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<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2015</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions 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="category"></name>
    </datatype>
    <datatype>
      <name name="error_description"></name>
    </datatype>
    <datatype>
      <name name="error_info"></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="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>Anno</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>Anno</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
          annotations</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="#column/1">column/1</seealso>,
          <seealso marker="#line/1">line/1</seealso>,
          <seealso marker="#location/1">location/1</seealso>, and
          <seealso marker="#text/1">text/1</seealso>.</p>
        <p>A <em>token</em> is a tuple containing information about
          syntactic category, the token annotations, 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 annotation. 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"/>
      <fsummary>Re-entrant scanner</fsummary>
      <type name="char_spec"/>
      <type name="return_cont"/>
      <type_desc name="return_cont">An opaque continuation</type_desc>
      <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="category" arity="1"/>
      <fsummary>Return the category</fsummary>
      <desc>
        <p>Returns the category of <c><anno>Token</anno></c>.
        </p>
      </desc>
    </func>
    <func>
      <name name="symbol" arity="1"/>
      <fsummary>Return the symbol</fsummary>
      <desc>
        <p>Returns the symbol of <c><anno>Token</anno></c>.
        </p>
      </desc>
    </func>
    <func>
      <name name="column" arity="1"/>
      <fsummary>Return the column</fsummary>
      <desc>
        <p>Returns the column of <c><anno>Token</anno></c>'s
          collection of annotations.
        </p>
      </desc>
    </func>
    <func>
      <name name="end_location" arity="1"/>
      <fsummary>Return the end location of the text</fsummary>
      <desc>
        <p>Returns the end location of the text of
          <c><anno>Token</anno></c>'s collection of annotations. If
            there is no text,
          <c>undefined</c> is returned.
        </p>
      </desc>
    </func>
    <func>
      <name name="line" arity="1"/>
      <fsummary>Return the line</fsummary>
      <desc>
        <p>Returns the line of <c><anno>Token</anno></c>'s collection
          of annotations.
        </p>
      </desc>
    </func>
    <func>
      <name name="location" arity="1"/>
      <fsummary>Return the location</fsummary>
      <desc>
        <p>Returns the location of <c><anno>Token</anno></c>'s
          collection of annotations.
        </p>
      </desc>
    </func>
    <func>
      <name name="text" arity="1"/>
      <fsummary>Return the text</fsummary>
      <desc>
        <p>Returns the text of <c><anno>Token</anno></c>'s collection
        of annotations. If there is no text, <c>undefined</c> is
        returned.
        </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_anno">erl_anno(3)</seealso>,
       <seealso marker="erl_parse">erl_parse(3)</seealso></p>
  </section>
</erlref>