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

<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>2002</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.

    </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.sgml</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 may specify options
      overriding the default settings, object properties, annotations,
      etc.</p>
    <p>Two keys are considered equal if they match (<c>=:=</c>). In other
      words, numbers are compared literally rather than by value, so that,
      for instance, <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 <c>get_all_values/2</c>, but each value is
          wrapped in a list unless it is already itself a list, and the
          resulting list of lists is concatenated. This is often useful for
          "incremental" options; e.g., <c>append_values(a, [{a, [1,2]}, {b, 0}, {a, 3}, {c, -1}, {a, [4]}])</c> will return the list
          <c>[1,2,3,4]</c>.</p>
      </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: <c>property/1</c>, <c>unfold/1</c>.</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:</p>
        <code type="none">
          expand([{{foo, true}, [bar, baz]}],
          [{foo, false}, fie, foo, fum])</code>
        <p>because <c>{foo, false}</c> shadows <c>foo</c>.</p>
        <p>Note 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: <c>normalize/2</c>.</p>
      </desc>
    </func>
    <func>
      <name name="get_all_values" arity="2"/>
      <fsummary></fsummary>
      <desc>
        <p>Similar to <c>get_value/2</c>, 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>
        <p>See also: <c>get_value/2</c>.</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>
          is returned.</p>
        <p>See also: <c>get_value/2</c>, <c>lookup/2</c>.</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> is returned.</p>
        <p>See also: <c>get_all_values/2</c>, <c>get_bool/2</c>,
          <c>get_value/2</c>, <c>lookup/2</c>.</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> is returned.</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: <c>get_bool/2</c>, <c>get_value/2</c>,
          <c>lookup_all/2</c>.</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: <c>lookup/2</c>.</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,
          the function <c>substitute_aliases/2</c> is applied using the
          given list of aliases; for a <c>negations</c> operation,
          <c>substitute_negations/2</c> is applied using the given
          negation list; for an <c>expand</c> operation, the function
          <c>expand/2</c> is applied using the given list of expansions.
          The final result is automatically compacted (cf.
          <c>compact/1</c>).</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: <c>compact/1</c>, <c>expand/2</c>,
          <c>substitute_aliases/2</c>, <c>substitute_negations/2</c>.</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, this returns <c>Key</c>, otherwise
	  the whole term <c><anno>PropertyIn</anno></c> is returned.</p>
        <p>See also: <c>property/2</c>.</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: <c>property/1</c>.</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 given keys,
          also with their original relative order preserved.</p>
        <p>Example:
          split([{c, 2}, {e, 1}, a, {c, 3, 4}, d, {b, 5}, b], [a, b, c])</p>
        <p>returns</p>
        <p>{[[a], [{b, 5}, b],[{c, 2}, {c, 3, 4}]], [{e, 1}, d]}</p>
      </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>Example: <c>substitute_aliases([{color, colour}], L)</c>
          will replace 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: <c>normalize/2</c>, <c>substitute_negations/2</c>.</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>, then
          if the entry was <c>{K1, true}</c> it will be replaced with
          <c>{K2, false}</c>, otherwise it will be replaced with
          <c>{K2, true}</c>, thus changing the name of the option and
          simultaneously negating the value given by
          <c>get_bool(ListIn)</c>. If the same <c>K1</c> occurs more
          than once in <c><anno>Negations</anno></c>, only the first occurrence is
          used.</p>
        <p>Example: <c>substitute_negations([{no_foo, foo}], L)</c>
          will replace 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: <c>get_bool/2</c>, <c>normalize/2</c>,
          <c>substitute_aliases/2</c>.</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>