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, 58 insertions, 22 deletions

diff --git a/compiler/utils/Outputable.lhs b/compiler/utils/Outputable.lhs
index cf99e12bcf..52262ec02e 100644
--- a/compiler/utils/Outputable.lhs
+++ b/compiler/utils/Outputable.lhs
@@ -16,7 +16,7 @@ module Outputable (
 	PprStyle, CodeStyle(..), PrintUnqualified, alwaysQualify,
 	getPprStyle, withPprStyle, withPprStyleDoc, pprDeeper, pprSetDepth,
 	codeStyle, userStyle, debugStyle, dumpStyle, asmStyle,
-	ifPprDebug, unqualStyle, 
+	ifPprDebug, qualName, qualModule,
 	mkErrStyle, defaultErrStyle, defaultDumpStyle, defaultUserStyle,
 
 	SDoc, 		-- Abstract
@@ -51,7 +51,8 @@ module Outputable (
 #include "HsVersions.h"
 
 
-import {-# SOURCE #-} 	Module( Module )
+import {-# SOURCE #-} 	Module( Module, modulePackageId, 
+                                ModuleName, moduleName )
 import {-# SOURCE #-} 	OccName( OccName )
 
 import StaticFlags	( opt_PprStyle_Debug, opt_PprUserLength )
@@ -99,33 +100,64 @@ data Depth = AllTheWay
            | PartWay Int	-- 0 => stop
 
 
-type PrintUnqualified = Module -> OccName -> Bool
-	-- This function tells when it's ok to print 
-	-- a (Global) name unqualified
+-- -----------------------------------------------------------------------------
+-- Printing original names
 
-alwaysQualify,neverQualify :: PrintUnqualified
-alwaysQualify m n = False
-neverQualify  m n = True
+-- When printing code that contains original names, we need to map the
+-- original names back to something the user understands.  This is the
+-- purpose of the pair of functions that gets passed around
+-- when rendering 'SDoc'.
+
+-- | given an /original/ name, this function tells you which module
+-- name it should be qualified with when printing for the user, if
+-- any.  For example, given @Control.Exception.catch@, which is in scope
+-- as @Exception.catch@, this fuction will return @Just "Exception"@.
+-- Note that the return value is a ModuleName, not a Module, because
+-- in source code, names are qualified by ModuleNames.
+type QualifyName = Module -> OccName -> Maybe ModuleName
+
+-- | For a given module, we need to know whether to print it with
+-- a package name to disambiguate it, and if so which package name should
+-- we use.
+type QualifyModule = Module -> Maybe PackageId
+
+type PrintUnqualified = (QualifyName, QualifyModule)
+
+alwaysQualifyNames :: QualifyName
+alwaysQualifyNames m n = Just (moduleName m)
+
+neverQualifyNames :: QualifyName
+neverQualifyNames m n = Nothing
+
+alwaysQualifyModules :: QualifyModule
+alwaysQualifyModules m = Just (modulePackageId m)
+
+neverQualifyModules :: QualifyModule
+neverQualifyModules m = Nothing
+
+alwaysQualify = (alwaysQualifyNames, alwaysQualifyModules)
+neverQualify  = (neverQualifyNames,  neverQualifyModules)
 
 defaultUserStyle = mkUserStyle alwaysQualify AllTheWay
 
 defaultDumpStyle |  opt_PprStyle_Debug = PprDebug
 		 |  otherwise          = PprDump
 
+-- | Style for printing error messages
 mkErrStyle :: PrintUnqualified -> PprStyle
--- Style for printing error messages
-mkErrStyle print_unqual = mkUserStyle print_unqual (PartWay opt_PprUserLength)
+mkErrStyle qual = mkUserStyle qual (PartWay opt_PprUserLength)
 
 defaultErrStyle :: PprStyle
 -- Default style for error messages
 -- It's a bit of a hack because it doesn't take into account what's in scope
 -- Only used for desugarer warnings, and typechecker errors in interface sigs
 defaultErrStyle 
-  | opt_PprStyle_Debug = mkUserStyle alwaysQualify AllTheWay
-  | otherwise	       = mkUserStyle alwaysQualify  (PartWay opt_PprUserLength)
+  | opt_PprStyle_Debug   = mkUserStyle alwaysQualify AllTheWay
+  | otherwise            = mkUserStyle alwaysQualify (PartWay opt_PprUserLength)
 
-mkUserStyle unqual depth |  opt_PprStyle_Debug = PprDebug
-	          	 |  otherwise          = PprUser unqual depth
+mkUserStyle unqual depth
+   | opt_PprStyle_Debug = PprDebug
+   | otherwise          = PprUser unqual depth
 \end{code}
 
 Orthogonal to the above printing styles are (possibly) some
@@ -152,22 +184,26 @@ withPprStyleDoc :: PprStyle -> SDoc -> Doc
 withPprStyleDoc sty d = d sty
 
 pprDeeper :: SDoc -> SDoc
-pprDeeper d (PprUser unqual (PartWay 0)) = Pretty.text "..."
-pprDeeper d (PprUser unqual (PartWay n)) = d (PprUser unqual (PartWay (n-1)))
-pprDeeper d other_sty        		 = d other_sty
+pprDeeper d (PprUser q (PartWay 0)) = Pretty.text "..."
+pprDeeper d (PprUser q (PartWay n)) = d (PprUser q (PartWay (n-1)))
+pprDeeper d other_sty        	    = d other_sty
 
 pprSetDepth :: Int -> SDoc -> SDoc
-pprSetDepth n d (PprUser unqual _) = d (PprUser unqual (PartWay n))
-pprSetDepth n d other_sty	   = d other_sty
+pprSetDepth n d (PprUser q _) = d (PprUser q (PartWay n))
+pprSetDepth n d other_sty     = d other_sty
 
 getPprStyle :: (PprStyle -> SDoc) -> SDoc
 getPprStyle df sty = df sty sty
 \end{code}
 
 \begin{code}
-unqualStyle :: PprStyle -> PrintUnqualified
-unqualStyle (PprUser    unqual _) m n = unqual m n
-unqualStyle other		  m n = False
+qualName :: PprStyle -> QualifyName
+qualName (PprUser (qual_name,_) _) m n = qual_name m n
+qualName other		           m n = Just (moduleName m)
+
+qualModule :: PprStyle -> QualifyModule
+qualModule (PprUser (_,qual_mod) _) m = qual_mod m
+qualModule other                    m = Just (modulePackageId m)
 
 codeStyle :: PprStyle -> Bool
 codeStyle (PprCode _)	  = True