[GHC] #5144: Pattern synonyms

[GHC]

#5144: Pattern synonyms
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        Reporter:  simonpj           |            Owner:  cactus
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:  _|_
       Component:  Compiler          |          Version:
      Resolution:                    |         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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Comment (by cactus):

 Exports now work with the following syntax:

 {{{
 module Foo (pattern P) where

 pattern P x y = (x, y, 42)
 }}}

 This exports `P` both as a pattern and as an expression (if it's
 bidirectional, like in the case here)

 The extra `pattern` keyword is needed because pattern names live in the
 constructor namespace, so it's perfectly valid to have

 {{{
 data P  = C
 pattern P = ()
 }}}

 (due to entirely non-interesting reasons, the implementation of exporting
 is not yet pushed to https://github.com/gergoerdi/ghc/tree/pattern-
 synonyms)

 Remaining work to do:

 * Sort out recursive pattern definitions. Currently these are rejected
 implicitly by an internal GHC error, not exactly ideal... also, non-
 recursive usages of pattern synonyms mentioning each other don't work, but
 I see no reason why we wouldn't want to allow it:

 {{{
 pattern P1 = P2
 pattern P2 = ()
 }}}

 * The typechecker for pattern synonym definitions is wrong: it returns
 monomorphic types (e.g. for `pattern Head x = x:_`, it infers type `t ->
 [t]` instead of `forall a. a -> [a]`. This is worked around by a horrible
 hack when typechecking pattern synonym usages in patterns. Richard
 Eisenberg and Simon Peyton Jones have already offered to help with this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5144#comment:20>
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