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.
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