Re: [GHC] #7842: Incorrect checking of let-bindings in recursive do

21 Jun 2013

      #7842: Incorrect checking of let-bindings in recursive do
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Reporter:  diatchki                   |          Owner:                         
    Type:  bug                        |         Status:  new                    
Priority:  normal                     |      Component:  Compiler (Type checker)
 Version:  7.7                        |       Keywords:                         
      Os:  Unknown/Multiple           |   Architecture:  Unknown/Multiple       
 Failure:  GHC rejects valid program  |      Blockedby:                         
Blocking:                             |        Related:                         
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Comment(by wvv):

 First, it is needed to add extension: Rank2Types:

 {{{
 {-# LANGUAGE RecursiveDo, Rank2Types #-}

 bug :: IO (Char,Bool)
 bug = mdo
    a <- return b
    let f = id
    let g :: (forall a. (a -> a) ) -> (Char, Bool)
        g f = (f 'a', f True)
    b <- return a
    return $ g f

 }}}

 but still error:

 {{{
     Couldn't match type `a0' with `a'
       because type variable `a' would escape its scope
     This (rigid, skolem) type variable is bound by
       a type expected by the context: a -> a
     The following variables have types that mention a0
       f :: a0 -> a0 (bound at test2.hs:34:8)
     Expected type: a -> a
       Actual type: a0 -> a0
     In the first argument of `g', namely `f'
     In the second argument of `($)', namely `g f'
     In a stmt of an 'mdo' block: return $ g f
 }}}

 Code

 {{{
 {-# LANGUAGE RecursiveDo, Rank2Types #-}

 bug :: IO (Char,Bool)
 bug = mdo
    let f = id
    let g :: (forall a. (a -> a) ) -> (Char, Bool)
        g f = (f 'a', f True)
    return $ g f
 }}}
  is OK.

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7842#comment:2
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