[GHC] #14288: ScopedTypeVariables with nexted foralls broken since 8.0.2

27 Sep 2017


      #14288: ScopedTypeVariables with nexted foralls broken since 8.0.2
-------------------------------------+-------------------------------------
           Reporter:                 |             Owner:  (none)
  MikolajKonarski                    |
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Linux
       Architecture:  x86_64         |   Type of failure:  GHC rejects
  (amd64)                            |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following example fails in 8.2.1 and 8.0.2. Works fine in 7.10.3. If
 that's intended (that only the first forall has extended scope), I didn't
 find any mention of that in GHC manual. The two commented out variants
 work fine in all.

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

 main :: IO ()
 main = do
 --  let f :: forall a ref. ref () -> ()
 --  let f :: forall ref. forall a. ref () -> ()
   let f :: forall a. forall ref. ref () -> ()
       f x = let r :: ref ()
                 r = x
             in ()
   return $ f (Just ())
 }}}

 The error (the same in 8.2.1 and 8.0.2) is:

 {{{#!hs
 $ ghc --make forall.hs
 [1 of 1] Compiling Main             ( forall.hs, forall.o )

 forall.hs:8:21: error:
     • Couldn't match type ‘ref’ with ‘ref1’
       ‘ref’ is a rigid type variable bound by
         the type signature for:
           f :: forall a (ref :: * -> *). ref () -> ()
         at forall.hs:6:29
       ‘ref1’ is a rigid type variable bound by
         the type signature for:
           r :: forall (ref1 :: * -> *). ref1 ()
         at forall.hs:7:22
       Expected type: ref1 ()
         Actual type: ref ()
     • In the expression: x
       In an equation for ‘r’: r = x
       In the expression:
         let
           r :: ref ()
           r = x
         in ()
     • Relevant bindings include
         r :: ref1 () (bound at forall.hs:8:17)
         x :: ref () (bound at forall.hs:7:9)
         f :: ref () -> () (bound at forall.hs:7:7)
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14288
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler