[GHC] #7461: Error messages about "do" statements contain false information

[GHC]

#7461: Error messages about "do" statements contain false information
-----------------------------+----------------------------------------------
Reporter:  EyalLotem         |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.6.1             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
-----------------------------+----------------------------------------------
 When GHC complains about a type-error in a "do" block, it says, for
 example:

 {{{
 main = putChar $ do
   getLine
   return 'x'
 }}}

 Results in an error:

 {{{
     Couldn't match type `IO Char' with `Char'
     Expected type: IO String -> IO Char -> Char
       Actual type: IO String -> IO Char -> IO Char
     In a stmt of a 'do' block: getLine
 }}}

 I think this error message is pretty directly saying: The '''Actual'''
 type of "getLine" is:

 {{{
 IO String -> IO Char -> IO Char
 }}}

 But that is of course non-sense! It is actually talking about the type of
 the (>>=) binding the "getLine" to the rest of the statements.  But the
 (>>=) is not "in the statement" at all, so the error message is plainly
 wrong.

 It would be much better to talk about the type of the entire do block, or
 maybe the type of the last stmt in the do block (which is the same).

 For example, the error could be replaced by:

 {{{
     Couldn't match type `IO Char' with `Char'
     Expected type: Char
       Actual type: IO Char
     In a stmt of a 'do' block: return 'x'
 }}}

 Or by:

 {{{
     Couldn't match type `IO Char' with `Char'
     Expected type: Char
       Actual type: IO Char
     In 'do' block: `do { getLine;
                          return 'x' }'
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7461>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler