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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?

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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