[GHC] #8779: Exhaustiveness checks for pattern synonyms

[GHC]

#8779: Exhaustiveness checks for pattern synonyms
-------------------------------------+-------------------------------------
        Reporter:  nomeata           |                Owner:  mpickering
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.8.1
  checker)                           |             Keywords:
      Resolution:                    |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2669
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by mpickering):

 Replying to [comment:53 goldfire]:
 > Replying to [comment:46 mpickering]:
 >
 > Thanks for this detailed reply! I'm happy with all your responses save
 one:
 >
 > > >  - The spec says "We verify that the result types of each
 constructor in a complete match agrees with each other." What does this
 sentence mean? Consider the possibility of pattern synonyms that match
 against functions, polymorphic patterns, and possibly even higher-rank
 patterns.
 > > >
 > >
 > > It means that we look at the result type of each conlike and then
 verify that the type constructor for each type is the same. In the case of
 a set containing polymorphic patterns, at least one pattern in the set
 must have a definite type or you must specify a type signature to fix the
 type for the whole set.
 >
 > I'm not sure what you mean here. Where do you specify the type
 signature? In the `COMPLETE` pragma?


 I updated the wiki page now with lots of examples.

 You can specify something like..

 {{{#!hs
 pattern P :: Maybe a
 pattern P = Nothing

 {-# COMPLETE P :: Maybe #-}
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8779#comment:55>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler