[GHC] #2178: GHC.Prim.Any replaces forall-bound type variables in GHCi

[GHC]

#2178: GHC.Prim.Any replaces forall-bound type variables in GHCi
--------------------+-------------------------------------------------------
 Reporter:  japple  |          Owner:         
     Type:  bug     |         Status:  closed 
 Priority:  normal  |      Milestone:         
Component:  GHCi    |        Version:  6.8.2  
 Severity:  normal  |     Resolution:  invalid
 Keywords:          |     Difficulty:  Unknown
 Testcase:          |   Architecture:  x86    
       Os:  Linux   |  
--------------------+-------------------------------------------------------
Changes (by simonpj):

  * status:  new => closed
  * difficulty:  => Unknown
  * resolution:  => invalid

Comment:

 Looks fine to me.  'see' has type 'forall a,b. IO (a->b)'.  You bind it,
 so 'g' must have type 't1 -> t2' for some types 't1' and 't2'. There is no
 generalisation in a bind, so GHC must choose some types for 't1' and
 't2'... and it has.

 It's essential for soundness that we don't generalise in a bind, otherwise
 we'd get the ML polymorphic reference problem.  If you say
 {{{
   r <- newIORef []
 }}}
 you do not want 'r :: forall a. IORef [a]'.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2178#comment:1>
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