From c5a9ed64dab05594bcf871716225ffcbaff6ca7f Mon Sep 17 00:00:00 2001 From: Simon Marlow Date: Thu, 24 Aug 2006 15:35:00 +0000 Subject: [PATCH] Update for changes to packages Not much has changed really: just the removal of the overlap restriction, and the re-instatement of the requirement that -package-name must be used when compiling a package now. --- docs/users_guide/packages.xml | 87 +++++++++++++++++++++++++++++------------ 1 file changed, 62 insertions(+), 25 deletions(-) diff --git a/docs/users_guide/packages.xml b/docs/users_guide/packages.xml index 3bd65c6..4d0cac7 100644 --- a/docs/users_guide/packages.xml +++ b/docs/users_guide/packages.xml @@ -137,9 +137,12 @@ exposed-modules: Network.BSD, base) need to be explicitly exposed using options. - This is a good way to insulate your program from differences - in the globally exposed packages, and being explicit about package - dependencies is a Good Thing. + This is a good way to insulate your program from + differences in the globally exposed packages, and being + explicit about package dependencies is a Good Thing. + Cabal always passes the + flag to GHC, for + exactly this reason. @@ -181,27 +184,31 @@ exposed-modules: Network.BSD, - The module overlap restriction - - The module names in a Haskell program must be distinct. - This doesn't sound like a severe restriction, but in a Haskell program - using multiple packages with interdependencies, difficulties can start to - arise. You should be aware of what the module overlap - restriction means, and how to avoid it. - - GHC knows which packages are in use by your - program: a package is in use if you imported something from it, or if it - is a dependency of some other package in use. There must be no conflicts - between the packages in use; a conflict is when two packages contain - a module with the same name. If - GHC detects a conflict, it will issue a message stating which packages - are in conflict, and which modules are overlapping. - - For example, a conflict might arise if you use two packages, say P - and Q, which respectively depend on two different versions of another - package, say R-1.0 and R-2.0. The - two versions of R are likely to contain at least some - of the same modules, so this situation would be a conflict. + Consequences of packages + + It is possible that by using packages you might end up with + a program that contains two modules with the same name: perhaps + you used a package P that has a hidden module + M, and there is also a module M in your program. Or perhaps the + dependencies of packages that you used contain some overlapping + modules. Perhaps the program even contains multiple versions of a + certain package, due to dependencies from other packages. + + None of these scenarios gives rise to an error on its + ownit used to in GHC 6.4, but not since + 6.6, but they may have some interesting + consequences. For instance, if you have a type + M.T from version 1 of package + P, then this is not the + same as the type M.T from version 2 of package + P, and GHC will report an error if you try to + use one where the other is expected. + + Formally speaking, in Haskell 98, an entity (function, type + or class) in a program is uniquely identified by the pair of the + module name in which it is defined and its name. In GHC, an + entity is uniquely defined by a triple: package, module, and + name. @@ -371,11 +378,41 @@ $ export GHC_PACKAGE_PATH=$HOME/.my-ghc-packages.conf: + To compile a module which is to be part of a new package, + use the -package-name option: + + + + + -package-name + option + + This option is added to the command line when + compiling a module that is destined to be part of package + foo. If this flag is omitted then the + default package main is assumed. + + Note: the argument to + should be the full package identifier for the package, + that is it should include the version number. For example: + -package mypkg-1.2. + + + + + Failure to use the -package-name option + when compiling a package will probably result in disaster, but + you will only discover later when you attempt to import modules + from the package. At this point GHC will complain that the + package name it was expecting the module to come from is not the + same as the package name stored in the .hi + file. + It is worth noting that on Windows, when each package is built as a DLL, since a reference to a DLL costs an extra indirection, intra-package references are cheaper than inter-package references. Of course, this applies to the - Main package as well. + main package as well. -- 1.7.10.4