[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:  (none)
            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 AntC):

 Replying to [comment:51 AntC]:
 > Pattern synonym used in an expression context could act as a smart
 constructor; and be used in a pattern context just for matching.
 >

 Ah, I see I can already do that. But that wasn't at all clear from the
 docos. So ignore comment:51, here's some notes to improve
 [https://downloads.haskell.org/~ghc/8.2.1/docs/html/users_guide/glasgow_exts.html
 #pattern-synonyms the User Guide] -- which mentions a wiki page, but I
 can't find that. Does it mean the implementation notes linked from
 comment:24?

 To make a smart constructor:
 {{{
 data Dumb = Dumb Int
 pattern Smart { nonneg } <- Dumb nonneg   where
   Smart nonneg = if nonneg >= 0 then (Dumb nonneg) else error "Smart
 constructor called on negative"

 }}}
 Things I noticed:
 * The User Guide syntax for explicit bidirectionals is not quite right: it
 says the lhs's are both `pat_lhs`, but you can only use Record syntax on
 the first line with `<-`; the second line with `=` must use Prefix or
 Infix.
 * You can put an arbitrary `expr` on rhs of the `=`. The User Guide says
 that, but gives no examples other than Data constructors.
 * On the rhs of `=`, the pattern/constructor doesn't have to be the same
 as on the `<-` line. Indeed you can go

 {{{
 data PosNeg = Pos Int | Neg Int
 pattern Smarter{ nonneg } <- Pos nonneg  where
   Smarter x = if x >= 0 then (Pos x) else (Neg x)

 }}}

 And I guess that possibility is why you can't use Record syntax on lhs of
 the `=`: you don't know which combo of field names applies until you know
 which data constructor you're producing.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8581#comment:52>
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