To simplify organisation and compilation, GHC keeps libraries in <Emphasis>packages</Emphasis>. Packages are also compiled into single libraries on Unix, and DLLs on Windows. The term ``package'' can be used pretty much synonymously with ``library'', except that an application also forms a package, the Main package.
</Para>
<ItemizedList>
<ListItem>
<Para>
A package is a group of modules. It may span many directories, or many packages may exist in a single directory. Packages may not be mutually recursive.
</Para>
</ListItem>
<ListItem>
<Para>
A path is considered “absolute” if it starts with
“<Filename>/</Filename>”, or
“<Filename>A:/</Filename>”, or
“<Filename>A:\</Filename>” (or
“<Filename>B:/</Filename>”,
“<Filename>B:\</Filename>” etc).
A package has a name (e.g. <Filename>std</Filename>)
</Para>
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<Para>
Patrick Sansom had a workshop paper about how all this is done (though
the details have changed quite a bit). <ULink URL="mailto:sansom@dcs.gla.ac.uk">Ask him</ULink> if you want a copy.
Each package is built into a single library (Unix; e.g. <Filename>libHSfoo.a</Filename>), or a single DLL (Windows; e.g. <Filename>HSfoo.dll</Filename>)
</Para>
</ListItem>
<ListItem>
<Para>
The <Option>-package-name foo</Option> flag tells GHC that the module being compiled is destined for package <Filename>foo</Filename>. If this is omitted, the default package, <Filename>Main</Filename>, is assumed.
</Para>
</ListItem>
<ListItem>
<Para>
The <Option>-package foo</Option> flag tells GHC to make available modules
from package <Filename>foo</Filename>. It replaces <Option>-syslib foo</Option>, which is now deprecated.
</Para>
</ListItem>
<ListItem>
<Para>
GHC does not maintain detailed cross-package dependency information.
It does remember which modules in other packages the current module
depends on, but not which things within those imported things.
</Para>
</ListItem>
</ItemizedList>
<Para>
All of this tidies up the Prelude enormously. The Prelude and
Standard Libraries are built into a single package called <Filename>std</Filename>. (This
is a change; the library is now called <Filename>libHSstd.a</Filename> instead of <Filename>libHS.a</Filename>).
</Para>
<Para>
It is worth noting that on Windows, because each package is built as a DLL, and 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. This is not normally the case on most Unices.
