Re: [GHC] #14203: GHC-inferred type signature doesn't actually typecheck

3 Nov 2017

      #14203: GHC-inferred type signature doesn't actually typecheck
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  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 RyanGlScott):

 OK, I found out how to do this. The trick is to make the kind of `a` in
 `F` visible:

 {{{#!hs
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 module Bug where

 type family F a
 data Foo a (z :: F a)

 bar :: forall a (z :: F a). Foo a z -> Int
 bar _ = 42
 }}}

 A similar trick works for the program in comment:1:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module Bug where

 import Data.Kind

 data TyFun :: * -> * -> *
 type a ~> b = TyFun a b -> *
 infixr 0 ~>
 type family Apply (f :: k1 ~> k2) (x :: k1) :: k2

 data Foo p x (z :: Apply (p :: a ~> *) (x :: a))

 data Bar where
   MkBar :: forall a (p :: TyFun a * -> *) (x :: a) (z :: Apply p x).
            Foo p x z -> Bar
 }}}

 Alas, this trick doesn't scale very well to the original program, since
 you'd need to give `p` as a visible argument to `Sing px` somehow. (It's
 not clear to me if even visible kind application would fix this problem.)

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14203#comment:14
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