[GHC] #14049: relax -Wmissing-import-lists to allow one module without implicit imports when using NoImplicitPrelude

[GHC]

#14049: relax -Wmissing-import-lists to allow one module without implicit imports
when using NoImplicitPrelude
-------------------------------------+-------------------------------------
           Reporter:  cdepillabout   |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 It would be nice to modify the `-Wmissing-import-lists` warning flag to
 allow one module with implicit imports when using the `NoImplicitPrelude`
 pragma.

 For example, imagine the following program:

 {{{#!hs
 {-# LANGUAGE NoImplicitPrelude #-}

 module Main where

 import MyCustomPrelude

 import Control.Monad.Trans.Reader (Reader, runReader)

 what :: Reader Int String -> String
 what x = runReader x 3

 main :: IO ()
 main = print "hello"
 }}}

 If this program is compiled with the `-Wmissing-import-lists` flag, the
 following warning will be produced:

 {{{
 example.hs:5:1: warning: [-Wmissing-import-lists]
     The module ‘MyCustomPrelude’ does not have an explicit import list
 }}}

 I think it would be nice if this warning did not appear.

 Ideally, when using `NoImplicitPrelude` and `-Wmissing-import-lists`
 together, I would want one import per module to be permitted not to be
 explicit.  This would make it possible to use an alternative Prelude and
 `-Wmissing-import-lists` at the same time.

 Alternatively, a new warning flag could be created with the above
 behavior.  Maybe it could be called something like `-Wmulti-implicit-
 imports` or `-Wmulti-open-imports`.  I would appreciate
 feedback/suggestions for the name.

 ----

 PureScript currently works like I have described above.  By default the
 `Prelude` module is not imported (so you must explicitly import it), and
 the compiler will warn you if you have more than one implicit import.  I
 really like this combination and it would be nice to be able to get the
 same thing in Haskell.

 In case anyone is interested,
 [https://github.com/purescript/purescript/issues/1869 here] is the issue
 for PureScript where they talked about making this change.

 ----

 If it seems like this is a good idea and other people are onboard, I'd be
 willing to implement it.  I think it would be a relatively easy change, so
 it would be good for a first-time contributor.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14049>
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