[GHC] #15050: ScopedTypeVariables could allow more programs

[GHC]

#15050: ScopedTypeVariables could allow more programs
-------------------------------------+-------------------------------------
        Reporter:  nomeata           |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.1
      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:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 > What does it mean for a type variable to be a type variable?

 Imagine the fully-typechecked Core program.  Now, does the source-level
 type variable name a type variable, or some other non-type-variable type?
 Example:
 {{{
 data T a = MkT a

 f1 :: T b -> Int
 f1 (MkT (x::z)) = ...   -- OK

 f2 :: T Boll -> Int
 f2 (MkT (x::z)) = ...   -- Not OK
 }}}
 In `f1` the source-level lexical type variable `z` names a type variable
 in Core.  The elaborated version of `f1` could be
 {{{
 f1 = /\b. \(x::T b). case x of { MkT (x::b) -> ... }
 }}}
 So `z` names `b`.

 In `f2`, you'll see that `z` names `Int`.

 I agree this is a questionable choice.  At the time I was worried that
 it'd be confusing to have a type variable that was just an alias for
 `Int`; that is not a type variable at all.  But in these days of GADTs and
 type equalities we are all used to that.  We'd make a different choice
 today.

 Feel free to make a GHC proposal to change this behaviour.  It'd be
 backward compatible!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15050#comment:6>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler