
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              Reporter:  Lemming     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 See the attached module.
 {{{
 $ cat TypeFunctionBijection.hs
 {-# LANGUAGE TypeFamilies #-}
 module TypeFunctionBijection where

 import Data.Int (Int8, Int16, Int32)

 type family Up a
 type instance Up Int8  = Int16
 type instance Up Int16 = Int32

 class (Up (Down a) ~ a) => Convert a where
    type Down a
    down :: a -> Down a

 instance Convert Int16 where
    type Down Int16 = Int8
    down = fromIntegral

 instance Convert Int32 where
    type Down Int32 = Int16
    down = fromIntegral

 x :: Int8
 x = down 8
 }}}
 {{{
 $ ghci-7.8.4 -Wall TypeFunctionBijection.hs
 GHCi, version 7.8.4: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 [1 of 1] Compiling TypeFunctionBijection ( TypeFunctionBijection.hs,
 interpreted )
 Ok, modules loaded: TypeFunctionBijection.
 *TypeFunctionBijection> :q
 Leaving GHCi.
 }}}
 {{{
 $ ghci-7.10.1 -Wall TypeFunctionBijection.hs
 GHCi, version 7.10.1: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling TypeFunctionBijection ( TypeFunctionBijection.hs,
 interpreted )

 TypeFunctionBijection.hs:24:5:
     Couldn't match expected type ‘Int8’ with actual type ‘Down a0’
     The type variable ‘a0’ is ambiguous
     In the expression: down 8
     In an equation for ‘x’: x = down 8
 Failed, modules loaded: none.
 Prelude> :q
 Leaving GHCi.
 }}}

 Up to GHC-7.8.4 I could make a type function like `Down` a bijection by
 adding equality constraints to the `Convert` class.
 In GHC-7.10.1 this fails.
 Is this a bug or a feature?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10634>
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