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+ <title>The GHC Commentary - Modules, ModuleNames and Packages</title>
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+ <h1>Modules, ModuleNames and Packages</h1>
+
+ <p>This section describes the datatypes <code>ModuleName</code>
+ <code>Module</code> and <code>PackageName</code> all available
+ from the module <a
+ href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/basicTypes/Module.lhs"><code>Module</code></a>.<p>
+
+ <h2>Packages</h2>
+
+ <p>A package is a collection of (zero or more) Haskell modules,
+ together with some information about external libraries, extra C
+ compiler options, and other things that this collection of modules
+ requires. When using DLLs on windows (or shared libraries on a
+ Unix system; currently unsupported), a package can consist of only
+ a single shared library of Haskell code; the reason for this is
+ described below.
+
+ <p>Packages are further described in the User's Guide <a
+ href="http://www.haskell.org/ghc/docs/latest/packages.html">here</a>.
+
+ <h2>The ModuleName type</h2>
+
+ <p>At the bottom of the hierarchy is a <code>ModuleName</code>,
+ which, as its name suggests, is simply the name of a module. It
+ is represented as a Z-encoded FastString, and is an instance of
+ <code>Uniquable</code> so we can build <code>FiniteMap</code>s
+ with <code>ModuleName</code>s as the keys.
+
+ <p>A <code>ModuleName</code> can be built from a
+ <code>String</code>, using the <code>mkModuleName</code> function.
+
+ <h2>The Module type</h2>
+
+ <p>For a given module, the compiler also needs to know whether the
+ module is in the <em>home package</em>, or in another package.
+ This distinction is important for two reasons:
+
+ <ul>
+ <li><p>When generating code to call a function in another package,
+ the compiler might have to generate a cross-DLL call, which is
+ different from an intra-DLL call (hence the restriction that the
+ code in a package can only reside in a single DLL).
+
+ <li><p>We avoid putting version information in an interface file
+ for entities defined in another package, on the grounds that other
+ packages are generally "stable". This also helps keep the size of
+ interface files down.
+ </ul>
+
+ <p>The <code>Module</code> type contains a <code>ModuleName</code>
+ and a <code>PackageInfo</code> field. The
+ <code>PackageInfo</code> indicates whether the given
+ <code>Module</code> comes from the current package or from another
+ package.
+
+ <p>To get the actual package in which a given module resides, you
+ have to read the interface file for that module, which contains
+ the package name (actually the value of the
+ <code>-package-name</code> flag when that module was built). This
+ information is currently unused inside the compiler, but we might
+ make use of it in the future, especially with the advent of
+ hierarchical modules, to allow the compiler to automatically
+ figure out which packages a program should be linked with, and
+ thus avoid the need to specify <code>-package</code> options on
+ the command line.
+
+ <p><code>Module</code>s are also instances of
+ <code>Uniquable</code>, and indeed the unique of a
+ <code>Module</code> is the same as the unique of the underlying
+ <code>ModuleName</code>.
+ </body>
+</html>
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