[GHC] #12426: Allow smart constructors their own types

[GHC]

#12426: Allow smart constructors their own types
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           Reporter:  dfeuer         |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 GHC currently offers one-way pattern synonyms allowing general
 deconstruction with nice syntax. And it offers bidirectional pattern
 synonyms, which toss in construction functions that start with capital
 letters. Unfortunately, it forces the construction and deconstruction
 sides to share a type. This is sometimes rather unfortunate. The
 documentation itself points out the limitation with regard to matching
 numeric literals (where deconstruction requires `Eq`). Another example is
 Edward Yang's `NF` type, whose smart constructor (but not deconstructor)
 requires `NFData`. I think the easiest fix is to allow a type signature in
 the `where` clause of a pattern synonym definition.

 {{{#!hs
 pattern X :: Eq a => a -> Foo
 pattern X q <- ... where
   X :: X a => a -> Foo
   X a = ...
 }}}

 Ideally, I'd like to offer the other one-way (constructor without pattern)
 as well, but I care less about that.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12426>
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