[GHC] #13571: Injective type family syntax accepted without TypeFamilyDependencies

13 Apr 2017

      #13571: Injective type family syntax accepted without TypeFamilyDependencies
-------------------------------------+-------------------------------------
           Reporter:  jstolarek      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Parser)                           |
           Keywords:  TypeFamilies,  |  Operating System:  Unknown/Multiple
  Injective                          |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This program compiles fine although it names the result type variable,
 which is a piece of syntax that should only be available with
 `TypeFamilyDependencies` (injective type families):

 {{{#!haskell
 {-# LANGUAGE TypeFamilies #-}

 module Foo where

 type family F a = r where
     F a = a
 }}}

 If I say:

 {{{#!haskell
 type family F a = r | r -> a where
     F a = a
 }}}

 only then the definition is rejected.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13571
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler