Typespecs

Elixir comes with a notation for declaring types and specifications. Elixir is a dynamically typed language, and as such, type specifications are never used by the compiler to optimize or modify code. Still, using type specifications is useful because

they provide documentation (for example, tools such as ExDoc show type specifications in the documentation)
they’re used by tools such as Dialyzer, that can analyze code with typespec to find type inconsistencies and possible bugs

Type specifications (sometimes referred to as typespecs) are defined in different contexts using the following attributes:

@type
@opaque
@typep
@spec
@callback
@macrocallback

See the “Defining a type” and “Defining a specification” sub-sections below for more information on defining types and typespecs.

Types and their syntax

The syntax Elixir provides for type specifications is similar to the one in Erlang. Most of the built-in types provided in Erlang (for example, pid()) are expressed in the same way: pid() (or simply pid). Parametrized types (such as list(integer)) are supported as well and so are remote types (such as Enum.t). Integers and atom literals are allowed as types (e.g., 1, :atom, or false). All other types are built out of unions of predefined types. Some shorthands are allowed, such as [...], <<>>, and {...}.

Basic types

type :: any()                   # the top type, the set of all terms
      | none()                  # the bottom type, contains no terms
      | atom()
      | map()                   # any map
      | pid()                   # process identifier
      | port()
      | reference()
      | struct()                # any struct
      | tuple()                 # tuple of any size

                                ## Numbers
      | float()
      | integer()
      | neg_integer()           # ..., -3, -2, -1
      | non_neg_integer()       # 0, 1, 2, 3, ...
      | pos_integer()           # 1, 2, 3, ...

                                                    ## Lists
      | list(type)                                  # proper list ([]-terminated)
      | nonempty_list(type)                         # non-empty proper list
      | maybe_improper_list(type1, type2)           # proper or improper list
      | nonempty_improper_list(type1, type2)        # improper list
      | nonempty_maybe_improper_list(type1, type2)  # non-empty proper or improper list

      | Literals                # Described in section "Literals"
      | Builtin                 # Described in section "Built-in types"
      | Remotes                 # Described in section "Remote types"
      | UserDefined             # Described in section "User-defined types"

Literals

The following literals are also supported in typespecs:

type ::                               ## Atoms
        :atom                         # atoms: :foo, :bar, ...
      | true | false | nil            # special atom literals

                                      ## Bitstrings
      | <<>>                          # empty bitstring
      | <<_::size>>                   # size is 0 or a positive integer
      | <<_::_*unit>>                 # unit is an integer from 1 to 256
      | <<_::size, _::_*unit>>

                                      ## Functions
      | (... -> type)                 # any arity, returns type
      | (() -> type)                  # 0-arity, returns type
      | (type1, type2 -> type)        # 2-arity, returns type

                                      ## Integers
      | 1                             # integer
      | 1..10                         # integer from 1 to 10

                                      ## Lists
      | [type]                        # list with any number of type elements
      | []                            # empty list
      | [...]                         # shorthand for nonempty_list(any())
      | [type, ...]                   # shorthand for nonempty_list(type)
      | [key: value_type]             # keyword list with key :key of value_type

                                              ## Maps
      | %{}                                   # empty map
      | %{key: value_type}                    # map with required key :key of value_type
      | %{required(key_type) => value_type}   # map with required pairs of key_type and value_type
      | %{optional(key_type) => value_type}   # map with optional pairs of key_type and value_type
      | %SomeStruct{}                         # struct with all fields of any type
      | %SomeStruct{key: value_type}          # struct with required key :key of value_type

                                      ## Tuples
      | {}                            # empty tuple
      | {:ok, type}                   # two-element tuple with an atom and any type

Built-in types

The following types are also provided by Elixir as shortcuts on top of the basic and literal types described above.

Built-in type	Defined as
`term()`	`any()`
`arity()`	`0..255`
`as_boolean(t)`	`t`
`binary()`	`<<_::_*8>>`
`bitstring()`	`<<_::_*1>>`
`boolean()`	`false` \| `true`
`byte()`	`0..255`
`char()`	`0..0x10FFFF`
`charlist()`	`[char()]`
`nonempty_charlist()`	`[char(), ...]`
`fun()`	`(... -> any)`
`function()`	`fun()`
`identifier()`	`pid()` \| `port()` \| `reference()`
`iodata()`	`iolist()` \| `binary()`
`iolist()`	`maybe_improper_list(byte() \| binary() \| iolist(), binary() \| [])`
`keyword()`	`[{atom(), any()}]`
`keyword(t)`	`[{atom(), t}]`
`list()`	`[any()]`
`nonempty_list()`	`nonempty_list(any())`
`maybe_improper_list()`	`maybe_improper_list(any(), any())`
`nonempty_maybe_improper_list()`	`nonempty_maybe_improper_list(any(), any())`
`mfa()`	`{module(), atom(), arity()}`
`module()`	`atom()`
`no_return()`	`none()`
`node()`	`atom()`
`number()`	`integer()` \| `float()`
`struct()`	`%{:__struct__ => atom(), optional(atom()) => any()}`
`timeout()`	`:infinity` \| `non_neg_integer()`

Remote types

Any module is also able to define its own types and the modules in Elixir are no exception. For example, the Range module defines a t/0 type that represents a range: this type can be referred to as Range.t/0. In a similar fashion, a string is String.t/0, any enumerable can be Enum.t/0, and so on.

Maps

The key types in maps are allowed to overlap, and if they do, the leftmost key takes precedence. A map value does not belong to this type if it contains a key that is not in the allowed map keys.

If you want to denote that keys that were not previously defined in the map are allowed, it is common to end a map type with optional(any) => any.

Notice that the syntactic representation of map() is %{optional(any) => any}, not %{}. The notation %{} specifies the singleton type for the empty map.

User-defined types

The @type, @typep, and @opaque module attributes can be used to define new types:

@type type_name :: type
@typep type_name :: type
@opaque type_name :: type

A type defined with @typep is private. An opaque type, defined with @opaque is a type where the internal structure of the type will not be visible, but the type is still public.

Types can be parameterized by defining variables as parameters; these variables can then be used to define the type.

@type dict(key, value) :: [{key, value}]

Defining a specification

A specification for a function can be defined as follows:

@spec function_name(type1, type2) :: return_type

Guards can be used to restrict type variables given as arguments to the function.

@spec function(arg) :: [arg] when arg: atom

If you want to specify more than one variable, you separate them by a comma.

@spec function(arg1, arg2) :: {arg1, arg2} when arg1: atom, arg2: integer

Type variables with no restriction can also be defined.

@spec function(arg) :: [arg] when arg: var

You can also name your arguments in a typespec using arg_name :: arg_type syntax. This is particularly useful in documentation as a way to differentiate multiple arguments of the same type (or multiple elements of the same type in a type definition):

@spec days_since_epoch(year :: integer, month :: integer, day :: integer) :: integer
@type color :: {red :: integer, green :: integer, blue :: integer}

Specifications can be overloaded just like ordinary functions.

@spec function(integer) :: atom
@spec function(atom) :: integer

Behaviours

Behaviours in Elixir (and Erlang) are a way to separate and abstract the generic part of a component (which becomes the behaviour module) from the specific part (which becomes the callback module).

A behaviour module defines a set of functions and macros (referred to as callbacks) that callback modules implementing that behaviour must export. This “interface” identifies the specific part of the component. For example, the GenServer behaviour and functions abstract away all the message-passing (sending and receiving) and error reporting that a “server” process will likely want to implement from the specific parts such as the actions that this server process has to perform.

To define a behaviour module, it’s enough to define one or more callbacks in that module. To define callbacks, the @callback and @macrocallback module attributes can be used (for function callbacks and macro callbacks respectively).

defmodule MyBehaviour do
  @callback my_fun(arg :: any) :: any
  @macrocallback my_macro(arg :: any) :: Macro.t
end

As seen in the example above, defining a callback is a matter of defining a specification for that callback, made of:

the callback name (my_fun or my_macro in the example)
the arguments that the callback must accept (arg :: any in the example)
the expected type of the callback return value

Optional callbacks

Optional callbacks are callbacks that callback modules may implement if they want to, but are not required to. Usually, behaviour modules know if they should call those callbacks based on configuration, or they check if the callbacks are defined with function_exported?/3 or macro_exported?/3.

Optional callbacks can be defined through the @optional_callbacks module attribute, which has to be a keyword list with function or macro name as key and arity as value. For example:

defmodule MyBehaviour do
  @callback vital_fun() :: any
  @callback non_vital_fun() :: any
  @macrocallback non_vital_macro(arg :: any) :: Macro.t
  @optional_callbacks non_vital_fun: 0, non_vital_macro: 1
end

One example of optional callback in Elixir’s standard library is GenServer.format_status/2.

Implementing behaviours

To specify that a module implements a given behaviour, the @behaviour attribute must be used:

defmodule MyBehaviour do
  @callback my_fun(arg :: any) :: any
end

defmodule MyCallbackModule do
  @behaviour MyBehaviour
  def my_fun(arg), do: arg
end

If a callback module that implements a given behaviour doesn’t export all the functions and macros defined by that behaviour, the user will be notified through warnings during the compilation process (no errors will happen).

Elixir’s standard library contains a few frequently used behaviours such as GenServer, Supervisor, and Application.

Notes

Elixir discourages the use of type t:string/0 as it might be confused with binaries which are referred to as “strings” in Elixir (as opposed to character lists). In order to use the type that is called t:string/0 in Erlang, one has to use the charlist/0 type which is a synonym for string. If you use string, you’ll get a warning from the compiler.

If you want to refer to the “string” type (the one operated on by functions in the String module), use String.t/0 type instead.