[GHC] #15334: (forall x. c x, forall x. d x) is not equivalent to forall x. (c x, d x)

[GHC]

#15334: (forall x. c x, forall x. d x) is not equivalent to forall x. (c x, d x)
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 When I write `foo :: (C a, D a b) => a -> b -> a`, I'm not thinking that
 my function expects a given tuple. I'm expecting a given `C a` and `D a
 b`. The parentheses-and-comma are just concrete syntax. I know it's not
 implemented that way, but that's the programmer's view.

 Along similar lines, I would expect `forall a b. (C a, D a b)` to be
 shorthand for `forall a b. C a` and `forall a b. D a b`. There's really no
 other sensible interpretation (as you point out), so let's just add this
 as a special case, just as `(C a, D a b) => ...` is a special syntax for
 `C a => D a b => ...`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15334#comment:6>
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