[GHC] #8581: Pattern synonym used in an expression context could have different constraints to pattern used in a pattern context

[GHC]

#8581: Pattern synonym used in an expression context could have different
constraints to pattern used in a pattern context
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        Reporter:  cactus            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by cactus):

 Replying to [comment:43 dfeuer]:
 > If we let the constructor have its own signature, can we drop the whole
 required constraints bit? If so, I think that would make things
 considerably less confusing.

 But the required constraints don't come from builders; a unidirectional
 pattern synonym can have just as much of a required context. The simplest
 example I can think of is


 {{{
 pattern P x <- (f -> x)
 }}}

 Here, any constraint of `f` on its argument's type will be a required
 constraint in `P`'s type.

 (Note that a special case of this is matching against overloaded literals,
 e.g. `pattern Z = 0`, which requires `(Num a, Eq a)`.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8581#comment:44>
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