[GHC] #15869: Discrepancy between seemingly equivalent type synonym and type family

[GHC]

#15869: Discrepancy between seemingly equivalent type synonym and type family
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.6.2
  checker)                           |
      Resolution:                    |             Keywords:  TypeFamilies
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):

 Lots of bugs here!

 1. I would expect `Apply1` to be rejected without an explicit `forall` in
 the equation, giving the polytype of `f`.

 2. The error message for `Apply2` shouldn't say that it expects for
 arguments to `f`. The rest of the error message is perhaps confusing, but
 it's not unreasonable.

 3. The version of `Apply2` with the explicit `forall` in the equation
 should be accepted.

 4. In the `g` and `h` with `KindOf1`: Their types should be `forall a -> a
 -> Type`, not what GHC is reporting.

 5. Ditto for `h` in the `KindOf2` case.

 Note that the inferred nature (as opposed to specified) of `a` is a non-
 bug: when you don't write a type signature, you don't get specified
 variables. But of course these should be required, not invisible or
 specified.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15869#comment:2>
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