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

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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2018</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>io_lib</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>io_lib</module>
  <modulesummary>I/O library functions.</modulesummary>
  <description>
    <p>This module contains functions for converting to and from
      strings (lists of characters). They are used for implementing the
      functions in the <seealso marker="io"><c>io</c></seealso> module.
      There is no guarantee that the
      character lists returned from some of the functions are flat,
      they can be deep lists. Function
      <seealso marker="lists#flatten/1"><c>lists:flatten/1</c></seealso>
      can be used for flattening deep lists.</p>
  </description>

  <datatypes>
    <datatype>
      <name name="chars"/>
    </datatype>
    <datatype>
      <name name="continuation"/>
      <desc><p>A continuation as returned by
        <seealso marker="#fread/3"><c>fread/3</c></seealso>.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="chars_limit"/>
    </datatype>
    <datatype>
      <name name="depth"/>
    </datatype>
    <datatype>
      <name name="fread_error"/>
    </datatype>
    <datatype>
      <name name="fread_item"/>
    </datatype>
    <datatype>
      <name name="latin1_string"/>
    </datatype>
    <datatype>
      <name name="format_spec"/>
      <desc><p>Where:</p>
      <list type="bulleted">
        <item><p><c>control_char</c> is the type of control
          sequence: <c>$P</c>, <c>$w</c>, and so on.</p>
        </item>
        <item><p><c>args</c> is a list of the arguments used by the
          control sequence, or an empty list if the control sequence
          does not take any arguments.</p>
        </item>
        <item><p><c>width</c> is the field width.</p>
        </item>
        <item><p><c>adjust</c> is the adjustment.</p>
        </item>
        <item><p><c>precision</c> is the precision of the printed
          argument.</p>
        </item>
        <item><p><c>pad_char</c> is the padding character.</p>
        </item>
        <item><p><c>encoding</c> is set to <c>true</c> if translation
          modifier <c>t</c> is present.</p>
        </item>
        <item><p><c>strings</c> is set to <c>false</c> if modifier
          <c>l</c> is present.</p>
        </item>
      </list>
      </desc>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="build_text" arity="1" since="OTP 18.0"/>
      <fsummary>Build the output text for a preparsed format list.</fsummary>
      <desc>
        <p>For details, see
          <seealso marker="#scan_format/2"><c>scan_format/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="char_list" arity="1"/>
      <fsummary>Test for a list of characters.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Term</anno></c> is a flat list of
          characters in the Unicode range, otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="deep_char_list" arity="1"/>
      <fsummary>Test for a deep list of characters.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Term</anno></c> is a, possibly deep,
          list of characters in the Unicode range, otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="deep_latin1_char_list" arity="1" since="OTP R16B"/>
      <fsummary>Test for a deep list of characters.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Term</anno></c> is a, possibly deep,
          list of characters in the ISO Latin-1 range, otherwise
          <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="format" arity="2"/>
      <name name="fwrite" arity="2"/>
      <fsummary>Write formatted output.</fsummary>
      <desc>
        <p>Returns a character list that represents <c><anno>Data</anno></c>
          formatted in accordance with <c><anno>Format</anno></c>.
          For a detailed description of the available formatting options, see
          <seealso marker="io#fwrite/1"><c>io:fwrite/1,2,3</c></seealso>.
          If the format string or argument list contains an error, a fault is
          generated.</p>
        <p>If and only if the Unicode translation modifier is used in the
          format string (that is, <c>~ts</c> or <c>~tc</c>), the resulting list
          can contain characters beyond the ISO Latin-1 character range
          (that is, numbers &gt; 255). If so, the
          result is still an ordinary Erlang <c>string()</c>, and can well be
          used in any context where Unicode data is allowed.</p>
      </desc>
    </func>

    <func>
      <name name="format" arity="3" since="OTP 21.0"/>
      <name name="fwrite" arity="3" since="OTP 21.0"/>
      <fsummary>Write formatted output.</fsummary>
      <desc>
        <p>Returns a character list that represents <c><anno>Data</anno></c>
          formatted in accordance with <c><anno>Format</anno></c> in
          the same way as
          <seealso marker="#fwrite/2"><c>fwrite/2</c></seealso> and
          <seealso marker="#format/2"><c>format/2</c></seealso>,
          but takes an extra argument, a list of options.</p>
          <p>Valid option:</p>
          <taglist>
            <tag><c>{chars_limit, <anno>CharsLimit</anno>}</c></tag>
            <item>
              <p>A soft limit on the number of characters returned.
                When the number of characters is reached, remaining
                structures are replaced by "<c>...</c>".
                <c><anno>CharsLimit</anno></c> defaults to -1, which
		means no limit on the number of characters returned.</p>
            </item>
          </taglist>
      </desc>
    </func>

    <func>
      <name name="fread" arity="2"/>
      <fsummary>Read formatted input.</fsummary>
      <desc>
        <p>Tries to read <c><anno>String</anno></c> in accordance with the
          control sequences in <c><anno>Format</anno></c>.
          For a detailed description of the available formatting options, see
          <seealso marker="io#fread/3"><c>io:fread/3</c></seealso>. It is
          assumed that <c><anno>String</anno></c> contains whole lines.</p>
        <p>The function returns:</p>
        <taglist>
          <tag><c>{ok, <anno>InputList</anno>,
            <anno>LeftOverChars</anno>}</c></tag>
          <item>
            <p>The string was read. <c><anno>InputList</anno></c> is the list
              of successfully matched and read items, and
              <c><anno>LeftOverChars</anno></c> are the input characters not
              used.</p>
          </item>
          <tag><c>{more, <anno>RestFormat</anno>, <anno>Nchars</anno>,
            <anno>InputStack</anno>}</c></tag>
          <item>
            <p>The string was read, but more input is needed to complete the
              original format string. <c><anno>RestFormat</anno></c> is the
              remaining format string, <c><anno>Nchars</anno></c> is the number
              of characters scanned, and <c><anno>InputStack</anno></c> is the
              reversed list of inputs matched up to that point.</p>
          </item>
          <tag><c>{error, <anno>What</anno>}</c></tag>
          <item>
            <p>The read operation failed and parameter <c><anno>What</anno></c>
              gives a hint about the error.</p>
          </item>
        </taglist>
        <p><em>Example:</em></p>
        <pre>
3> <input>io_lib:fread("~f~f~f", "15.6 17.3e-6 24.5").</input>
{ok,[15.6,1.73e-5,24.5],[]}</pre>
      </desc>
    </func>

    <func>
      <name name="fread" arity="3"/>
      <fsummary>Re-entrant formatted reader</fsummary>
      <desc>
        <p>This is the re-entrant formatted reader. The continuation of
          the first call to the functions must be <c>[]</c>. For a complete
          description of how the re-entrant input scheme works, see
          Armstrong, Virding, Williams: 'Concurrent Programming in
          Erlang', Chapter 13.</p>
        <p>The function returns:</p>
        <taglist>
          <tag><c>{done, <anno>Result</anno>,
            <anno>LeftOverChars</anno>}</c></tag>
          <item>
            <p>The input is complete. The result is one of the following:</p>
            <taglist>
              <tag><c>{ok, <anno>InputList</anno>}</c></tag>
              <item>
                <p>The string was read. <c><anno>InputList</anno></c> is the
                  list of successfully matched and read items, and
                  <c><anno>LeftOverChars</anno></c> are the remaining
                  characters.</p>
              </item>
              <tag><c>eof</c></tag>
              <item>
                <p>End of file was encountered.
                  <c><anno>LeftOverChars</anno></c> are the input characters not
                  used.</p>
              </item>
              <tag><c>{error, <anno>What</anno>}</c></tag>
              <item>
                <p>An error occurred and parameter <c><anno>What</anno></c>
                  gives a hint about the error.</p>
              </item>
            </taglist>
          </item>
          <tag><c>{more, <anno>Continuation</anno>}</c></tag>
          <item>
            <p>More data is required to build a term.
              <c><anno>Continuation</anno></c> must be passed to <c>fread/3</c>
              when more data becomes available.</p>
          </item>
        </taglist>
      </desc>
    </func>

    <func>
      <name name="indentation" arity="2"/>
      <fsummary>Indentation after printing string.</fsummary>
      <desc>
        <p>Returns the indentation if <c><anno>String</anno></c> has been
          printed, starting at <c><anno>StartIndent</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="latin1_char_list" arity="1" since="OTP R16B"/>
      <fsummary>Test for a list of ISO Latin-1 characters.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Term</anno></c> is a flat list of
          characters in the ISO Latin-1 range, otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="nl" arity="0"/>
      <fsummary>Write a newline.</fsummary>
      <desc>
        <p>Returns a character list that represents a new line character.</p>
      </desc>
    </func>

    <func>
      <name name="print" arity="1"/>
      <name name="print" arity="4"/>
      <fsummary>Pretty print a term.</fsummary>
      <desc>
        <p>Returns a list of characters that represents
          <c><anno>Term</anno></c>, but breaks representations longer
          than one line into many lines and indents each line sensibly.
          Also tries to detect and output lists of printable characters
          as strings.</p>
        <list type="bulleted">
          <item><c><anno>Column</anno></c> is the starting column; defaults
            to 1.</item>
          <item><c><anno>LineLength</anno></c> is the maximum line length;
            defaults to 80.</item>
          <item><c><anno>Depth</anno></c> is the maximum print depth;
            defaults to -1, which means no limitation.</item>
        </list>
      </desc>
    </func>

    <func>
      <name name="printable_latin1_list" arity="1" since="OTP R16B"/>
      <fsummary>Test for a list of printable ISO Latin-1 characters.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Term</anno></c> is a flat list of
          printable ISO Latin-1 characters, otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="printable_list" arity="1"/>
      <fsummary>Test for a list of printable characters.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Term</anno></c> is a flat list of
          printable characters, otherwise <c>false</c>.</p>
        <p>What is a printable character in this case is determined by
          startup flag <c>+pc</c> to the Erlang VM; see
          <seealso marker="io#printable_range/0">
          <c>io:printable_range/0</c></seealso> and 
          <seealso marker="erts:erl"><c>erl(1)</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="printable_unicode_list" arity="1" since="OTP R16B"/>
      <fsummary>Test for a list of printable Unicode characters.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Term</anno></c> is a flat list of
          printable Unicode characters, otherwise <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="scan_format" arity="2" since="OTP 18.0"/>
      <fsummary>Parse all control sequences in the format string.</fsummary>
      <desc>
        <p>Returns a list corresponding to the specified format string,
          where control sequences have been replaced with
          corresponding tuples. This list can be passed to:</p>
        <list type="bulleted">
          <item>
            <p><seealso marker="#build_text/1"><c>build_text/1</c></seealso>
              to have the same effect as <c>format(Format, Args)</c></p>
          </item>
          <item>
            <p><seealso marker="#unscan_format/1">
              <c>unscan_format/1</c></seealso> to get the corresponding pair
              of <c>Format</c> and <c>Args</c> (with every <c>*</c> and
              corresponding argument expanded to numeric values)</p>
          </item>
        </list>
        <p>A typical use of this function is to replace unbounded-size
          control sequences like <c>~w</c> and <c>~p</c> with the
          depth-limited variants <c>~W</c> and <c>~P</c> before
          formatting to text in, for example, a logger.</p>
      </desc>
    </func>

    <func>
      <name name="unscan_format" arity="1" since="OTP 18.0"/>
      <fsummary>Revert a preparsed format list to a plain character list
        and a list of arguments.</fsummary>
      <desc>
        <p>For details, see
          <seealso marker="#scan_format/2"><c>scan_format/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="write" arity="1"/>
      <name name="write" arity="2" clause_i="1"/>
      <name name="write" arity="2" clause_i="2"/>
      <fsummary>Write a term.</fsummary>
      <desc>
        <p>Returns a character list that represents <c><anno>Term</anno></c>.
          Option <c><anno>Depth</anno></c> controls the depth of the
          structures written. When the specified depth is reached,
          everything below this level is replaced by "<c>...</c>".
          <c><anno>Depth</anno></c> defaults to -1, which means
          no limitation. Option <c><anno>CharsLimit</anno></c> puts a
          soft limit on the number of characters returned. When the
          number of characters is reached, remaining structures are
          replaced by "<c>...</c>". <c><anno>CharsLimit</anno></c>
          defaults to -1, which means no limit on the number of
          characters returned.</p>
        <p><em>Example:</em></p>
        <pre>
1> <input>lists:flatten(io_lib:write({1,[2],[3],[4,5],6,7,8,9})).</input>
"{1,[2],[3],[4,5],6,7,8,9}"
2> <input>lists:flatten(io_lib:write({1,[2],[3],[4,5],6,7,8,9}, 5)).</input>
"{1,[2],[3],[...],...}"
3> <input>lists:flatten(io_lib:write({[1,2,3],[4,5],6,7,8,9}, [{chars_limit,20}])).</input>
"{[1,2|...],[4|...],...}"</pre>
      </desc>
    </func>

    <func>
      <name name="write_atom" arity="1"/>
      <fsummary>Write an atom.</fsummary>
      <desc>
        <p>Returns the list of characters needed to print atom
          <c><anno>Atom</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="write_atom_as_latin1" arity="1" since="OTP 20.0"/>
      <fsummary>Write an atom.</fsummary>
      <desc>
        <p>Returns the list of characters needed to print atom
          <c><anno>Atom</anno></c>. Non-Latin-1 characters
          are escaped.</p>
      </desc>
    </func>

    <func>
      <name name="write_char" arity="1"/>
      <fsummary>Write a character.</fsummary>
      <desc>
        <p>Returns the list of characters needed to print a character
          constant in the Unicode character set.</p>
      </desc>
    </func>

    <func>
      <name name="write_char_as_latin1" arity="1" since="OTP R16B"/>
      <fsummary>Write a character.</fsummary>
      <desc>
        <p>Returns the list of characters needed to print a character
          constant in the Unicode character set. Non-Latin-1 characters
          are escaped.</p>
      </desc>
    </func>

    <func>
      <name name="write_latin1_char" arity="1" since="OTP R16B"/>
      <fsummary>Write an ISO Latin-1 character.</fsummary>
      <desc>
        <p>Returns the list of characters needed to print a character
          constant in the ISO Latin-1 character set.</p>
      </desc>
    </func>

    <func>
      <name name="write_latin1_string" arity="1" since="OTP R16B"/>
      <fsummary>Write an ISO Latin-1 string.</fsummary>
      <desc>
        <p>Returns the list of characters needed to print
          <c><anno>Latin1String</anno></c> as a string.</p>
      </desc>
    </func>

    <func>
      <name name="write_string" arity="1"/>
      <fsummary>Write a string.</fsummary>
      <desc>
        <p>Returns the list of characters needed to print
          <c><anno>String</anno></c> as a string.</p>
      </desc>
    </func>

    <func>
      <name name="write_string_as_latin1" arity="1" since="OTP R16B"/>
      <fsummary>Write a string.</fsummary>
      <desc>
        <p>Returns the list of characters needed to print
          <c><anno>String</anno></c> as a string. Non-Latin-1
          characters are escaped.</p>
      </desc>
    </func>
  </funcs>
</erlref>