Records — Erlang System Documentation v27.0

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# View Source Records

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.

More examples are provided in Programming Examples.

## Defining Records

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, `undefined` is used.

    -record(Name, {Field1 [= Expr1],
                   ...
                   FieldN [= ExprN]}).

The default value for a field is an arbitrary expression, except that it must not use any variables.

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.

If a record is used in several modules, it is recommended that the record definition is placed in an include file.

> #### Change

> Starting from Erlang/OTP 26, records can be defined in the Erlang shell using the syntax described in this section. In earlier releases, it was necessary to use the `shell` built-in function `rd/2`.

## Creating Records

The following expression creates a new `Name` record where the value of each field `FieldI` is the value of evaluating the corresponding expression `ExprI`:

    #Name{Field1=Expr1, ..., FieldK=ExprK}

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.

If several fields are to be assigned the same value, the following construction can be used:

    #Name{Field1=Expr1, ..., FieldK=ExprK, _=ExprL}

Omitted fields then get the value of evaluating `ExprL` instead of their default values. This feature is primarily intended to be used to create patterns for ETS and Mnesia match functions.

Example:

    -record(person, {name, phone, address}).

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

## Accessing Record Fields

    Expr#Name.Field

Returns the value of the specified field. `Expr` is to evaluate to a `Name` record.

Example:

    -record(person, {name, phone, address}).

    get_person_name(Person) ->
        Person#person.name.

The following expression returns the position of the specified field in the tuple representation of the record:

    #Name.Field

Example:

    -record(person, {name, phone, address}).

    lookup(Name, List) ->
        lists:keyfind(Name, #person.name, List).

## Updating Records

    Expr#Name{Field1=Expr1, ..., FieldK=ExprK}

`Expr` is to evaluate to a `Name` record. A copy of this record is returned, with the value of each specified field `FieldI` changed to the value of evaluating the corresponding expression `ExprI`. All other fields retain their old values.

## Records in Guards

Since record expressions are expanded to tuple expressions, creating records and accessing record fields are allowed in guards. However, all subexpressions (for initializing fields), must be valid guard expressions as well.

Examples:

    handle(Msg, State) when Msg =:= #msg{to=void, no=3} ->
        ...

    handle(Msg, State) when State#state.running =:= true ->
        ...

There is also a type test BIF `is_record(Term, RecordTag)`.

Example:

    is_person(P) when is_record(P, person) ->
        true;
    is_person(_P) ->
        false.

## Records in Patterns

A pattern that matches a certain record is created in the same way as a record is created:

    #Name{Field1=Expr1, ..., FieldK=ExprK}

In this case, one or more of `Expr1` ... `ExprK` can be unbound variables.

## Nested Records

Assume the following record definitions:

    -record(nrec0, {name = "nested0"}).
    -record(nrec1, {name = "nested1", nrec0=#nrec0{}}).
    -record(nrec2, {name = "nested2", nrec1=#nrec1{}}).

    N2 = #nrec2{},

Accessing or updating nested records can be written without parentheses:

    "nested0" = N2#nrec2.nrec1#nrec1.nrec0#nrec0.name,
        N0n = N2#nrec2.nrec1#nrec1.nrec0#nrec0{name = "nested0a"},

which is equivalent to:

    "nested0" = ((N2#nrec2.nrec1)#nrec1.nrec0)#nrec0.name,
    N0n = ((N2#nrec2.nrec1)#nrec1.nrec0)#nrec0{name = "nested0a"},

> #### Change

> Before Erlang/OTP R14, parentheses were necessary when accessing or updating nested records.

## Internal Representation of Records

Record expressions are translated to tuple expressions during compilation. A record defined as:

    -record(Name, {Field1, ..., FieldN}).

is internally represented by the tuple:

    {Name, Value1, ..., ValueN}

Here each `ValueI` is the default value for `FieldI`.

To each module using records, a pseudo function is added during compilation to obtain information about records:

    record_info(fields, Record) -> [Field]
    record_info(size, Record) -> Size

`Size` is the size of the tuple representation, that is, one more than the number of fields.

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