Generalise Package Support

This patch pushes through one fundamental change: a module is now
identified by the pair of its package and module name, whereas
previously it was identified by its module name alone.  This means
that now a program can contain multiple modules with the same name, as
long as they belong to different packages.

This is a language change - the Haskell report says nothing about
packages, but it is now necessary to understand packages in order to
understand GHC's module system.  For example, a type T from module M
in package P is different from a type T from module M in package Q.
Previously this wasn't an issue because there could only be a single
module M in the program.

The "module restriction" on combining packages has therefore been
lifted, and a program can contain multiple versions of the same
package.

Note that none of the proposed syntax changes have yet been
implemented, but the architecture is geared towards supporting import
declarations qualified by package name, and that is probably the next
step.

It is now necessary to specify the package name when compiling a
package, using the -package-name flag (which has been un-deprecated).
Fortunately Cabal still uses -package-name.

Certain packages are "wired in".  Currently the wired-in packages are:
base, haskell98, template-haskell and rts, and are always referred to
by these versionless names.  Other packages are referred to with full
package IDs (eg. "network-1.0").  This is because the compiler needs
to refer to entities in the wired-in packages, and we didn't want to
bake the version of these packages into the comiler.  It's conceivable
that someone might want to upgrade the base package independently of
GHC.

Internal changes:

  - There are two module-related types:

        ModuleName      just a FastString, the name of a module
        Module          a pair of a PackageId and ModuleName

    A mapping from ModuleName can be a UniqFM, but a mapping from Module
    must be a FiniteMap (we provide it as ModuleEnv).

  - The "HomeModules" type that was passed around the compiler is now
    gone, replaced in most cases by the current package name which is
    contained in DynFlags.  We can tell whether a Module comes from the
    current package by comparing its package name against the current
    package.

  - While I was here, I changed PrintUnqual to be a little more useful:
    it now returns the ModuleName that the identifier should be qualified
    with according to the current scope, rather than its original
    module.  Also, PrintUnqual tells whether to qualify module names with
    package names (currently unused).

Docs to follow.

1 files changed, 46 insertions, 2 deletions

diff --git a/compiler/main/PackageConfig.hs b/compiler/main/PackageConfig.hs
index e19a10dbc5..bfd2f34496 100644
--- a/compiler/main/PackageConfig.hs
+++ b/compiler/main/PackageConfig.hs
@@ -6,14 +6,21 @@ module PackageConfig (
 	-- * PackageId
 	PackageId, 
 	mkPackageId, stringToPackageId, packageIdString, packageConfigId,
-	packageIdFS, fsToPackageId, 
+	packageIdFS, fsToPackageId,  unpackPackageId,
 	
 	-- * The PackageConfig type: information about a package
 	PackageConfig,
 	InstalledPackageInfo(..), showPackageId,
 	Version(..),
 	PackageIdentifier(..),
-	defaultPackageConfig
+	defaultPackageConfig,
+
+	-- * Wired-in PackageIds
+	basePackageId,
+	rtsPackageId,
+	haskell98PackageId,
+	thPackageId,
+	mainPackageId
   ) where
 
 #include "HsVersions.h"
@@ -22,6 +29,7 @@ import Distribution.InstalledPackageInfo
 import Distribution.Package
 import Distribution.Version
 import FastString
+import Text.ParserCombinators.ReadP ( readP_to_S )
 
 -- -----------------------------------------------------------------------------
 -- Our PackageConfig type is just InstalledPackageInfo from Cabal.  Later we
@@ -66,4 +74,40 @@ mkPackageId = stringToPackageId . showPackageId
 packageConfigId :: PackageConfig -> PackageId
 packageConfigId = mkPackageId . package
 
+unpackPackageId :: PackageId -> Maybe PackageIdentifier
+unpackPackageId p
+  = case [ pid | (pid,"") <- readP_to_S parsePackageId str ] of
+        []      -> Nothing
+        (pid:_) -> Just pid
+  where str = packageIdString p
+
+-- -----------------------------------------------------------------------------
+-- Package Ids that are wired in
+
+-- Certain packages are "known" to the compiler, in that we know about certain
+-- entities that reside in these packages, and the compiler needs to 
+-- declare static Modules and Names that refer to these packages.  Hence
+-- the wired-in packages can't include version numbers, since we don't want
+-- to bake the version numbers of these packages into GHC.
+--
+-- So here's the plan.  Wired-in packages are still versioned as
+-- normal in the packages database, and you can still have multiple
+-- versions of them installed.  However, for each invocation of GHC,
+-- only a single instance of each wired-in package will be recognised
+-- (the desired one is selected via -package/-hide-package), and GHC
+-- will use the unversioned PackageId below when referring to it,
+-- including in .hi files and object file symbols.  Unselected
+-- versions of wired-in packages will be ignored, as will any other
+-- package that depends directly or indirectly on it (much as if you
+-- had used -ignore-package).
+
+basePackageId      = fsToPackageId FSLIT("base")
+rtsPackageId	   = fsToPackageId FSLIT("rts")
+haskell98PackageId = fsToPackageId FSLIT("haskell98")
+thPackageId        = fsToPackageId FSLIT("template-haskell")
+
+-- This is the package Id for the program.  It is the default package
+-- Id if you don't specify a package name.  We don't add this prefix
+-- to symbol name, since there can be only one main package per program.
+mainPackageId	   = fsToPackageId FSLIT("main")