[GHC] #9085: Inaccessible equations in a closed type family should be a warning, not an error

7 May 2014

      #9085: Inaccessible equations in a closed type family should be a warning, not an
error
------------------------------------+-------------------------------------
       Reporter:  goldfire          |             Owner:  goldfire
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 If I say

 {{{
 foo _     = ()
 foo False = ()
 }}}

 I get

 {{{
 /Users/rae/temp/Bug.hs:2:1: Warning:
     Pattern match(es) are overlapped
     In an equation for ‘foo’: foo False = ...
 }}}

 But, if I say

 {{{
 {-# LANGUAGE TypeFamilies #-}

 type family Foo a where
   Foo x    = ()
   Foo Bool = ()
 }}}

 I get

 {{{
 /Users/rae/temp/Bug.hs:5:3:
     Inaccessible family instance equation:
       Foo Bool = ()
     In the equations for closed type family ‘Foo’
     In the type family declaration for ‘Foo’
 }}}

 If this issues a warning at the term level, it probably should do the same
 at the type level.

 This was my design decision that I'm changing, and I will fix. The warning
 will be controlled by the same flag that controls the term-level version.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9085
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