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Data Types

data_types.xml

Erlang provides a number of data types, which are listed in this section.

## Terms

A piece of data of any data type is called a term.

## Number

There are two types of numeric literals, integers and floats. Besides the conventional notation, there are two Erlang-specific notations:

$char
ASCII value or unicode code-point of the character char. base#value
Integer with the base base, that must be an integer in the range 2..36.

Examples:

    1> 42.
    42
    2> $A.
    65
    3> $\n.
    10
    4> 2#101.
    5
    5> 16#1f.
    31
    6> 2.3.
    2.3
    7> 2.3e3.
    2.3e3
    8> 2.3e-3.
    0.0023

## Atom

An atom is a literal, a constant with name. An atom is to be enclosed in single quotes (') if it does not begin with a lower-case letter or if it contains other characters than alphanumeric characters, underscore (_), or @.

Examples:

    hello
    phone_number
    'Monday'
    'phone number'

## Bit Strings and Binaries

A bit string is used to store an area of untyped memory.

Bit strings are expressed using the bit syntax.

Bit strings that consist of a number of bits that are evenly divisible by eight, are called binaries

Examples:

    1> <<10,20>>.
    <<10,20>>
    2> <<"ABC">>.
    <<"ABC">>
    1> <<1:1,0:1>>.
    <<2:2>>

For more examples, see Programming Examples.

## Reference

A reference is a term that is unique in an Erlang runtime system, created by calling make_ref/0.

## Fun

A fun is a functional object. Funs make it possible to create an anonymous function and pass the function itself -- not its name -- as argument to other functions.

Example:

    1> Fun1 = fun (X) -> X+1 end.
    #Fun<erl_eval.6.39074546>
    2> Fun1(2).
    3

Read more about funs in Fun Expressions. For more examples, see Programming Examples.

## Port Identifier

A port identifier identifies an Erlang port.

open_port/2, which is used to create ports, returns a value of this data type.

Read more about ports in Ports and Port Drivers.

## Pid

A process identifier, pid, identifies a process.

The following BIFs, which are used to create processes, return values of this data type:

spawn/1,2,3,4 spawn_link/1,2,3,4 spawn_opt/4

Example:

    1> spawn(m, f, []).
    <0.51.0>

In the following example, the BIF self() returns the pid of the calling process:

    -module(m).
    -export([loop/0]).

    loop() ->
        receive
            who_are_you ->
                io:format("I am ~p~n", [self()]),
                loop()
        end.

    1> P = spawn(m, loop, []).
    <0.58.0>
    2> P ! who_are_you.
    I am <0.58.0>
    who_are_you

Read more about processes in Processes.

## Tuple

A tuple is a compound data type with a fixed number of terms:

    {Term1,...,TermN}

Each term Term in the tuple is called an element. The number of elements is said to be the size of the tuple.

There exists a number of BIFs to manipulate tuples.

Examples:

    1> P = {adam,24,{july,29}}.
    {adam,24,{july,29}}
    2> element(1,P).
    adam
    3> element(3,P).
    {july,29}
    4> P2 = setelement(2,P,25).
    {adam,25,{july,29}}
    5> tuple_size(P).
    3
    6> tuple_size({}).
    0

## Map

A map is a compound data type with a variable number of key-value associations:

    #{Key1=>Value1,...,KeyN=>ValueN}

Each key-value association in the map is called an association pair. The key and value parts of the pair are called elements. The number of association pairs is said to be the size of the map.

There exists a number of BIFs to manipulate maps.

Examples:

    1> M1 = #{name=>adam,age=>24,date=>{july,29}}.
    #{age => 24,date => {july,29},name => adam}
    2> maps:get(name,M1).
    adam
    3> maps:get(date,M1).
    {july,29}
    4> M2 = maps:update(age,25,M1).
    #{age => 25,date => {july,29},name => adam}
    5> map_size(M).
    3
    6> map_size(#{}).
    0

A collection of maps processing functions can be found in maps manual page in STDLIB.

Read more about maps in Map Expressions.

Maps are considered to be experimental during Erlang/OTP R17.

## List

A list is a compound data type with a variable number of terms.

    [Term1,...,TermN]

Each term Term in the list is called an element. The number of elements is said to be the length of the list.

Formally, a list is either the empty list [] or consists of a head (first element) and a tail (remainder of the list). The tail is also a list. The latter can be expressed as [H|T]. The notation [Term1,...,TermN] above is equivalent with the list [Term1|[...|[TermN|[]]]].

Example:

[] is a list, thus
[c|[]] is a list, thus
[b|[c|[]]] is a list, thus
[a|[b|[c|[]]]] is a list, or in short [a,b,c]

A list where the tail is a list is sometimes called a proper list. It is allowed to have a list where the tail is not a list, for example, [a|b]. However, this type of list is of little practical use.

Examples:

    1> L1 = [a,2,{c,4}].
    [a,2,{c,4}]
    2> [H|T] = L1.
    [a,2,{c,4}]
    3> H.
    a
    4> T.
    [2,{c,4}]
    5> L2 = [d|T].
    [d,2,{c,4}]
    6> length(L1).
    3
    7> length([]).
    0

A collection of list processing functions can be found in the lists manual page in STDLIB.

## String

Strings are enclosed in double quotes ("), but is not a data type in Erlang. Instead, a string "hello" is shorthand for the list [$h,$e,$l,$l,$o], that is, [104,101,108,108,111].

Two adjacent string literals are concatenated into one. This is done in the compilation, thus, does not incur any runtime overhead.

Example:

    "string" "42"

is equivalent to

    "string42"

## Record

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. However, a record is not a true data type. Instead, 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 shell(3) manual page in STDLIB).

Examples:

    -module(person).
    -export([new/2]).

    -record(person, {name, age}).

    new(Name, Age) ->
        #person{name=Name, age=Age}.

    1> person:new(ernie, 44).
    {person,ernie,44}

Read more about records in Records. More examples can be found in Programming Examples.

## Boolean

There is no Boolean data type in Erlang. Instead the atoms true and false are used to denote Boolean values.

Examples:

    1> 2 =< 3.
    true
    2> true or false.
    true

## Escape Sequences

Within strings and quoted atoms, the following escape sequences are recognized:

Sequence Description
\b Backspace
\d Delete
\e Escape
\f Form feed
\n Newline
\r Carriage return
\s Space
\t Tab
\v Vertical tab
\XYZ, \YZ, \Z Character with octal representation XYZ, YZ or Z
\xXY Character with hexadecimal representation XY
\x{X...} Character with hexadecimal representation; X... is one or more hexadecimal characters
\^a...\^z
\^A...\^Z Control A to control Z
\' Single quote
\" Double quote
\\ Backslash
Recognized Escape Sequences

## Type Conversions

There are a number of BIFs for type conversions.

Examples:

    1> atom_to_list(hello).
    "hello"
    2> list_to_atom("hello").
    hello
    3> binary_to_list(<<"hello">>).
    "hello"
    4> binary_to_list(<<104,101,108,108,111>>).
    "hello"
    5> list_to_binary("hello").
    <<104,101,108,108,111>>
    6> float_to_list(7.0).
    "7.00000000000000000000e+00"
    7> list_to_float("7.000e+00").
    7.0
    8> integer_to_list(77).
    "77"
    9> list_to_integer("77").
    77
    10> tuple_to_list({a,b,c}).
    [a,b,c]
    11> list_to_tuple([a,b,c]).
    {a,b,c}
    12> term_to_binary({a,b,c}).
    <<131,104,3,100,0,1,97,100,0,1,98,100,0,1,99>>
    13> binary_to_term(<<131,104,3,100,0,1,97,100,0,1,98,100,0,1,99>>).
    {a,b,c}
    14> binary_to_integer(<<"77">>).
    77
    15> integer_to_binary(77).
    <<"77">>
    16> float_to_binary(7.0).
    <<"7.00000000000000000000e+00">>
    17> binary_to_float(<<"7.000e+00">>).
    7.0