[GHC] #15020: PatternSynonyms: Problems with quantified constraints / foralls

[GHC]

#15020: PatternSynonyms: Problems with quantified constraints / foralls
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      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 simonpj):

 Interesting.  Would you expect the following to work?
 {{{
 data Lam2 a = Lam2 (forall mm. Monad mm => mm a)
             | Other
 newtype Forall f = Forall (forall xx. f xx)

 mLam2 :: Forall Lam2 -> r -> ((forall xx mm. Monad mm => mm xx) -> r) -> r
 mLam2 (Forall (Lam2 lam)) d k = k lam
 mLam2 (Forall Other)      d k = d
 }}}
 Here `mLam2` is the sort of matching function that GHC builds for `PLam2`.
 And indeed `mLam2` is rejected because `lam` is not polymorphic enough.

 Is that so unreasonable?

 (I checked why `mkLam3` works; it's because teh pattern matching is pushed
 into the argument of the continuation, something like this
 {{{
 mLam3 :: Forall Lam2 -> r -> ((forall xx mm. Monad mm => mm xx) -> r) -> r
 mLam3 x d k = k (case x of { Forall (Lam2 lam) -> lam })
 }}}
 But this won't work when we want to bind multiple arguments, or when
 the match can fail.)

 I don't see an easy way to improve this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15020#comment:2>
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