[GHC] #8582: Record syntax for pattern synonyms

[GHC]

#8582: Record syntax for pattern synonyms
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        Reporter:  cactus            |                   Owner:  mpickering
            Type:  feature request   |                  Status:  new
        Priority:  high              |               Milestone:  8.0.1
       Component:  Compiler          |                 Version:
      Resolution:                    |                Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:  5144              |                Blocking:
 Related Tickets:                    |  Differential Revisions:  Phab:D1152
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Comment (by mpickering):

 I think the confusion here is that pattern synonyms is an undescriptive
 name for the feature. I prefer to think of (bidirectional) pattern
 synonyms as data constructors which are not (yet) associated with a
 particular type.

 It is better then to say that instead of "In general, a pattern synonym
 should behave exactly like its expansion." that "In general, a pattern
 synonym should behave exactly like the relevant data constructor". For
 example, a bidirectional prefix pattern synonym should behave like a
 prefix data constructor, an infix pattern synonym should behave like an
 infix data constructor and a record pattern synonym should behave like a
 record data constructor.

 When we introduce records the expansion idea falls apart a bit. For normal
 prefix pattern synonyms there is one way to pattern match and one way to
 construct (which matches the expansion). With records there are a few ways
 to pattern match, a few ways to construct and also the possibility to
 update. This means the syntax has to diverge from the expansion idea as
 which expansion do we choose?

 If we define a synonym `P` and datatype `Q` as follows,

 {{{#!hs

 pattern MkP :: Int -> Int -> Q
 pattern MkP{x, y} = MkQ x1 y1

 data Q = MkQ { x1 :: Int, y1 :: Int }
 }}}

 then we expect `MkP` to behave precisely as `MkQ` modulo field naming. To
 be clear these situations are as follows.

 * Construction (`MkP 0 0`)
 * Construction with record syntax (`MkP { x = 0, y = 1 }`)
 * Matching (`foo (MkP c l) = ...`)
 * Matching with normal record syntax (`foo (MkP {x = l, y = c}) = ...`)
 * Matching with field puns (`foo (MkP {x, y}) = ...`)
 * Updating with record syntax (`(MkP 0 0) { x = 1 }`)
 * Using record selectors (`x (MkP 0 0)`)

 For a unidirectional synonym, we define selectors and the matching part
 but not updates or construction.

 Is that clearer? I think the best specification for this feature is in
 terms of ordinary records as the goal is to get as close as possible to
 normal record data constructors.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8582#comment:12>
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