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<chapter>
  <header>
    <copyright>
      <year>2003</year><year>2017</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
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      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
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    <title>Records</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>records.xml</file>
  </header>
  <p>A record is a data structure for storing a fixed number of
    elements. It has named fields and is similar to a struct in C.
    Record expressions are translated to tuple expressions during
    compilation. Therefore, record expressions are not understood by
    the shell unless special actions are taken. For details, see the
    <seeerl marker="stdlib:shell">shell(3)</seeerl>
    manual page in STDLIB.</p>
  <p>More examples are provided in
    <seeguide marker="system/programming_examples:records">
    Programming Examples</seeguide>.</p>

  <section>
    <title>Defining Records</title>
    <p>A record definition consists of the name of the record,
      followed by the field names of the record. Record and field names
      must be atoms. Each field can be given an optional default value.
      If no default value is supplied, <c>undefined</c> is used.</p>
    <pre>
-record(Name, {Field1 [= Value1],
               ...
               FieldN [= ValueN]}).</pre>
    <p>A record definition can be placed anywhere among the attributes
      and function declarations of a module, but the definition must
      come before any usage of the record.</p>
    <p>If a record is used in several modules, it is recommended that
      the record definition is placed in an include file.</p>
  </section>

  <section>
    <title>Creating Records</title>
    <p>The following expression creates a new <c>Name</c> record where
      the value of each field <c>FieldI</c> is the value of evaluating
      the corresponding expression <c>ExprI</c>:</p>
    <pre>
#Name{Field1=Expr1,...,FieldK=ExprK}</pre>
    <p>The fields can be in any order, not necessarily the same order as
      in the record definition, and fields can be omitted. Omitted
      fields get their respective default value instead.</p>
    <p>If several fields are to be assigned the same value,
      the following construction can be used:</p>
    <pre>
#Name{Field1=Expr1,...,FieldK=ExprK, _=ExprL}</pre>
    <p>Omitted fields then get the value of evaluating <c>ExprL</c>
    instead of their default values. This feature is primarily
    intended to be used to create patterns for ETS and Mnesia match
    functions.</p>
    <p><em>Example:</em></p>
    <pre>
-record(person, {name, phone, address}).

...

lookup(Name, Tab) ->
    ets:match_object(Tab, #person{name=Name, _='_'}).</pre>
  </section>

  <section>
    <title>Accessing Record Fields</title>
    <pre>
Expr#Name.Field</pre>
    <p>Returns the value of the specified field. <c>Expr</c> is to
      evaluate to a <c>Name</c> record.</p>
    <p>The following expression returns the position of the specified
      field in the tuple representation of the record:</p>
    <pre>
#Name.Field</pre>
    <p><em>Example:</em></p>
    <pre>
-record(person, {name, phone, address}).

...

lookup(Name, List) ->
    lists:keysearch(Name, #person.name, List).</pre>
  </section>

  <section>
    <title>Updating Records</title>
    <pre>
Expr#Name{Field1=Expr1,...,FieldK=ExprK}</pre>
    <p><c>Expr</c> is to evaluate to a <c>Name</c> record. A
      copy of this record is returned, with the value of each specified field
      <c>FieldI</c> changed to the value of evaluating the corresponding
      expression <c>ExprI</c>. All other fields retain their old
      values.</p>
    <p></p>
  </section>

  <section>
    <title>Records in Guards</title>
    <p>Since record expressions are expanded to tuple expressions,
      creating records and accessing record fields are allowed in
      guards. However all subexpressions, for example, for field
      initiations, must be valid guard expressions as well.</p>
    <p><em>Examples:</em></p>
    <code type="none">
handle(Msg, State) when Msg==#msg{to=void, no=3} ->
    ...

handle(Msg, State) when State#state.running==true ->
    ...</code>
    <p>There is also a type test BIF <c>is_record(Term, RecordTag)</c>.</p>
    <p><em>Example:</em></p>
    <pre>
is_person(P) when is_record(P, person) ->
    true;
is_person(_P) ->
    false.</pre>
  </section>

  <section>
    <title>Records in Patterns</title>
    <p>A pattern that matches a certain record is created in the same
      way as a record is created:</p>
    <pre>
#Name{Field1=Expr1,...,FieldK=ExprK}</pre>
    <p>In this case, one or more of <c>Expr1</c>...<c>ExprK</c> can be
      unbound variables.</p>
  </section>

  <section>
    <title>Nested Records</title>
    <p>Beginning with Erlang/OTP R14, parentheses when accessing or updating nested
      records can be omitted. Assume the following record
      definitions:</p>
    <pre>
-record(nrec0, {name = "nested0"}).
-record(nrec1, {name = "nested1", nrec0=#nrec0{}}).
-record(nrec2, {name = "nested2", nrec1=#nrec1{}}).

N2 = #nrec2{},
    </pre>
    <p>Before R14, parentheses were needed as follows:</p>
    <pre>
"nested0" = ((N2#nrec2.nrec1)#nrec1.nrec0)#nrec0.name,
N0n = ((N2#nrec2.nrec1)#nrec1.nrec0)#nrec0{name = "nested0a"},
    </pre>
    <p>Since R14, the following can also be written:</p>
    <pre>
"nested0" = N2#nrec2.nrec1#nrec1.nrec0#nrec0.name,
N0n = N2#nrec2.nrec1#nrec1.nrec0#nrec0{name = "nested0a"},</pre>
  </section>

  <section>
    <title>Internal Representation of Records</title>
    <p>Record expressions are translated to tuple expressions during
      compilation. A record defined as:</p>
    <pre>
-record(Name, {Field1,...,FieldN}).</pre>
    <p>is internally represented by the tuple:</p>
    <pre>
{Name,Value1,...,ValueN}</pre>
    <p>Here each <c>ValueI</c> is the default value for <c>FieldI</c>.</p>
    <p>To each module using records, a pseudo function is added
      during compilation to obtain information about records:</p>
    <pre>
record_info(fields, Record) -> [Field]
record_info(size, Record) -> Size</pre>
    <p><c>Size</c> is the size of the tuple representation, that is,
      one more than the number of fields.</p>
    <p>In addition, <c>#Record.Name</c> returns the index in the tuple
      representation of <c>Name</c> of the record <c>Record</c>.</p>
    <p><c>Name</c> must be an atom.</p>
  </section>
</chapter>

