[GHC] #14529: Refactor ConDecl

[GHC]

#14529: Refactor ConDecl
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
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 goldfire):

 The problem with `LHsQTyVars` is that every other instance of that type
 stores the user-written tyvars of a tycon declaration. Thus, the
 `kcHsTyVarBndrs` function that checks them asks for information about
 CUSKs, etc., which is utterly wrong for data constructors. (The reason I
 refactored this is that I've had to make some changes to `kcHsTyVarBndrs`
 in my type-level implication constraints patch, such that it no longer
 works at all for datacons.) Really, the right type for GADT constructors
 is `HsImplicitBndrs`. The problem is that you'd then have to have a tiered
 structure, where a `ConDeclGADT` stores an `HsImplicitBndrs` wrapped
 around a `ConDeclGADTDetails` or some such. It's a bit ugly. But the
 structure would be just right.

 Alternatives would be to have a `con_implicit_qvars :: [IdP pass],
 con_explicit_qvars :: [LHsTyVarBndr pass]`, or a new function to consume a
 `LHsQTyVars` from a datacon (instead of `kcHsTyVarBndrs`).

 Also, why change the `Maybe ...` to a `Bool` (with an invariant) to say
 whether or not there's a forall? The `Maybe` seems closer to the truth.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14529#comment:3>
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