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lfe_types(7) FreeBSD Miscellaneous Information Manual lfe_types(7)

lfe_types - LFE Types and Functions Specifications

This is a description of the type syntax.

LFE type Erlang type
(any) any()
(none) none()
(atom) atom()
(integer) integer()
(range i1 i2) I1..I2
(float) float()
(bitstring m n) <<_:M,_:_*N>>
(binary) <<_:0,_:_*8>>
(bitstring) <<_:0,_:_*1>>
(nil) [] %% nil
... ...
(lambda any <type>) fun((...) -> <type>)
(lambda () <type>) fun(() -> <type>)
(lambda (<tlist>) <type>) fun((<tlist>) -> <type>)
(map) map()
#M() #{}
#M(<key> <value> ...) #{<pairlist>}
(tuple) tuple()
#() {}
#(<tlist>) {<tlist>}
(UNION <tlist>) <type> | <type>

Apart from the predefined types in the Erlang type system we also have the following predefined types which cannot be redefined: UNION, call, lambda and range. The usage of bitstring, tuple and map have also been extended.

Note that the type #M() is the empty map and the type #() is the empty tuple. We can still use the older (map <key valuelist>) and (tuple <tlist>) formats when declaring types for maps and tuples.

The general form of bitstrings is (bitstring m n) which denotes a bitstring which starts with m bits and continues with segments of n bits. (binary) is a short form for a sequence of bytes while (bitstring) is a short form for a sequence of bits. There is currently no short form for an empty binary, (bitstring 0 0) must be used.

(deftype (type-name) type-def)

(defopaque (type-name) type-def)

(deftype (type-name par1 par2) type-def)

(defopaque (type-name par1 par2) type-def)

For unparameterised types the parentheses around the type name are optional. An example:

(deftype foo (tuple 'foo (integer) (list)))
(deftype bar (tuple 'bar (integer) (list)))
    

(defrecord rec (field1 default1 type1) (field2 default2) (field3))

Fields with type annotations MUST give a default value and fields without type annotations get the default type (any).

(defspec (func-name arity) function-spec ...)

where

function-spec = (arg-type-list ret-type)
function-spec = (arg-type-list ret-type constraint-list)
function-spec = #M(arg-types arg-type-list ret-type ret-type)
function-spec = #M(arg-types arg-type-list ret-type ret-type

contraints constraint-list) arg-type-list = (arg-type ...) constraint-list = (constraint ...) constraint = (var var-type)

For multiple types add more function specs. The parentheses around the function name and the arity are optional. For example from the docs:

(defspec foo ([(pos_integer)] (pos_integer)))
(defspec (foo 1)

([(pos_integer)] (pos_integer))
([(integer)] (integer))) (defspec (remove-if 2)
([(lambda ((any)) (boolean)) (list)] (list)))

Or with constraints:

(defspec id ((X) X ((X (tuple)))))
(defspec (foo 1)

([(tuple X (integer))] X ((X (atom))))
([(list Y)] Y ((Y (number))))) (defspec (remove-if 2)
([pred (list)] (list) [(pred (lambda ((any)) (boolean)))]))

Note that a constraint variable doesn’t need to start with an upper-case like an Erlang variable, though in some case it may be easier to read.

Note we are using the alternate list form with [ ] instead of parentheses to make it easier to see the function arguments.

Types can also be defined in the module declaration, for example:

(defmodule this-module

...
(type ((foo-type) (tuple 'foo (integer) (list)))
((bar-type) (tuple 'bar (integer) (list))))
(spec ((foo 1) ([(integer)] (foo-type)))
((id 1) ([x] x ((x (tuple))))))
...)

Robert Virding.

2021

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