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, 11 insertions, 5 deletions

diff --git a/compiler/iface/IfaceEnv.lhs b/compiler/iface/IfaceEnv.lhs
index c7e78b3d45..3eceaa0f04 100644
--- a/compiler/iface/IfaceEnv.lhs
+++ b/compiler/iface/IfaceEnv.lhs
@@ -35,9 +35,9 @@ import Name		( Name, nameUnique, nameModule,
 import NameSet		( NameSet, emptyNameSet, addListToNameSet )
 import OccName		( OccName, isTupleOcc_maybe, tcName, dataName, mapOccEnv, occNameFS,
 			  lookupOccEnv, unitOccEnv, extendOccEnv, extendOccEnvList )
-import PrelNames	( gHC_PRIM, pREL_TUP )
-import Module		( Module, emptyModuleEnv, 
-			  lookupModuleEnv, extendModuleEnv_C )
+import PrelNames	( gHC_PRIM, dATA_TUP )
+import Module		( Module, emptyModuleEnv, ModuleName, modulePackageId,
+			  lookupModuleEnv, extendModuleEnv_C, mkModule )
 import UniqFM           ( lookupUFM, addListToUFM )
 import FastString       ( FastString )
 import UniqSupply	( UniqSupply, splitUniqSupply, uniqFromSupply, uniqsFromSupply )
@@ -230,7 +230,7 @@ newIPName occ_name_ip
 \begin{code}
 lookupOrigNameCache :: OrigNameCache -> Module -> OccName -> Maybe Name
 lookupOrigNameCache nc mod occ
-  | mod == pREL_TUP || mod == gHC_PRIM,		-- Boxed tuples from one, 
+  | mod == dATA_TUP || mod == gHC_PRIM,		-- Boxed tuples from one, 
     Just tup_info <- isTupleOcc_maybe occ	-- unboxed from the other
   = 	-- Special case for tuples; there are too many
 	-- of them to pre-populate the original-name cache
@@ -340,7 +340,7 @@ lookupIfaceTc other_tc	    = return (ifaceTyConName other_tc)
 
 lookupIfaceExt :: IfaceExtName -> IfL Name
 lookupIfaceExt (ExtPkg  mod occ)   = lookupOrig mod occ
-lookupIfaceExt (HomePkg mod occ _) = lookupOrig mod occ
+lookupIfaceExt (HomePkg mod occ _) = lookupHomePackage mod occ
 lookupIfaceExt (LocalTop occ)	   = lookupIfaceTop occ
 lookupIfaceExt (LocalTopSub occ _) = lookupIfaceTop occ
 
@@ -349,6 +349,12 @@ lookupIfaceTop :: OccName -> IfL Name
 lookupIfaceTop occ
   = do	{ env <- getLclEnv; lookupOrig (if_mod env) occ }
 
+lookupHomePackage :: ModuleName -> OccName -> IfL Name
+lookupHomePackage mod_name occ
+  = do	{ env <- getLclEnv; 
+        ; let this_pkg = modulePackageId (if_mod env)
+        ; lookupOrig (mkModule this_pkg mod_name) occ }
+
 newIfaceName :: OccName -> IfL Name
 newIfaceName occ
   = do	{ uniq <- newUnique