[GHC] #14112: bang patterns on pattern synonyms? (left vs right hand sides)

[GHC]

#14112: bang patterns on pattern synonyms? (left vs right hand sides)
-------------------------------------+-------------------------------------
        Reporter:  carter            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      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:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Ryan has it right. The syntax is
 {{{
 pattern P x y z = <pattern>
 }}}
 Hence bangs are allowed. For the implicitly-bidirectional case we have to
 turn the `<pattern>` into an expression.  Currently, that conversion
 simply discards the bangs, which is the behaviour you are seeing.
 Silently ignoring the bangs is arguably rather undesirable.  (The
 conversion also silently ignores a `~`, but that's probably correct
 behaviour.)

 It would make more sense to do one of these:

 A. Reject an implicitly-bidirectional pattern with bangs, on the grounds
 that it's not invertible (like view patterns, say); suggest using an
 explicitly-bidirectional pattern instead.

 B. Auto-generate the bang'd definition that Ryan writes above.
 {{{
   MkPair1 !x !y = MkPair x y
 }}}
   That is, in the `ImplicitBidirectional` case, if we see `!x` in the
 pattern, use a `!x`, instead of `x` as the argument pattern of the
 builder.

 I think I prefer (B).   It's easy to implement: see `mk_match_group` in
 `TcPatSyn.tcPatSynBuilderBind` .  You'd need to alter `tcPatToExpr` to
 return the banged variables.  (Ignore bangs on constructors e.g. `Just
 !(Just x)`; they are no-ops.)

 Don't forget to specify all this in the user manual.

 Volunteers?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14112#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler