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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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Most of the other users of the fptools build system have migrated to
Cabal, and with the move to darcs we can now flatten the source tree
without losing history, so here goes.
The main change is that the ghc/ subdir is gone, and most of what it
contained is now at the top level. The build system now makes no
pretense at being multi-project, it is just the GHC build system.
No doubt this will break many things, and there will be a period of
instability while we fix the dependencies. A straightforward build
should work, but I haven't yet fixed binary/source distributions.
Changes to the Building Guide will follow, too.
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