Re: [GHC] #16209: GHC behaves differently when binding has separate or inlined type declaration

21 Jan 2019

      #16209: GHC behaves differently when binding has separate or inlined type
declaration
-------------------------------------+-------------------------------------
        Reporter:  fghibellini       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
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:                    |
-------------------------------------+-------------------------------------
Description changed by fghibellini:

Old description:
...
{{{#!hs
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
main = do
    let a :: forall t. Functor t => t () = undefined
    let g :: forall u. Functor u => u () = a
    putStrLn "success"
}}}
fails with:
{{{
source_file.hs:6:44:
    Couldn't match expected type ‘forall (u :: * -> *).
                                  Functor u =>
                                  u ()’
                with actual type ‘t0 ()’
    When checking that: t0 ()
      is more polymorphic than: forall (u :: * -> *). Functor u => u ()
    In the expression: a
}}}
but the following compiles successfully
{{{#!hs
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
main = do
    let a :: forall t. Functor t => t () = undefined
    let g :: forall u. Functor u => u ()
        g = a
    putStrLn "success"
}}}
On GHC 7.10 it works as described above. On newer ones it fails even on
the "functioning code".
{{{
a.hs:5:44: error:
    • Cannot instantiate unification variable ‘a0’
      with a type involving foralls:
        forall (t :: * -> *). Functor t => t ()
        GHC doesn't yet support impredicative polymorphism
    • In the expression: undefined
      In a pattern binding:
        a :: forall (t :: * -> *). Functor t => t () = undefined
      In the expression:
        do let a :: forall t. Functor t => t () = undefined
           let g :: forall u. Functor u => u ()
               g = a
           putStrLn "success"
}}}
New description:

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}

 main = do
     let a :: forall t. Functor t => t () = undefined
     let g :: forall u. Functor u => u () = a
     putStrLn "success"
 }}}

 fails with:

 {{{
 source_file.hs:6:44:
     Couldn't match expected type ‘forall (u :: * -> *).
                                   Functor u =>
                                   u ()’
                 with actual type ‘t0 ()’
     When checking that: t0 ()
       is more polymorphic than: forall (u :: * -> *). Functor u => u ()
     In the expression: a
 }}}

 but the following compiles successfully

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}

 main = do
     let a :: forall t. Functor t => t () = undefined
     let g :: forall u. Functor u => u ()
         g = a
     putStrLn "success"
 }}}

 On GHC 7.10 it works as described above. On 8.4.4 it fails even on the
 "functioning code".

 {{{
 a.hs:5:44: error:
     • Cannot instantiate unification variable ‘a0’
       with a type involving foralls:
         forall (t :: * -> *). Functor t => t ()
         GHC doesn't yet support impredicative polymorphism
     • In the expression: undefined
       In a pattern binding:
         a :: forall (t :: * -> *). Functor t => t () = undefined
       In the expression:
         do let a :: forall t. Functor t => t () = undefined
            let g :: forall u. Functor u => u ()
                g = a
            putStrLn "success"
 }}}

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16209#comment:1
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