Commit c5a9ed64 authored by Simon Marlow's avatar Simon Marlow
Browse files

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.
parent e7ac000c
......@@ -137,9 +137,12 @@ exposed-modules: Network.BSD,
<literal>base</literal>) need to be explicitly exposed using
<option>-package</option> options.</para>
<para>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.</para>
<para>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
<option>-hide-all-packages</option> flag to GHC, for
exactly this reason.</para>
......@@ -181,27 +184,31 @@ exposed-modules: Network.BSD,
<sect2 id="package-overlaps">
<title>The module overlap restriction</title>
<para>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.</para>
<para>GHC knows which packages are <emphasis>in use</emphasis> 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.</para>
<para>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 <literal>R-1.0</literal> and <literal>R-2.0</literal>. The
two versions of <literal>R</literal> are likely to contain at least some
of the same modules, so this situation would be a conflict.</para>
<title>Consequences of packages</title>
<para>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 <emphasis>hidden</emphasis> 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.</para>
<para>None of these scenarios gives rise to an error on its
own<footnote><para>it used to in GHC 6.4, but not since
6.6</para></footnote>, but they may have some interesting
consequences. For instance, if you have a type
<literal>M.T</literal> from version 1 of package
<literal>P</literal>, then this is <emphasis>not</emphasis> the
same as the type <literal>M.T</literal> from version 2 of package
<literal>P</literal>, and GHC will report an error if you try to
use one where the other is expected.</para>
<para>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
<sect2 id="package-databases">
......@@ -371,11 +378,41 @@ $ export GHC_PACKAGE_PATH=$HOME/.my-ghc-packages.conf:</screen>
<para>To compile a module which is to be part of a new package,
use the <literal>-package-name</literal> option:</para>
<term><option>-package-name <replaceable>foo</replaceable></option></term>
<para>This option is added to the command line when
compiling a module that is destined to be part of package
<literal>foo</literal>. If this flag is omitted then the
default package <literal>main</literal> is assumed.</para>
<para>Note: the argument to <option>-package-name</option>
should be the full package identifier for the package,
that is it should include the version number. For example:
<literal>-package mypkg-1.2</literal>.</para>
<para>Failure to use the <literal>-package-name</literal> 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 <literal>.hi</literal>
<para>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
<filename>Main</filename> package as well.</para>
<filename>main</filename> package as well.</para>
<sect2 id="package-management">
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