[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 dfeuer):

 Replying to [comment:34 simonpj]:
 > Let's not think about implementation before we have a ''design''.   I
 have not read the entire thread again, but I'm pretty convinced that
 >
 > * We can't have two different types, one for construction and one for
 pattern matching
 >
 > I think it'll just be too confusing to have two types.  It's bad enough
 to have this provided/required stuff without, in addition, having a
 completely separate type for construction.  Are you seriously proposing to
 have two signatures for each pattern synonym?  (Optionally, I assume.)

 I respectfully disagree. Pattern synonyms are not, and likely will never
 be, written by many beginners. And very few programmers are likely to need
 to write terribly many of them. I think, therefore, that making the type
 checker enforce some sort of "reasonableness" on them is a considerably
 lower priority than making them powerful enough to do what librarians need
 them to do. I ran into this yesterday writing a pattern synonym for Edward
 Yang's `NF` type (in the `nf` package), which needs a constraint on
 construction but not on matching.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8581#comment:37>
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