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<h1>3 The Erlang mode for Emacs</h1>
<h3><a name="id64082">3.1
Purpose</a></h3>
<p>The purpose of this user guide is to introduce you to the
Erlang mode for Emacs and gives some relevant background
information of the functions and features. See also <span class="bold_code"><a href="erlang.el.html">Erlang mode reference manual</a></span> The
purpose of the Erlang mode itself is to facilitate the developing
process for the Erlang programmer.</p>
<h3><a name="id64101">3.2
Pre-requisites</a></h3>
<p>Basic knowledge of Emacs and Erlang/OTP. </p>
<h3><a name="id64113">3.3
Elisp</a></h3>
<p>There are two Elisp modules included in this tool package
for Emacs. There is erlang.el that defines the actual erlang mode
and there is erlang-start.el that makes some nice initializations.</p>
<h3><a name="id64126">3.4
Setup on UNIX</a></h3>
<p>To set up the Erlang Emacs mode on a UNIX systems, edit/create
the file <span class="code">.emacs</span> in the your home directory.</p>
<p>Below is a complete example of what should be added to a user's
<span class="code">.emacs</span> provided that OTP is installed in the directory
<span class="code">/usr/local/otp </span>: </p>
<div class="example"><pre>
(setq load-path (cons "/usr/local/otp/lib/tools-<ToolsVer>/emacs"
load-path))
(setq erlang-root-dir "/usr/local/otp")
(setq exec-path (cons "/usr/local/otp/bin" exec-path))
(require 'erlang-start)
</pre></div>
<h3><a name="id64160">3.5
Setup on Windows </a></h3>
<p>To set up the Erlang Emacs mode on a Windows systems,
edit/create the file <span class="code">.emacs</span>, the location of the file
depends on the configuration of the system. If the <strong>HOME</strong>
environment variable is set, Emacs will look for the
<span class="code">.emacs</span> file in the directory indicated by the
<strong>HOME</strong> variable. If <strong>HOME</strong> is not set, Emacs
will look for the <span class="code">.emacs</span> file in <span class="code">C:\ </span>.</p>
<p>Below is a complete example of what should be added to a user's
<span class="code">.emacs</span> provided that OTP is installed in the directory
<span class="code">C:\Program Files\erl<Ver></span>: </p>
<div class="example"><pre>
(setq load-path (cons "C:/Program Files/erl<Ver>/lib/tools-<ToolsVer>/emacs"
load-path))
(setq erlang-root-dir "C:/Program Files/erl<Ver>")
(setq exec-path (cons "C:/Program Files/erl<Ver>/bin" exec-path))
(require 'erlang-start)
</pre></div>
<div class="note">
<div class="label">Note</div>
<div class="content"><p>
<p>In .emacs, the slash character "/" can be used as path
separator. But if you decide to use the backslash character "\",
please not that you must use double backslashes, since they are
treated as escape characters by Emacs.</p>
</p></div>
</div>
<h3><a name="id64228">3.6
Indentation</a></h3>
<p>The "Oxford Advanced Learners Dictionary of Current English" says the
following about the word "indent":</p>
<p>"start (a line of print or writing) farther from
the margin than the others".</p>
<p>The Erlang mode does, of course, provide this feature. The layout
used is based on the common use of the language.</p>
<p>It is strongly recommend to use this feature and avoid to indent lines
in a nonstandard way. Some motivations are:</p>
<ul>
<li>Code using the same layout is easy to read and maintain. </li>
<li>Since several features of Erlang mode is based on the
standard layout they might not work correctly if a nonstandard layout
is used. </li>
</ul>
<p>The indentation features can be used to reindent large sections
of a file. If some lines use nonstandard indentation they will
be reindented.</p>
<h3><a name="id64277">3.7
Editing</a></h3>
<ul>
<li>
<strong><span class="code">M-x erlang-mode RET</span></strong> - This command activates
the Erlang major mode for the current buffer. When this
mode is active the mode line contain the word "Erlang".</li>
</ul>
<p>When the Erlang mode is correctly installed, it is
automatically activated when a file ending in <span class="code">.erl</span> or
<span class="code">.hrl</span> is opened in Emacs.</p>
<p>When a file is saved the name in the <span class="code">-module().</span> line is
checked against the file name. Should they mismatch Emacs can
change the module specifier so that it matches the file name.
By default, the user is asked before the change is performed.</p>
<p>An "electric" command is a character that in addition to just
inserting the character performs some type of action. For
example the ";" character is typed in a situation where is ends
a function clause a new function header is generated. The electric
commands are as follows: </p>
<ul>
<li>
<strong><span class="code">erlang-electric-comma</span></strong> - Insert a comma
character and possibly a new indented line. </li>
<li>
<strong><span class="code">erlang-electric-semicolon</span></strong> - Insert a
semicolon character and possibly a prototype for the next line.</li>
<li>
<strong><span class="code">erlang-electric-gt</span></strong> - "Insert a '>'-sign
and possible a new indented line.</li>
</ul>
<p>To disable all electric commands set the variable
<span class="code">erlang-electric-commands</span> to the empty list. In short,
place the following line in your <span class="code">.emacs</span>-file:</p>
<div class="example"><pre>
(setq erlang-electric-commands '())</pre></div>
<h3><a name="id64373">3.8
Syntax highlighting</a></h3>
<p>It is possible for Emacs to use colors when displaying a buffer. By
"syntax highlighting", we mean that syntactic components, for example
keywords and function names, will be colored.</p>
<p>The basic idea of syntax highlighting is to make the structure of a
program clearer. For example, the highlighting will make it easier to
spot simple bugs. Have not you ever written a variable in lower-case
only? With syntax highlighting a variable will colored while atoms
will be shown with the normal text color.</p>
<h3><a name="id64396">3.9
Tags</a></h3>
<a name="tags"></a>
<p>Tags is a standard Emacs package used to record information
about source files in large development projects. In addition to
listing the files of a project, a tags file normally contains
information about all functions and variables that are defined.
By far, the most useful command of the tags system is its ability
to find the definition of functions in any file in the project.
However the Tags system is not limited to this feature, for
example, it is possible to do a text search in all files in a
project, or to perform a project-wide search and replace.</p>
<p>In order to use the Tags system a file named <span class="code">TAGS</span> must be
created. The file can be seen as a database over all functions,
records, and macros in all files in the project. The
<span class="code">TAGS</span> file can be created using two different methods for
Erlang. The first is the standard Emacs utility "etags", the
second is by using the Erlang module <span class="code">tags</span>.</p>
<h3><a name="id64430">3.10
Etags</a></h3>
<p><span class="code">etags</span> is a program that is part of the Emacs
distribution. It is normally executed from a command line, like
a unix shell or a DOS box.</p>
<p>The <span class="code">etags</span> program of fairly modern versions of Emacs and XEmacs
has native support for Erlang. To check if your version does include
this support, issue the command <span class="code">etags --help</span> at a the command
line prompt. At the end of the help text there is a list of supported
languages. Unless Erlang is a member of this list I suggest that you
should upgrade to a newer version of Emacs.</p>
<p>As seen in the help text -- unless you have not upgraded your
Emacs yet (well, what are you waiting around here for? Off you go and
upgrade!) -- <span class="code">etags</span> associate the file extensions <span class="code">.erl</span>
and <span class="code">.hrl</span> with Erlang.</p>
<p>Basically, the <span class="code">etags</span> utility is ran using the following form:</p>
<div class="example"><pre>
etags file1.erl file2.erl</pre></div>
<p>This will create a file named <span class="code">TAGS</span> in the current directory.</p>
<p>The <span class="code">etags</span> utility can also read a list of files from its
standard input by supplying a single dash in place of the file
names. This feature is useful when a project consists of a
large number of files. The standard UNIX command <span class="code">find</span>
can be used to generate the list of files, e.g:</p>
<div class="example"><pre>
find . -name "*.[he]rl" -print | etags -</pre></div>
<p>The above line will create a <span class="code">TAGS</span> file covering all the
Erlang source files in the current directory, and in the
subdirectories below.</p>
<p>Please see the GNU Emacs Manual and the etags man page for more
info.</p>
<h3><a name="id64528">3.11
Shell</a></h3>
<p>The look and feel on an Erlang shell inside Emacs should be the
same as in a normal Erlang shell. There is just one major
difference, the cursor keys will actually move the cursor around
just like in any normal Emacs buffer. The command line history
can be accessed by the following commands: </p>
<ul>
<li>
<strong><span class="code">C-up </span></strong> or <strong><span class="code">M-p </span></strong>
(<span class="code">comint-previous-input</span>) -
Move to the previous line in the input history.</li>
<li>
<strong><span class="code">C-down </span></strong> or <strong><span class="code">M-n </span></strong>
(<span class="code">comint-next-input</span>) - Move to the next line in the
input history.</li>
</ul>
<p>If the Erlang shell buffer would be killed the command line
history is saved to a file. The command line history is
automatically retrieved when a new Erlang shell is started.</p>
<h3><a name="id64585">3.12
Compilation</a></h3>
<p>The classic edit-compile-bugfix cycle for Erlang is to edit the
source file in an editor, save it to a file and switch to an
Erlang shell. In the shell the compilation command is given.
Should the compilation fail you have to bring out the editor and
locate the correct line.</p>
<p>With the Erlang editing mode the entire edit-compile-bugfix cycle can
be performed without leaving Emacs. Emacs can order Erlang to compile
a file and it can parse the error messages to automatically place the
point on the erroneous lines.</p>
</div>
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