[GHC] #15050: ScopedTypeVariables could allow more programs

[GHC]

#15050: ScopedTypeVariables could allow more programs
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        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:                    |
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Comment (by nomeata):

 Here is another case where `ScopedTypeVariables` are less powerful than
 one might expect. This works:
 {{{
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}

 type family F a where F Bool = Int

 data T a where MkT :: forall b a. b ~ F a => b -> T a
 foo :: T Bool -> ()
 foo (MkT (_ :: Int)) = ()
 }}}
 but the equivalent(?) formulation using `TypeFamilies` does not:
 {{{
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FunctionalDependencies #-}

 class C b a | a -> b
 instance C Int Bool
 data T a where MkT :: forall b a. C b a => b -> T a
 foo :: T Bool -> ()
 foo (MkT (_ :: Int)) = ()
 }}}
 gives
 {{{
 /tmp/Foo.hs:11:11: error:
     • Couldn't match expected type ‘b’ with actual type ‘Int’
       ‘b’ is a rigid type variable bound by
         a pattern with constructor: MkT :: forall b a. C b a => b -> T a,
         in an equation for ‘foo’
         at /tmp/Foo.hs:11:6-19
     • When checking that the pattern signature: Int
         fits the type of its context: b
       In the pattern: _ :: Int
       In the pattern: MkT (_ :: Int)
 }}}
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15050#comment:10>
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