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

16 Apr 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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 I have run into a problem with the type-checking of recursive do blocks,
 which reduces to the following example:

 {{{
 {-# LANGUAGE RecursiveDo #-}
 module Bug where

 bug :: (Int -> IO Int) -> IO (Bool, Char)
 bug m =
   mdo i <- m i1      -- RECURSION

       let i1 :: Int
           i1 = i     -- RECURSION

           -- This appears to be monomorphic, despite the type signature.
           f :: b -> b
           f x = x

       return (f True, f 'a')
 }}}

 This program is rejected with the errors shown below.  The problem appears
 to be that somehow `f` has become monomorphic, despite its type-signature.
 This seems to happen only when `f` is part of a `let` block that is also
 involved in the recursion.

 Here is the error reported by GHC 7.7.20130215:

 {{{
 Bug.hs:15:23:
     Couldn't match expected type `Char' with actual type `Bool'
     In the return type of a call of `f'
     In the expression: f 'a'
     In the first argument of `return', namely `(f True, f 'a')'

 Bug.hs:15:25:
     Couldn't match expected type `Bool' with actual type `Char'
     In the first argument of `f', namely 'a'
     In the expression: f 'a'
     In the first argument of `return', namely `(f True, f 'a')'
 }}}

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