[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     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.9
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  GHC         |  Related Tickets:
  accepts invalid program            |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
Description changed by ezyang:

Old description:

> 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.

New description:

 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. (A
 counter argument is that this is confusing for users, who will see the use
 of a declaration, browse to the local hsig file, and not see it defined;
 thus this mode of use should be disallowed.)

 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.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9764#comment:1>
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