path: root/lib/stdlib/doc/src/erl_scan.xml

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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2013</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>erl_scan</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_scan.sgml</file>
  </header>
  <module>erl_scan</module>
  <modulesummary>The Erlang Token Scanner</modulesummary>
  <description>
    <p>This module contains functions for tokenizing characters into
      Erlang tokens.</p>
  </description>
  <datatypes>
    <datatype>
      <name name="attribute_info"></name>
    </datatype>
    <datatype>
      <name name="attributes"></name>
    </datatype>
    <datatype>
      <name name="attributes_data"></name>
    </datatype>
    <datatype>
      <name name="category"></name>
    </datatype>
    <datatype>
      <name name="column"></name>
    </datatype>
    <datatype>
      <name name="error_description"></name>
    </datatype>
    <datatype>
      <name name="error_info"></name>
    </datatype>
    <datatype>
      <name name="info_line"></name>
    </datatype>
    <datatype>
      <name name="info_location"></name>
    </datatype>
    <datatype>
      <name name="line"></name>
    </datatype>
    <datatype>
      <name name="location"></name>
    </datatype>
    <datatype>
      <name name="option"></name>
    </datatype>
    <datatype>
      <name name="options"></name>
    </datatype>
    <datatype>
      <name name="symbol"></name>
    </datatype>
    <datatype>
      <name name="resword_fun"></name>
    </datatype>
    <datatype>
      <name name="token"></name>
    </datatype>
    <datatype>
      <name name="token_info"></name>
    </datatype>
    <datatype>
      <name name="tokens"></name>
    </datatype>
    <datatype>
      <name name="tokens_result"></name>
    </datatype>
  </datatypes>
  <funcs>
    <func>
      <name name="string" arity="1"/>
      <name name="string" arity="2"/>
      <name name="string" arity="3"/>
      <fsummary>Scan a string and return the Erlang tokens</fsummary>
      <desc>
        <p>Takes the list of characters <c><anno>String</anno></c> and tries to
          scan (tokenize) them. Returns <c>{ok, <anno>Tokens</anno>,
           <anno>EndLocation</anno>}</c>,
          where <c><anno>Tokens</anno></c> are the Erlang tokens from
          <c><anno>String</anno></c>. <c><anno>EndLocation</anno></c>
          is the first location after the last token.</p>
        <p><c>{error, <anno>ErrorInfo</anno>, <anno>ErrorLocation</anno>}</c>
          is returned if an error occurs.
          <c><anno>ErrorLocation</anno></c> is the first location after
          the erroneous token.</p>
        <p><c>string(<anno>String</anno>)</c> is equivalent to
          <c>string(<anno>String</anno>, 1)</c>, and
          <c>string(<anno>String</anno>,
          <anno>StartLocation</anno>)</c> is equivalent to
          <c>string(<anno>String</anno>,
          <anno>StartLocation</anno>, [])</c>.</p>
        <p><c><anno>StartLocation</anno></c> indicates the initial location
          when scanning starts. If <c><anno>StartLocation</anno></c> is a line
          <c>attributes()</c> as well as <c><anno>EndLocation</anno></c> and
          <c><anno>ErrorLocation</anno></c> will be lines. If
          <c><anno>StartLocation</anno></c> is a pair of a line and a column
          <c>attributes()</c> takes the form of an opaque compound
          data type, and <c><anno>EndLocation</anno></c> and
          <c><anno>ErrorLocation</anno></c>
          will be pairs of a line and a column. The <em>token
          attributes</em> contain information about the column and the
          line where the token begins, as well as the text of the
          token (if the <c>text</c> option is given), all of which can
          be accessed by calling <seealso
          marker="#token_info/1">token_info/1,2</seealso> or <seealso
          marker="#attributes_info/1">attributes_info/1,2</seealso>.</p>
        <p>A <em>token</em> is a tuple containing information about
          syntactic category, the token attributes, and the actual
          terminal symbol. For punctuation characters (e.g. <c>;</c>,
          <c>|</c>) and reserved words, the category and the symbol
          coincide, and the token is represented by a two-tuple.
          Three-tuples have one of the following forms: <c>{atom,
          Info, atom()}</c>,
          <c>{char, Info, integer()}</c>, <c>{comment, Info,
          string()}</c>, <c>{float, Info, float()}</c>, <c>{integer,
          Info, integer()}</c>, <c>{var, Info, atom()}</c>,
          and <c>{white_space, Info, string()}</c>.</p>
        <p>The valid options are:</p>
        <taglist>
        <tag><c>{reserved_word_fun, reserved_word_fun()}</c></tag>
        <item><p>A callback function that is called when the scanner
          has found an unquoted atom. If the function returns
          <c>true</c>, the unquoted atom itself will be the category
          of the token; if the function returns <c>false</c>,
          <c>atom</c> will be the category of the unquoted atom.</p>
        </item>
        <tag><c>return_comments</c></tag>
        <item><p>Return comment tokens.</p>
        </item>
        <tag><c>return_white_spaces</c></tag>
        <item><p>Return white space tokens. By convention, if there is
          a newline character, it is always the first character of the
          text (there cannot be more than one newline in a white space
          token).</p>
        </item>
        <tag><c>return</c></tag>
        <item><p>Short for <c>[return_comments, return_white_spaces]</c>.</p>
        </item>
        <tag><c>text</c></tag>
        <item><p>Include the token's text in the token attributes. The
          text is the part of the input corresponding to the token.</p>
        </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="tokens" arity="3"/>
      <name name="tokens" arity="4"/>
      <type name="char_spec"/>
      <type name="return_cont"/>
      <type_desc name="return_cont">An opaque continuation</type_desc>
      <fsummary>Re-entrant scanner</fsummary>
      <desc>
        <p>This is the re-entrant scanner which scans characters until
          a <em>dot</em> ('.' followed by a white space) or
          <c>eof</c> has been reached. It returns:</p>
        <taglist>
          <tag><c>{done, <anno>Result</anno>, <anno>LeftOverChars</anno>}</c>
          </tag>
          <item>
            <p>This return indicates that there is sufficient input
              data to get a result. <c><anno>Result</anno></c> is:</p>
            <taglist>
              <tag><c>{ok, Tokens, EndLocation}</c>
              </tag>
              <item>
                <p>The scanning was successful. <c>Tokens</c>
                  is the list of tokens including <em>dot</em>.</p>
              </item>
              <tag><c>{eof, EndLocation}</c></tag>
              <item>
                <p>End of file was encountered before any more tokens.</p>
              </item>
              <tag><c>{error, ErrorInfo, EndLocation}</c>
              </tag>
              <item>
                <p>An error occurred. <c><anno>LeftOverChars</anno></c>
                  is the remaining characters of the input data,
                  starting from <c>EndLocation</c>.</p>
              </item>
            </taglist>
          </item>
          <tag><c>{more, <anno>Continuation1</anno>}</c></tag>
          <item>
            <p>More data is required for building a term.
              <c><anno>Continuation1</anno></c> must be passed in a new call to
              <c>tokens/3,4</c> when more data is available.</p>
          </item>
        </taglist>
        <p>The <c><anno>CharSpec</anno></c> <c>eof</c> signals end of file.
        <c><anno>LeftOverChars</anno></c> will then take the value <c>eof</c>
          as well.</p>
        <p><c>tokens(<anno>Continuation</anno>, <anno>CharSpec</anno>,
          <anno>StartLocation</anno>)</c> is equivalent to
          <c>tokens(<anno>Continuation</anno>, <anno>CharSpec</anno>,
          <anno>StartLocation</anno>, [])</c>.</p>
        <p>See <seealso marker="#string/3">string/3</seealso> for a
          description of the various options.</p>
      </desc>
    </func>
    <func>
      <name name="reserved_word" arity="1"/>
      <fsummary>Test for a reserved word</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Atom</anno></c> is an Erlang
         reserved word, otherwise <c>false</c>.</p>
      </desc>
    </func>
    <func>
      <name name="token_info" arity="1"/>
      <fsummary>Return information about a token</fsummary>
      <desc>
        <p>Returns a list containing information about the token
          <c><anno>Token</anno></c>. The order of the
          <c><anno>TokenInfoTuple</anno></c>s is not
          defined. See <seealso
          marker="#token_info/2">token_info/2</seealso> for
          information about specific
          <c><anno>TokenInfoTuple</anno></c>s.</p>
        <p>Note that if <c>token_info(Token, TokenItem)</c> returns
          <c>undefined</c> for some <c>TokenItem</c>, the
          item is not included in <c><anno>TokenInfo</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="token_info" arity="2" clause_i="1"/>
      <name name="token_info" arity="2" clause_i="2"/>
      <type name="token_item"/>
      <type name="attribute_item"/>
      <fsummary>Return information about a token</fsummary>
      <desc>
        <p>Returns a list containing information about the token
          <c><anno>Token</anno></c>. If one single
          <c><anno>TokenItem</anno></c> is given the returned value is
          the corresponding
          <c>TokenInfoTuple</c>, or <c>undefined</c> if the
          <c>TokenItem</c> has no value. If a list of
          <c><anno>TokenItem</anno></c>s is given the result is a list of
          <c><anno>TokenInfoTuple</anno></c>. The
          <c><anno>TokenInfoTuple</anno></c>s will
          appear with the corresponding <c><anno>TokenItem</anno></c>s in
          the same order as the <c><anno>TokenItem</anno></c>s
	  appear in the list of <c>TokenItem</c>s.
          <c><anno>TokenItem</anno></c>s with no value are not included
          in the list of <c><anno>TokenInfoTuple</anno></c>.</p>
	<p>The following <c><anno>TokenInfoTuple</anno></c>s with corresponding
	   <c><anno>TokenItem</anno></c>s are valid:</p>
        <taglist>
          <tag><c>{category, <seealso marker="#type-category">
            category()</seealso>}</c></tag>
          <item><p>The category of the token.</p>
          </item>
          <tag><c>{column, <seealso marker="#type-column">
            column()</seealso>}</c></tag>
          <item><p>The column where the token begins.</p>
          </item>
          <tag><c>{length, integer() > 0}</c></tag>
          <item><p>The length of the token's text.</p>
          </item>
          <tag><c>{line, <seealso marker="#type-line">
            line()</seealso>}</c></tag>
          <item><p>The line where the token begins.</p>
          </item>
          <tag><c>{location, <seealso marker="#type-location">
            location()</seealso>}</c></tag>
          <item><p>The line and column where the token begins, or
            just the line if the column unknown.</p>
          </item>
          <tag><c>{symbol, <seealso marker="#type-symbol">
            symbol()</seealso>}</c></tag>
          <item><p>The token's symbol.</p>
          </item>
          <tag><c>{text, string()}</c></tag>
          <item><p>The token's text.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="attributes_info" arity="1"/>
      <fsummary>Return information about token attributes</fsummary>
      <desc>
        <p>Returns a list containing information about the token
          attributes <c><anno>Attributes</anno></c>. The order of the
          <c><anno>AttributeInfoTuple</anno></c>s is not defined.
          See <seealso
          marker="#attributes_info/2">attributes_info/2</seealso> for
          information about specific
          <c><anno>AttributeInfoTuple</anno></c>s.</p>
        <p>Note that if <c>attributes_info(Token, AttributeItem)</c>
          returns <c>undefined</c> for some <c>AttributeItem</c> in
          the list above, the item is not included in
          <c><anno>AttributesInfo</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="attributes_info" arity="2" clause_i="1"/>
      <name name="attributes_info" arity="2" clause_i="2"/>
      <fsummary>Return information about a token attributes</fsummary>
      <type name="attribute_item"/>
      <desc>
        <p>Returns a list containing information about the token
          attributes <c><anno>Attributes</anno></c>. If one single
          <c><anno>AttributeItem</anno></c> is given the returned value is the
          corresponding <c><anno>AttributeInfoTuple</anno></c>,
          or <c>undefined</c> if the <c><anno>AttributeItem</anno></c>
          has no value. If a list of <c><anno>AttributeItem</anno></c>
          is given the result is a list of
          <c><anno>AttributeInfoTuple</anno></c>.
          The <c><anno>AttributeInfoTuple</anno></c>s
          will appear with the corresponding <c><anno>AttributeItem</anno></c>s
          in the same order as the <c><anno>AttributeItem</anno></c>s
          appear in the list of <c><anno>AttributeItem</anno></c>s.
          <c><anno>AttributeItem</anno></c>s with no
          value are not included in the list of
	  <c><anno>AttributeInfoTuple</anno></c>.</p>
	<p>The following <c><anno>AttributeInfoTuple</anno></c>s with
          corresponding <c><anno>AttributeItem</anno></c>s are valid:</p>
        <taglist>
          <tag><c>{column, <seealso marker="#type-column">
            column()</seealso>}</c></tag>
          <item><p>The column where the token begins.</p>
          </item>
          <tag><c>{length, integer() > 0}</c></tag>
          <item><p>The length of the token's text.</p>
          </item>
          <tag><c>{line, <seealso marker="#type-line">
            line()</seealso>}</c></tag>
          <item><p>The line where the token begins.</p>
          </item>
          <tag><c>{location, <seealso marker="#type-location">
            location()</seealso>}</c></tag>
          <item><p>The line and column where the token begins, or
            just the line if the column unknown.</p>
          </item>
          <tag><c>{text, string()}</c></tag>
          <item><p>The token's text.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="set_attribute" arity="3"/>
      <fsummary>Set a token attribute value</fsummary>
      <desc>
        <p>Sets the value of the <c>line</c> attribute of the token
          attributes <c><anno>Attributes</anno></c>.</p>
        <p>The <c><anno>SetAttributeFun</anno></c> is called with the value of
          the <c>line</c> attribute, and is to return the new value of
          the <c>line</c> attribute.</p>
      </desc>
    </func>
    <func>
      <name name="format_error" arity="1"/>
      <fsummary>Format an error descriptor</fsummary>
      <desc>
        <p>Takes an <c><anno>ErrorDescriptor</anno></c> and returns
          a string which
          describes the error or warning. This function is usually
          called implicitly when processing an <c>ErrorInfo</c>
          structure (see below).</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Error Information</title>
    <p>The <c>ErrorInfo</c> mentioned above is the standard
      <c>ErrorInfo</c> structure which is returned from all IO
      modules. It has the following format:</p>
    <code type="none">
{ErrorLocation, Module, ErrorDescriptor}</code>
    <p>A string which describes the error is obtained with the
      following call:</p>
    <code type="none">
Module:format_error(ErrorDescriptor)</code>
  </section>

  <section>
    <title>Notes</title>
    <p>The continuation of the first call to the re-entrant input
      functions must be <c>[]</c>. Refer to Armstrong, Virding and
      Williams, 'Concurrent Programming in Erlang', Chapter 13, for a
      complete description of how the re-entrant input scheme works.</p>
  </section>

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