Re: [GHC] #13839: GHC warnings do not respect the default module header

16 Nov 2018

      #13839: GHC warnings do not respect the default module header
-------------------------------------+-------------------------------------
        Reporter:  Feuerbach         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by Feuerbach):

 * status:  infoneeded => new

Old description:
...
If I compile this program with `-Wall`, I get no warnings:

...
{{{#!hs
type T = Int
main :: IO ()
main = return ()
}}}
If I add a module header:
{{{#!hs
module Main(main) where
type T = Int
main :: IO ()
main = return ()
}}}
I now get a warning:
{{{
ghcbug2.hs:3:1: warning: [-Wunused-top-binds]
    Defined but not used: type constructor or class ‘T’
}}}
Yet, according to Haskell2010 (section 5.1):
...
An abbreviated form of module, consisting only of the module body, is
permitted. If this is used, the header is assumed to be ‘module
Main(main) where’.
Therefore, the right behaviour should be to print a warning in the first
case as well.
...
An abbreviated form of module, consisting only of the module body, is
New description:

 If I load this program in ghci with `-Wall`, I get no warnings:

 {{{#!hs
 type T = Int

 main :: IO ()
 main = return ()
 }}}

 If I add a module header:

 {{{#!hs
 module Main(main) where

 type T = Int

 main :: IO ()
 main = return ()
 }}}

 I now get a warning:

 {{{
 ghcbug2.hs:3:1: warning: [-Wunused-top-binds]
     Defined but not used: type constructor or class ‘T’
 }}}

 Yet, according to Haskell2010 (section 5.1):

 permitted. If this is used, the header is assumed to be ‘module Main(main)
 where’.

 Therefore, the right behaviour should be to print a warning in the first
 case as well.

 If I compile the program instead of loading it in ghci, I correctly get
 the warning in both cases.

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13839#comment:3
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