[GHC] #9793: Some as-patterns could be accepted in pattern synonyms

[GHC]

#9793: Some as-patterns could be accepted in pattern synonyms
-------------------------------------+-------------------------------------
       Reporter:  cactus             |                   Owner:  cactus
           Type:  feature request    |                  Status:  new
       Priority:  normal             |               Milestone:  7.10.1
      Component:  Compiler (Type     |                 Version:  7.8.3
  checker)                           |        Operating System:
       Keywords:  pattern synonyms   |  Unknown/Multiple
   Architecture:  Unknown/Multiple   |         Type of failure:
     Difficulty:  Easy (less than 1  |  None/Unknown
  hour)                              |               Test Case:
     Blocked By:                     |                Blocking:
Related Tickets:                     |  Differential Revisions:
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 Currently all as-patterns are rejected in pattern synonym definitions, to
 avoid situations like


 {{{
 pattern P x y <- x@(y:_)
 }}}

 since there's no valid reason to be able to then write something like

 {{{
 f [True] False = ...
 }}}

 this would just lead to confusion.

 However, I think we could accept as-patterns where the body of the as-
 pattern doesn't contain any variable bindings that are accessible via the
 pattern synonym. In other words, this should be OK:

 {{{
 pattern P x <- x@(y:_)
 }}}

 since it's exactly equivalent to

 {{{
 pattern P x <- x@(_:_)
 }}}

 which I don't think is as confusing as the other example.

 I haven't really made up my mind yet if these should be bidirectional; but
 they certainly could be, by just ignoring the body of the as-pattern in
 the wrapper; so the following two would be equivalent:

 {{{
 pattern P1 x = [x@(y:_)]
 pattern P2 x <- [x@(y:_)]
   where
     P2 x = [x]
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9793>
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