Erlang is a dynamically typed language. Still, it comes with a language extension for declaring sets of Erlang terms to form a particular type, effectively forming a specific sub-type of the set of all Erlang terms.
Subsequently, these types can be used to specify types of record fields and the argument and return types of functions.
Type information can be used to document function interfaces,
provide more information for bug detection tools such as
Types describe sets of Erlang terms.
Types consist and are built from a set of predefined types (e.g.
For integers and atoms, we allow for singleton types (e.g. the integers
atom() | 'bar' | integer() | 42
describes the same set of terms as the type union:
atom() | integer()
Because of sub-type relations that exist between types, types form a lattice where the topmost element, any(), denotes the set of all Erlang terms and the bottom-most element, none(), denotes the empty set of terms.
The set of predefined types and the syntax for types is given below:
> | <<>> | <<_:Erlang_Integer>> %% Base size | <<_:_*Erlang_Integer>> %% Unit size | <<_:Erlang_Integer, _:_*Erlang_Integer>> Fun :: fun() %% any function | fun((...) -> Type) %% any arity, returning Type | fun(() -> Type) | fun((TList) -> Type) Integer :: integer() | Erlang_Integer %% ..., -1, 0, 1, ... 42 ... | Erlang_Integer..Erlang_Integer %% specifies an integer range List :: list(Type) %% Proper list ([]-terminated) | improper_list(Type1, Type2) %% Type1=contents, Type2=termination | maybe_improper_list(Type1, Type2) %% Type1 and Type2 as above Tuple :: tuple() %% stands for a tuple of any size | {} | {TList} TList :: Type | Type, TList ]]>
Because lists are commonly used, they have shorthand type notations.
The type
Notice that the shorthand for
For convenience, the following types are also built-in. They can be thought as predefined aliases for the type unions also shown in the table. (Some type unions below slightly abuse the syntax of types.)
Users are not allowed to define types with the same names as the predefined or built-in ones. This is checked by the compiler and its violation results in a compilation error. (For bootstrapping purposes, it can also result to just a warning if this involves a built-in type which has just been introduced.)
nonempty_maybe_improper_list(Type) :: nonempty_maybe_improper_list(Type, any()) nonempty_maybe_improper_list() :: nonempty_maybe_improper_list(any())
where the following two types define the set of Erlang terms one would expect:
nonempty_improper_list(Type1, Type2) nonempty_maybe_improper_list(Type1, Type2)
Also for convenience, we allow for record notation to be used. Records are just shorthands for the corresponding tuples.
Record :: #Erlang_Atom{} | #Erlang_Atom{Fields}
Records have been extended to possibly contain type information.
This is described in the sub-section
As seen, the basic syntax of a type is an atom followed by closed parentheses. New types are declared using '-type' and '-opaque' compiler attributes as in the following:
-type my_struct_type() :: Type. -opaque my_opaq_type() :: Type.
where the type name is an atom (
Type declarations can also be parameterized by including type variables between the parentheses. The syntax of type variables is the same as Erlang variables (starts with an upper case letter). Naturally, these variables can - and should - appear on the RHS of the definition. A concrete example appears below:
-type orddict(Key, Val) :: [{Key, Val}].
A module can export some types in order to declare that other modules are allowed to refer to them as remote types. This declaration has the following form:
-export_type([T1/A1, ..., Tk/Ak]).where the Ti's are atoms (the name of the type) and the Ai's are their arguments. An example is given below:
-export_type([my_struct_type/0, orddict/2]).Assuming that these types are exported from module
mod:my_struct_type() mod:orddict(atom(), term())One is not allowed to refer to types which are not declared as exported.
Types declared as
The types of record fields can be specified in the declaration of the record. The syntax for this is:
-record(rec, {field1 :: Type1, field2, field3 :: Type3}).
For fields without type annotations, their type defaults to any(). I.e., the above is a shorthand for:
-record(rec, {field1 :: Type1, field2 :: any(), field3 :: Type3}).
In the presence of initial values for fields, the type must be declared after the initialization as in the following:
-record(rec, {field1 = [] :: Type1, field2, field3 = 42 :: Type3}).
Naturally, the initial values for fields should be compatible
with (i.e. a member of) the corresponding types.
This is checked by the compiler and results in a compilation error
if a violation is detected. For fields without initial values,
the singleton type
-record(rec, {f1 = 42 :: integer(), f2 :: float(), f3 :: 'a' | 'b'}). -record(rec, {f1 = 42 :: integer(), f2 :: 'undefined' | float(), f3 :: 'undefined' | 'a' | 'b'}).
For this reason, it is recommended that records contain initializers, whenever possible.
Any record, containing type information or not, once defined, can be used as a type using the syntax:
#rec{}
In addition, the record fields can be further specified when using a record type by adding type information about the field in the following manner:
#rec{some_field :: Type}
Any unspecified fields are assumed to have the type in the original record declaration.
A specification (or contract) for a function is given using the new
compiler attribute
-spec Module:Function(ArgType1, ..., ArgTypeN) -> ReturnType.
The arity of the function has to match the number of arguments, or else a compilation error occurs.
This form can also be used in header files (.hrl) to declare type information for exported functions. Then these header files can be included in files that (implicitly or explicitly) import these functions.
For most uses within a given module, the following shorthand suffices:
-spec Function(ArgType1, ..., ArgTypeN) -> ReturnType.
Also, for documentation purposes, argument names can be given:
-spec Function(ArgName1 :: Type1, ..., ArgNameN :: TypeN) -> RT.
A function specification can be overloaded.
That is, it can have several types, separated by a semicolon (
-spec foo(T1, T2) -> T3 ; (T4, T5) -> T6.
A current restriction, which currently results in a warning (OBS: not an error) by the compiler, is that the domains of the argument types cannot be overlapping. For example, the following specification results in a warning:
-spec foo(pos_integer()) -> pos_integer() ; (integer()) -> integer().
Type variables can be used in specifications to specify relations for the input and output arguments of a function. For example, the following specification defines the type of a polymorphic identity function:
-spec id(X) -> X.
However, note that the above specification does not restrict the input and output type in any way. We can constrain these types by guard-like subtype constraints:
-spec id(X) -> X when is_subtype(X, tuple()).
or equivalently by the more succinct and more modern form of the above:
-spec id(X) -> X when X :: tuple().
and provide bounded quantification. Currently, the
The scope of an
-spec foo({X, integer()}) -> X when X :: atom() ; ([Y]) -> Y when Y :: number().
Some functions in Erlang are not meant to return; either because they define servers or because they are used to throw exceptions as the function below:
my_error(Err) -> erlang:throw({error, Err}).
For such functions we recommend the use of the special no_return() type for their "return", via a contract of the form:
-spec my_error(term()) -> no_return().