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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year>
      <year>2011</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.

  The Initial Developer of the Original Code is Ericsson AB.
    </legalnotice>

    <title>erl_pp</title>
    <prepared>Robert Virding</prepared>
    <responsible>Bjarne D&auml;cker</responsible>
    <docno>1</docno>
    <approved>Bjarne D&auml;cker</approved>
    <checked></checked>
    <date>97-01-24</date>
    <rev>B</rev>
    <file>erl_pp.sgml</file>
  </header>
  <module>erl_pp</module>
  <modulesummary>The Erlang Pretty Printer</modulesummary>
  <description>
    <p>The functions in this module are used to generate
      aesthetically attractive representations of abstract
      forms, which are suitable for printing. All functions return (possibly deep)
      lists of characters and generate an error if the form is wrong.</p>
    <p>All functions can have an optional argument which specifies a hook
      that is called if an attempt is made to print an unknown form.</p>
  </description>
  <funcs>
    <func>
      <name>form(Form) -> DeepCharList</name>
      <name>form(Form, HookFunction) -> DeepCharList</name>
      <fsummary>Pretty print a form</fsummary>
      <type>
        <v>Form = term()</v>
        <v>HookFunction = see separate description below.</v>
        <v>DeepCharList = [char()|DeepCharList]</v>
      </type>
      <desc>
        <p>Pretty prints a 
          <c>Form</c> which is an abstract form of a type which is
          returned by <c>erl_parse:parse_form</c>.</p>
      </desc>
    </func>
    <func>
      <name>attribute(Attribute) -> DeepCharList</name>
      <name>attribute(Attribute, HookFunction) -> DeepCharList</name>
      <fsummary>Pretty print an attribute</fsummary>
      <type>
        <v>Attribute = term()</v>
        <v>HookFunction = see separate description below.</v>
        <v>DeepCharList = [char()|DeepCharList]</v>
      </type>
      <desc>
        <p>The same as <c>form</c>, but only for the attribute
          <c>Attribute</c>.</p>
      </desc>
    </func>
    <func>
      <name>function(Function) -> DeepCharList</name>
      <name>function(Function, HookFunction) -> DeepCharList</name>
      <fsummary>Pretty print a function</fsummary>
      <type>
        <v>Function = term()</v>
        <v>HookFunction = see separate description below.</v>
        <v>DeepCharList = [char()|DeepCharList]</v>
      </type>
      <desc>
        <p>The same as <c>form</c>, but only for the function
          <c>Function</c>.</p>
      </desc>
    </func>
    <func>
      <name>guard(Guard) -> DeepCharList</name>
      <name>guard(Guard, HookFunction) -> DeepCharList</name>
      <fsummary>Pretty print a guard</fsummary>
      <type>
        <v>Form = term()</v>
        <v>HookFunction = see separate description below.</v>
        <v>DeepCharList = [char()|DeepCharList]</v>
      </type>
      <desc>
        <p>The same as <c>form</c>, but only for the guard test
          <c>Guard</c>.</p>
      </desc>
    </func>
    <func>
      <name>exprs(Expressions) -> DeepCharList</name>
      <name>exprs(Expressions, HookFunction) -> DeepCharList</name>
      <name>exprs(Expressions, Indent, HookFunction) -> DeepCharList</name>
      <fsummary>Pretty print <c>Expressions</c></fsummary>
      <type>
        <v>Expressions = term()</v>
        <v>HookFunction = see separate description below.</v>
        <v>Indent = integer()</v>
        <v>DeepCharList = [char()|DeepCharList]</v>
      </type>
      <desc>
        <p>The same as <c>form</c>, but only for the sequence of
          expressions in <c>Expressions</c>.</p>
      </desc>
    </func>
    <func>
      <name>expr(Expression) -> DeepCharList</name>
      <name>expr(Expression, HookFunction) -> DeepCharList</name>
      <name>expr(Expression, Indent, HookFunction) -> DeepCharList</name>
      <name>expr(Expression, Indent, Precedence, HookFunction) ->-> DeepCharList</name>
      <fsummary>Pretty print one <c>Expression</c></fsummary>
      <type>
        <v>Expression = term()</v>
        <v>HookFunction = see separate description below.</v>
        <v>Indent = integer()</v>
        <v>Precedence = </v>
        <v>DeepCharList = [char()|DeepCharList]</v>
      </type>
      <desc>
        <p>This function prints one expression. It is useful for implementing hooks (see below).</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Unknown Expression Hooks</title>
    <p>The optional argument <c>HookFunction</c>, shown in the functions described above,  defines a function which is called when an unknown form occurs where there
      should be a valid expression. It can have the following formats:</p>
    <taglist>
      <tag><c>Function</c></tag>
      <item>
        <p>The hook function is called by:</p>
        <code type="none">
Function(Expr, 
         CurrentIndentation,
         CurrentPrecedence,
         HookFunction)        </code>
      </item>
      <tag><c>none</c></tag>
      <item>
        <p>There is no hook function</p>
      </item>
    </taglist>
    <p>The called hook function should return a (possibly deep) list
      of characters. <c>expr/4</c> is useful in a hook.
      </p>
    <p>If <c>CurrentIndentation</c> is negative, there will be no line
      breaks and only a space is used as a separator.</p>
  </section>

  <section>
    <title>Bugs</title>
    <p>It should be possible to have hook functions for unknown forms
      at places other than expressions.</p>
  </section>

  <section>
    <title>See Also</title>
    <p><seealso marker="io">io(3)</seealso>,
      <seealso marker="erl_parse">erl_parse(3)</seealso>,
      <seealso marker="erl_eval">erl_eval(3)</seealso></p>
  </section>
</erlref>