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<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>2004</year><year>2009</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and limitations
under the License.
</legalnotice>
<title>packages</title>
<prepared>Kenneth Lundin</prepared>
<responsible>Kenneth Lundin</responsible>
<docno>1</docno>
<approved>Kenneth Lundin</approved>
<checked></checked>
<date>2004-09-07</date>
<rev>A</rev>
<file>packages.sgml</file>
</header>
<module>packages</module>
<modulesummary>Packages in Erlang</modulesummary>
<description>
<warning><p>
Packages has since it was introduced more than 5 years ago been an
experimental feature. Use it at your own risk, we do not
actively maintain and develop this feature. It might however be
supported some
day.
</p>
<p>
In spite of this packages work quite well, but there are some
known issues in tools and other parts where packages don't work well.
</p>
</warning>
<p><em>Introduction</em></p>
<p>Packages are simply namespaces for modules.
All old Erlang modules automatically belong to the top level
("empty-string") namespace, and do not need any changes.</p>
<p>The full name of a packaged module is written as e.g.
"<c>fee.fie.foe.foo</c>",
i.e., as atoms separated by periods,
where the package name is the part up to
but not including the last period;
in this case "<c>fee.fie.foe</c>".
A more concrete example is the module <c>erl.lang.term</c>,
which is in the
package <c>erl.lang</c>.
Package names can have any number of segments, as in
<c>erl.lang.list.sort</c>.
The atoms in the name can be quoted, as in <c>foo.'Bar'.baz</c>,
or even the
whole name, as in <c>'foo.bar.baz'</c> but the concatenation of
atoms and
periods must not contain two consecutive period characters or
end with a period,
as in <c>'foo..bar'</c>, <c>foo.'.bar'</c>, or <c>foo.'bar.'</c>.
The periods must not be followed by whitespace.</p>
<p>The code loader maps module names onto the file system directory
structure.
E.g., the module <c>erl.lang.term</c> corresponds to a file
<c>.../erl/lang/term.beam</c>
in the search path.
Note that the name of the actual object file corresponds to
the last part only of the full module name.
(Thus, old existing modules such as <c>lists</c>
simply map to <c>.../lists.beam</c>, exactly as before.)</p>
<p>A packaged module in a file "<c>foo/bar/fred.erl</c>" is declared
as:</p>
<code type="none">
-module(foo.bar.fred).</code>
<p>This can be compiled and loaded from the Erlang shell using
<c>c(fred)</c>, if
your current directory is the same as that of the file.
The object file will be named <c>fred.beam</c>.</p>
<p>The Erlang search path works exactly as before,
except that the package segments will be appended to each
directory in the path in order to find the
file. E.g., assume the path is <c>["/usr/lib/erl", "/usr/local/lib/otp/legacy/ebin", "/home/barney/erl"]</c>.
Then, the code for a module named <c>foo.bar.fred</c> will be
searched for
first as <c>"/usr/lib/erl/foo/bar/fred.beam"</c>, then
<c>"/usr/local/lib/otp/legacy/ebin/foo/bar/fred.beam"</c>
and lastly <c>"/home/barney/erl/foo/bar/fred.beam"</c>.
A module
like <c>lists</c>, which is in the top-level package,
will be looked for as <c>"/usr/lib/erl/lists.beam"</c>,
<c>"/usr/local/lib/otp/legacy/ebin/lists.beam"</c> and
<c>"/home/barney/erl/lists.beam"</c>.</p>
<p><em>Programming</em></p>
<p>Normally, if a call is made from one module to another,
it is assumed that the
called module belongs to the same package as the source module.
The compiler
automatically expands such calls. E.g., in:</p>
<code type="none">
-module(foo.bar.m1).
-export([f/1]).
f(X) -> m2:g(X).</code>
<p><c>m2:g(X)</c> becomes a call to <c>foo.bar.m2</c>
If this is not what was intended, the call can be written
explicitly, as in</p>
<code type="none">
-module(foo.bar.m1).
-export([f/1]).
f(X) -> fee.fie.foe.m2:g(X).</code>
<p>Because the called module is given with an explicit package name,
no expansion is done in this case.</p>
<p>If a module from another package is used repeatedly in a module,
an import declaration can make life easier:</p>
<code type="none">
-module(foo.bar.m1).
-export([f/1, g/1]).
-import(fee.fie.foe.m2).
f(X) -> m2:g(X).
g(X) -> m2:h(X).</code>
<p>will make the calls to <c>m2</c> refer to <c>fee.fie.foe.m2</c>.
More generally, a declaration <c>-import(Package.Module).</c>
will cause calls to <c>Module</c>
to be expanded to <c>Package.Module</c>.</p>
<p>Old-style function imports work as normal (but full module
names must be
used); e.g.:</p>
<code type="none">
-import(fee.fie.foe.m2, [g/1, h/1]).</code>
<p>however, it is probably better to avoid this form of import
altogether in new
code, since it makes it hard to see what calls are really "remote".</p>
<p>If it is necessary to call a module in the top-level package
from within a
named package, the module name can be written either with an
initial period as
in e.g. "<c>.lists</c>", or with an empty initial atom, as in
"<c>''.lists</c>".
However, the best way is to use an import declaration -
this is most obvious to
the eye, and makes sure we don't forget adding a period somewhere:</p>
<code type="none">
-module(foo.bar.fred).
-export([f/1]).
-import(lists).
f(X) -> lists:reverse(X).</code>
<p>The dot-syntax for module names can be used in any expression.
All segments must
be constant atoms, and the result must be a well-formed
package/module name.
E.g.:</p>
<code type="none">
spawn(foo.bar.fred, f, [X])</code>
<p>is equivalent to <c>spawn('foo.bar.fred', f, [X])</c>.</p>
<p><em>The Erlang Shell</em></p>
<p>The shell also automatically expands remote calls,
however currently no
expansions are made by default.
The user can change the behaviour by using the <c>import/1</c>
shell command (or its abbreviation <c>use/1</c>). E.g.:</p>
<pre>
1> <input>import(foo.bar.m).</input>
ok
2> <input>m:f().</input></pre>
<p>will evaluate <c>foo.bar.m:f()</c>.
If a new import is made of the same name,
this overrides any previous import.
(It is likely that in the future, some
system packages will be pre-imported.)</p>
<p>In addition, the shell command <c>import_all/1</c>
(and its alias <c>use_all/1</c>)
imports all modules currently found in the path for a given
package name. E.g.,
assuming the files "<c>.../foo/bar/fred.beam</c>",
"<c>.../foo/bar/barney.beam</c>"
and "<c>.../foo/bar/bambam.beam</c>" can be found from our current
path,</p>
<pre>
1> <input>import_all(foo.bar).</input></pre>
<p>will make <c>fred</c>, <c>barney</c> and <c>bambam</c>
expand to <c>foo.bar.fred</c>,
<c>foo.bar.barney</c> and <c>foo.bar.bambam</c>, respectively.</p>
<p>Note: The compiler does not have an "import all" directive, for the
reason that Erlang has no compile time type checking.
E.g. if the wrong search
path is used at compile time, a call <c>m:f(...)</c>
could be expanded to <c>foo.bar.m:f(...)</c>
without any warning, instead of the intended
<c>frob.ozz.m:f(...)</c>, if
package <c>foo.bar</c> happens to be found first in the path.
Explicitly
declaring each use of a module makes for safe code.</p>
</description>
</erlref>
