[GHC] #9764: Home package modules silently override available modules from package database

[GHC]

#9764: Home package modules silently override available modules from package
database
-------------------------------------+-------------------------------------
       Reporter:  ezyang             |                   Owner:  ezyang
           Type:  feature request    |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:  GHC
     Blocked By:                     |  accepts invalid program
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 To see this, define a module with the same name as some module in base.
 GHC will compile this:

 {{{
 module Data.List where
   foo = 2

 module A where
   import Data.List
   x = print foo
 }}}

 This behavior is intentional; the most compelling reason of behaving this
 way is that if you are calling `ghc` without `-hide-all-packages`, some
 random package you installed which just happened to provide a module name
 that conflicted with yours could cause your code to stop compiling if we
 treated the package database and home packages equally.

 However, this behavior seems undesirable when `-hide-all-packages` is
 provided; at the very least, it seems like you might want to provide a
 warning that you're defining a module which conflicts with a module from a
 package you have exposed.

 This behavior will be further undesirable when signatures come into the
 picture. Now, I will want to include a package containing a signature for
 `A`, and FURTHERMORE I may want to declare a signature `A.hsig` in the
 local package which augments this signature with some extra declarations I
 need. Under the current behavior, the externally imported signature is
 just ignored entirely, even though I wanted them to be merged together.

 My proposal is to switch the behavior so that we don't prefer home modules
 if `-hide-all-packages` is provided. What's not great about this proposal
 is that it adds yet another discrepancy between bare GHC use and GHC use
 with `-hide-all-packages`. I'm interested to know what people would like.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9764>
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