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

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>2002</year><year>2017</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>proplists</title>
    <prepared>Patrik Nyblom</prepared>
    <responsible>Kenneth Lundin</responsible>
    <docno>1</docno>
    <approved>Kenneth Lundin</approved>
    <checked></checked>
    <date>2002-09-28</date>
    <rev>A</rev>
    <file>proplists.xml</file>
  </header>
  <module>proplists</module>
  <modulesummary>Support functions for property lists.</modulesummary>
  <description>
    <p>Property lists are ordinary lists containing entries in the form
      of either tuples, whose first elements are keys used for lookup and
      insertion, or atoms, which work as shorthand for tuples
      <c>{Atom, true}</c>. (Other terms are allowed in the lists, but are
      ignored by this module.) If there is more than one entry in a list for a
      certain key, the first occurrence normally overrides any later
      (irrespective of the arity of the tuples).</p>

    <p>Property lists are useful for representing inherited properties,
      such as options passed to a function where a user can specify options
      overriding the default settings, object properties, annotations,
      and so on.</p>

    <p>Two keys are considered equal if they match (<c>=:=</c>). That is,
      numbers are compared literally rather than by value, so that,
      for example, <c>1</c> and <c>1.0</c> are different keys.</p>
  </description>

  <datatypes>
    <datatype>
      <name name="property"/>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="append_values" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Similar to
          <seealso marker="#get_all_values/2"><c>get_all_values/2</c></seealso>,
          but each value is wrapped in a list unless it is already itself a
          list. The resulting list of lists is concatenated. This is often
          useful for "incremental" options.</p>
        <p><em>Example:</em></p>
        <code type="none">
append_values(a, [{a, [1,2]}, {b, 0}, {a, 3}, {c, -1}, {a, [4]}])</code>
        <p>returns:</p>
        <code type="none">
[1,2,3,4]</code>
      </desc>
    </func>

    <func>
      <name name="compact" arity="1"/>
      <fsummary></fsummary>
      <desc>
        <p>Minimizes the representation of all entries in the list. This is
          equivalent to <c><![CDATA[[property(P) || P <- ListIn]]]></c>.</p>
        <p>See also
          <seealso marker="#property/1"><c>property/1</c></seealso>,
          <seealso marker="#unfold/1"><c>unfold/1</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="delete" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Deletes all entries associated with <c><anno>Key</anno></c> from
          <c><anno>List</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="expand" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Expands particular properties to corresponding sets of
          properties (or other terms). For each pair <c>{<anno>Property</anno>,
          <anno>Expansion</anno>}</c> in <c><anno>Expansions</anno></c>: if
          <c>E</c> is the first entry in <c><anno>ListIn</anno></c> with the
          same key as <c><anno>Property</anno></c>, and <c>E</c> and
          <c><anno>Property</anno></c> have equivalent normal forms, then
          <c>E</c> is replaced with the terms in <c><anno>Expansion</anno></c>,
          and any following entries with the same key are deleted from
          <c><anno>ListIn</anno></c>.</p>
        <p>For example, the following expressions all return
          <c>[fie, bar, baz, fum]</c>:</p>
        <code type="none">
expand([{foo, [bar, baz]}], [fie, foo, fum])
expand([{{foo, true}, [bar, baz]}], [fie, foo, fum])
expand([{{foo, false}, [bar, baz]}], [fie, {foo, false}, fum])</code>
        <p>However, no expansion is done in the following call
          because <c>{foo, false}</c> shadows <c>foo</c>:</p>
        <code type="none">
expand([{{foo, true}, [bar, baz]}], [{foo, false}, fie, foo, fum])</code>
        <p>Notice that if the original property term is to be preserved in the
          result when expanded, it must be included in the expansion list. The
          inserted terms are not expanded recursively. If
          <c><anno>Expansions</anno></c> contains more than one property with
          the same key, only the first occurrence is used.</p>
        <p>See also
          <seealso marker="#normalize/2"><c>normalize/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="get_all_values" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Similar to 
          <seealso marker="#get_value/2"><c>get_value/2</c></seealso>,
          but returns the list of values for <em>all</em> entries
          <c>{Key, Value}</c> in <c><anno>List</anno></c>. If no such entry
          exists, the result is the empty list.</p>
      </desc>
    </func>

    <func>
      <name name="get_bool" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns the value of a boolean key/value option. If
          <c>lookup(<anno>Key</anno>, <anno>List</anno>)</c> would yield
          <c>{<anno>Key</anno>, true}</c>, this function returns <c>true</c>,
          otherwise <c>false</c>.</p>
        <p>See also
          <seealso marker="#get_value/2"><c>get_value/2</c></seealso>,
          <seealso marker="#lookup/2"><c>lookup/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="get_keys" arity="1"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns an unordered list of the keys used in
          <c><anno>List</anno></c>, not containing duplicates.</p>
      </desc>
    </func>

    <func>
      <name name="get_value" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Equivalent to
          <c>get_value(<anno>Key</anno>, <anno>List</anno>, undefined)</c>.</p>
      </desc>
    </func>

    <func>
      <name name="get_value" arity="3"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns the value of a simple key/value property in
          <c><anno>List</anno></c>. If <c>lookup(<anno>Key</anno>,
          <anno>List</anno>)</c> would yield <c>{<anno>Key</anno>, Value}</c>,
          this function returns the corresponding <c>Value</c>, otherwise
          <c><anno>Default</anno></c>.</p>
        <p>See also
          <seealso marker="#get_all_values/2"><c>get_all_values/2</c></seealso>,
          <seealso marker="#get_bool/2"><c>get_bool/2</c></seealso>,
          <seealso marker="#get_value/2"><c>get_value/2</c></seealso>,
          <seealso marker="#lookup/2"><c>lookup/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="is_defined" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>List</anno></c> contains at least
          one entry associated with <c><anno>Key</anno></c>, otherwise
          <c>false</c>.</p>
      </desc>
    </func>

    <func>
      <name name="lookup" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns the first entry associated with <c><anno>Key</anno></c> in
          <c><anno>List</anno></c>, if one exists, otherwise returns
          <c>none</c>. For an atom <c>A</c> in the list, the tuple
          <c>{A, true}</c> is the entry associated with <c>A</c>.</p>
        <p>See also
          <seealso marker="#get_bool/2"><c>get_bool/2</c></seealso>,
          <seealso marker="#get_value/2"><c>get_value/2</c></seealso>,
          <seealso marker="#lookup_all/2"><c>lookup_all/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="lookup_all" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Returns the list of all entries associated with
          <c><anno>Key</anno></c> in <c><anno>List</anno></c>. If no such entry
          exists, the result is the empty list.</p>
        <p>See also
          <seealso marker="#lookup/2"><c>lookup/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="normalize" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Passes <c><anno>ListIn</anno></c> through a sequence of
          substitution/expansion stages. For an <c>aliases</c> operation,
          function <seealso marker="#substitute_aliases/2">
          <c>substitute_aliases/2</c></seealso> is applied using the
          specified list of aliases:</p>
        <list type="bulleted">
          <item>
            <p>For a <c>negations</c> operation, <c>substitute_negations/2</c>
              is applied using the specified negation list.</p>
          </item>
          <item>
            <p>For an <c>expand</c> operation, function
              <seealso marker="#expand/2"><c>expand/2</c></seealso>
              is applied using the specified list of expansions.</p>
          </item>
        </list>
        <p>The final result is automatically compacted (compare
          <seealso marker="#compact/1"><c>compact/1</c></seealso>).</p>
        <p>Typically you want to substitute negations first, then aliases,
          then perform one or more expansions (sometimes you want to pre-expand
          particular entries before doing the main expansion). You might want
          to substitute negations and/or aliases repeatedly, to allow such
          forms in the right-hand side of aliases and expansion lists.</p>
        <p>See also <seealso marker="#substitute_negations/2">
          <c>substitute_negations/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="property" arity="1"/>
      <fsummary></fsummary>
      <desc>
        <p>Creates a normal form (minimal) representation of a property. If
          <c><anno>PropertyIn</anno></c> is <c>{Key, true}</c>, where
          <c>Key</c> is an atom, <c>Key</c> is returned, otherwise
          the whole term <c><anno>PropertyIn</anno></c> is returned.</p>
        <p>See also
          <seealso marker="#property/2"><c>property/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="property" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Creates a normal form (minimal) representation of a simple key/value
          property. Returns <c><anno>Key</anno></c> if <c><anno>Value</anno></c>
          is <c>true</c> and <c><anno>Key</anno></c> is an atom, otherwise a
          tuple <c>{<anno>Key</anno>, <anno>Value</anno>}</c> is returned.</p>
        <p>See also
          <seealso marker="#property/1"><c>property/1</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="split" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Partitions <c><anno>List</anno></c> into a list of sublists and a
          remainder. <c><anno>Lists</anno></c> contains one sublist for each key
          in <c><anno>Keys</anno></c>, in the corresponding order. The relative
          order of the elements in each sublist is preserved from the original
          <c><anno>List</anno></c>. <c><anno>Rest</anno></c> contains the
          elements in <c><anno>List</anno></c> that are not associated with any
          of the specified keys,
          also with their original relative order preserved.</p>
        <p><em>Example:</em></p>
        <code type="none">
split([{c, 2}, {e, 1}, a, {c, 3, 4}, d, {b, 5}, b], [a, b, c])</code>
        <p>returns:</p>
        <code type="none">
{[[a], [{b, 5}, b],[{c, 2}, {c, 3, 4}]], [{e, 1}, d]}</code>
      </desc>
    </func>

    <func>
      <name name="substitute_aliases" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Substitutes keys of properties. For each entry in
          <c><anno>ListIn</anno></c>, if it is associated with some key
          <c>K1</c> such that <c>{K1, K2}</c> occurs in
          <c><anno>Aliases</anno></c>, the
          key of the entry is changed to <c>K2</c>. If the same
          <c>K1</c> occurs more than once in <c><anno>Aliases</anno></c>, only
          the first occurrence is used.</p>
        <p>For example, <c>substitute_aliases([{color, colour}], L)</c>
          replaces all tuples <c>{color, ...}</c> in <c>L</c>
          with <c>{colour, ...}</c>, and all atoms <c>color</c>
          with <c>colour</c>.</p>
        <p>See also
          <seealso marker="#normalize/2"><c>normalize/2</c></seealso>,
          <seealso marker="#substitute_negations/2">
          <c>substitute_negations/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="substitute_negations" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Substitutes keys of boolean-valued properties and
          simultaneously negates their values. For each entry in
          <c><anno>ListIn</anno></c>, if it is associated with some key
          <c>K1</c> such that <c>{K1, K2}</c> occurs in
          <c><anno>Negations</anno></c>: if the entry was
          <c>{K1, true}</c>, it is replaced with <c>{K2, false}</c>, otherwise
          with <c>{K2, true}</c>, thus changing the name of the option and
          simultaneously negating the value specified by
          <seealso marker="#get_bool/2">
          <c>get_bool(Key, <anno>ListIn</anno>)</c></seealso>.
          If the same <c>K1</c> occurs more than once in
          <c><anno>Negations</anno></c>, only the first occurrence is used.</p>
        <p>For example, <c>substitute_negations([{no_foo, foo}], L)</c>
          replaces any atom <c>no_foo</c> or tuple
          <c>{no_foo, true}</c> in <c>L</c> with <c>{foo, false}</c>,
          and any other tuple <c>{no_foo, ...}</c> with <c>{foo, true}</c>.</p>
        <p>See also
          <seealso marker="#get_bool/2"><c>get_bool/2</c></seealso>,
          <seealso marker="#normalize/2"><c>normalize/2</c></seealso>,
          <seealso marker="#substitute_aliases/2">
          <c>substitute_aliases/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="unfold" arity="1"/>
      <fsummary></fsummary>
      <desc>
        <p>Unfolds all occurrences of atoms in <c><anno>ListIn</anno></c> to
          tuples <c>{Atom, true}</c>.</p>
      </desc>
    </func>
  </funcs>
</erlref>