[GHC] #13893: Improved Help for Control.Concurrent and Control.Exception when "variable not in scope"

[GHC]

#13893: Improved Help for Control.Concurrent and Control.Exception when "variable
not in scope"
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        Reporter:  vanto             |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by vanto):

 Replying to [[span(style=color: #FF0000, slyfox )]]:\\
 >Why take preference of packages coming with ghc in this case?\\
 Because these are the main libraries, of course. Those related to Haskell
 standard Prelude. (e.g Data.Char, Control.Applicative, Control.Monad,
 Data.Monoid, Data.Foldable and so on)\\\\

 >It's not always the case that user wants to use base's primitives.\\
 so what?\\\\

 >I would say suggesting incorrect module imports would lead to more
 confusion.\\
 You do not trust the compiler?\\
 Dr. Bjarne Stroustrup said that you should always trust the answers given
 by the compiler. And he's right. So that GHC gives the right answers, you
 have to program it in this way. it is obvious.\\
 If the compiler is able to recognize the variables that belong to a module
 ( which I quote above) it must be able to suggest the appropriate
 module.\\
 __And to go further if the compiler is able to suggest the right module it
 must be able to compile it independently of the programmer.__\\
 The compiler is able to recognize many of the leading pragmas (file-header
 LANGUAGE pragmas).

 Experiencing yourself by writing a few expressions that need their pragmas
 to work, but do not write them and compile the file.
 You will be surprised to see how well the compiler is able to recognize
 them and to propose them to the programmer to insert them in the program.
 Even changes the order of the functions in the file. Moves the first
 function to the end for example and writes the function from end to
 beginning. Done with functions using various pragmas and you will be even
 more surprised.\\
 __What the compiler can do is marvellous.__\\\\

 >Which procedure should define which packages (and modules) are blessed to
 deliver suggestions for symbols?\\
 I do not quite understand the question.\\
 But if you talk about the code that makes it work it would be the same
 procedure which defines the pragmas I suppose or another choice that
 remains to be defined.\\\\

 >As ticket's subject is worded why Control.Concurrent and
 Control.Exception should be threated special?\\
 Not at all. These modules are examples. The subject is badly defined in
 the ticket.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13893#comment:5>
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