2 Data Types

2.1 Terms

Erlang provides a number of data types which are listed in this chapter. A piece of data of any data type is called a term.

2.2 Number

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

$char
ASCII value of the character char.
base#value
Integer with the base base, which must be an integer in the range 2..36.
In Erlang 5.2/OTP R9B and earlier versions, the allowed range is 2..16.

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

An atom is a literal, a constant with name. An atom should 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'

2.4 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 which consists of a number of bits which is evenly divisible by eight are called Binaries

Examples:

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

More examples can be found in Programming Examples.

2.5 Reference

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

2.6 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. More examples can be found in Programming Examples.

2.7 Port Identifier

A port identifier identifies an Erlang port. open_port/2, which is used to create ports, will return a value of this type.

Read more about ports in Ports and Port Drivers.

2.8 Pid

A process identifier, pid, identifies a process. spawn/1,2,3,4, spawn_link/1,2,3,4 and spawn_opt/4, which are used to create processes, return values of this type. Example:

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

The BIF self() returns the pid of the calling process. Example:

-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

2.9 Tuple

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

2.10 List

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) which is also a list. The latter can be expressed as [H|T]. The notation [Term1,...,TermN] above is actually shorthand for 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 STDLIB module lists.

2.11 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 at compile-time and does not incur any runtime overhead. Example:

"string" "42"

is equivalent to

"string42"

2.12 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, 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. See shell(3) for details.

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.

2.13 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

2.14 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

Table 2.1: Recognized Escape Sequences.

2.15 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}