1 Records

1.1  Records vs Tuples

The main advantage of using records instead of tuples is that fields in a record are accessed by name, whereas fields in a tuple are accessed by position. To illustrate these differences, suppose that we want to represent a person with the tuple {Name, Address, Phone}.

We must remember that the Name field is the first element of the tuple, the Address field is the second element, and so on, in order to write functions which manipulate this data. For example, to extract data from a variable P which contains such a tuple we might write the following code and then use pattern matching to extract the relevant fields.

Name = element(1, P),
Address = element(2, P),
...

Code like this is difficult to read and understand and errors occur if we get the numbering of the elements in the tuple wrong. If we change the data representation by re-ordering the fields, or by adding or removing a field, then all references to the person tuple, wherever they occur, must be checked and possibly modified.

Records allow us to refer to the fields by name and not position. We use a record instead of a tuple to store the data. If we write a record definition of the type shown below, we can then refer to the fields of the record by name.

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

For example, if P is now a variable whose value is a person record, we can code as follows in order to access the name and address fields of the records.

Name = P#person.name,
Address = P#person.address,
...

Internally, records are represented using tagged tuples:

{person, Name, Phone, Address}

1.2  Defining a Record

This definition of a person will be used in many of the examples which follow. It contains three fields, name, phone and address. The default values for name and phone is "" and [], respectively. The default value for address is the atom undefined, since no default value is supplied for this field:

-record(person, {name = "", phone = [], address}).

We have to define the record in the shell in order to be able use the record syntax in the examples:

> rd(person, {name = "", phone = [], address}).
person

This is due to the fact that record definitions are available at compile time only, not at runtime. See shell(3) for details on records in the shell.

1.3  Creating a Record

A new person record is created as follows:

> #person{phone=[0,8,2,3,4,3,1,2], name="Robert"}.
#person{name = "Robert",phone = [0,8,2,3,4,3,1,2],address = undefined}

Since the address field was omitted, its default value is used.

There is a new feature introduced in Erlang 5.1/OTP R8B, with which you can set a value to all fields in a record, overriding the defaults in the record specification. The special field _, means "all fields not explicitly specified".

> #person{name = "Jakob", _ = '_'}.
#person{name = "Jakob",phone = '_',address = '_'}

It is primarily intended to be used in ets:match/2 and mnesia:match_object/3, to set record fields to the atom '_'. (This is a wildcard in ets:match/2.)

1.4  Accessing a Record Field

> P = #person{name = "Joe", phone = [0,8,2,3,4,3,1,2]}.
#person{name = "Joe",phone = [0,8,2,3,4,3,1,2],address = undefined}
> P#person.name.
"Joe"

1.5  Updating a Record

> P1 = #person{name="Joe", phone=[1,2,3], address="A street"}.
#person{name = "Joe",phone = [1,2,3],address = "A street"}
> P2 = P1#person{name="Robert"}.
#person{name = "Robert",phone = [1,2,3],address = "A street"}

1.6  Type Testing

The following example shows that the guard succeeds if P is record of type person.

foo(P) when is_record(P, person) -> a_person;
foo(_) -> not_a_person.

1.7  Pattern Matching

Matching can be used in combination with records as shown in the following example:

> P3 = #person{name="Joe", phone=[0,0,7], address="A street"}.
#person{name = "Joe",phone = [0,0,7],address = "A street"}
> #person{name = Name} = P3, Name.
"Joe"

The following function takes a list of person records and searches for the phone number of a person with a particular name:

find_phone([#person{name=Name, phone=Phone} | _], Name) ->
    {found,  Phone};
find_phone([_| T], Name) ->
    find_phone(T, Name);
find_phone([], Name) ->
    not_found.

The fields referred to in the pattern can be given in any order.

1.8  Nested Records

The value of a field in a record might be an instance of a record. Retrieval of nested data can be done stepwise, or in a single step, as shown in the following example:

-record(name, {first = "Robert", last = "Ericsson"}).
-record(person, {name = #name{}, phone}).
demo() ->
  P = #person{name= #name{first="Robert",last="Virding"}, phone=123},
  First = (P#person.name)#name.first.

In this example, demo() evaluates to "Robert".

1.9  Example

%% File: person.hrl
%%-----------------------------------------------------------
%% Data Type: person
%% where:
%%    name:  A string (default is undefined).
%%    age:   An integer (default is undefined).
%%    phone: A list of integers (default is []).
%%    dict:  A dictionary containing various information 
%%           about the person. 
%%           A {Key, Value} list (default is the empty list).
%%------------------------------------------------------------
-record(person, {name, age, phone = [], dict = []}).
-module(person).
-include("person.hrl").
-compile(export_all). % For test purposes only.
%% This creates an instance of a person.
%%   Note: The phone number is not supplied so the
%%         default value [] will be used.
make_hacker_without_phone(Name, Age) ->
   #person{name = Name, age = Age, 
           dict = [{computer_knowledge, excellent}, 
                   {drinks, coke}]}.
%% This demonstrates matching in arguments
print(#person{name = Name, age = Age,
              phone = Phone, dict = Dict}) ->
  io:format("Name: ~s, Age: ~w, Phone: ~w ~n" 
            "Dictionary: ~w.~n", [Name, Age, Phone, Dict]).
%% Demonstrates type testing, selector, updating.
birthday(P) when record(P, person) -> 
   P#person{age = P#person.age + 1}.
register_two_hackers() ->
   Hacker1 = make_hacker_without_phone("Joe", 29),
   OldHacker = birthday(Hacker1),
   % The central_register_server should have 
   % an interface function for this.
   central_register_server ! {register_person, Hacker1},
   central_register_server ! {register_person, 
             OldHacker#person{name = "Robert", 
                              phone = [0,8,3,2,4,5,3,1]}}.