[GHC] #8968: Pattern synonyms and GADTs

[GHC]

#8968: Pattern synonyms and GADTs
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        Reporter:  kosmikus                   |            Owner:
            Type:  bug                        |           Status:  new
        Priority:  normal                     |        Milestone:
       Component:  Compiler (Type checker)    |          Version:
      Resolution:                             |  7.8.1-rc2
Operating System:  Unknown/Multiple           |         Keywords:
 Type of failure:  GHC rejects valid program  |     Architecture:
       Test Case:                             |  Unknown/Multiple
        Blocking:                             |       Difficulty:  Unknown
                                              |       Blocked By:
                                              |  Related Tickets:
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Comment (by simonpj):

 > I could just pretend that `P` is another constructor of `T`:
 > {{{
 > data T a b where
 >   MkT :: a -> T a Bool
 >   MkX :: T a b
 >   P   :: T Bool Bool
 > }}}
 Alas, no.  If the definition of T really did have that constructor P, then
 GADT-style type refinement would take place when you match on P.  So for
 example, this would typecheck:
 {{{
 f :: T a b -> a -> Bool
 f P v = v && True
 }}}
 But obviously the expansion does not:
 {{{
 f :: T a b -> a -> Bool
 f (MkT x) v = v && True
 }}}
 Moreover, these are terribly simple examples.  In general a pattern
 synonym might mention dozens of constructor from dozens of types.  Which
 of those types would you like to pretend the pattern synonym constructor
 is from?

 This is what I mean by modularity: no matter how complicated the right
 hand side of the pattern synonym, I want the client (of the library
 defining the pattern synonym) to be able to use it by knowing only its
 type -- just like he can for a function exported by a library.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8968#comment:13>
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