[GHC] #15142: GHC HEAD regression: tcTyVarDetails

[GHC]

#15142: GHC HEAD regression: tcTyVarDetails
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  closed
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:  TypeInType,
      Resolution:  fixed             |  TypeFamilies, CUSKs
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Compile-time      |            Test Case:  indexed-
  crash or panic                     |  types/should_compile/T15142
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 I agree with everything in that Note.

 Why have the explicit `forall`? Because perhaps the user wants inference,
 not CUSKness. By making a decision based on the `forall`, then the user
 can get what they want. For example:

 {{{#!hs
 data T1 :: k1 k2 -> Type where
   MkT1 :: forall (k1 :: Type -> Type) (k2 :: Type) (x :: k1 k2). T1 x
 }}}

 There are two possible meanings for this. Meaning 1 (M1):

 {{{#!hs
 T1 :: forall (k1 :: Type -> Type) (k2 :: Type). k1 k2 -> Type
 MkT1 :: forall (k1 :: Type -> Type) (k2 :: Type) (x :: k1 k2). T1 k1 k2 x
 }}}

 Meaning 2 (M2):

 {{{#!hs
 T1 :: forall (k :: Type) (k1 :: k -> Type) (k2 :: k). k1 k2 -> Type
 MkT1 :: forall (k :: Type) (k1 :: k -> Type) (k2 :: k) (x :: k1 k2). (k ~#
 Type) -> T1 k k1 k2 x
 }}}

 In M1, we do not quantify `T1`'s kind further, and the data constructor is
 not GADT-like. In M2, though, we ''do'' generalize, making `MkT1` a GADT
 constructor packing an equality.

 M1 is what happens if `T1` does not have a CUSK. `M2` is what happens when
 it does. Having CUSKness depend on the presence of the `forall` allows
 users to choose which of these interpretations to use. We ''could''
 absolutely, remove it, but then I don't know how to write `T1` with
 meaning M1.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15142#comment:21>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler