Data Types

## 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.
    2.30000
    7> 2.3e3.
    2300.00
    8> 2.3e-3.
    2.30000e-3

### 2.3 Atom

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 Binary

A binary is used to store an area of untyped memory.

Binaries are expressed using the bit syntax.

Examples:

    1> <<10,20>>.
    <<10,20>>
    2> <<"ABC">>.
    <<65,66,67>>

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

Read more about processes in Processes.

### 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> size(P).
    3
    6> 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:

Recognized Escape Sequences.
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
\^a...\^z \^A...\^Z | control A to control Z
\' | single quote
\" | double quote
\\ | backslash

### 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").
    7.00000
    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}

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