Programmers may declare new named data structures with the following procedure:
:- foreign_type
Type_name = Type,
…,
Type_name = Type.
where Type_name is an atom, and Type defines either an atomic or compound type, or is a previously-defined type name.
In Prolog, atomic types are represented by the natural atomic term
(integer, float, or atom). Compound structures are represented by
terms whose functor is the name of the type, and whose only argument
is the address of the data. So a term foo(123456) represents the
thing of type foo that exists at machine address 123456. And a term
integer(123456) represents the integer that lives in memeory at
address 123456, not the number 123456.
For types that are not named, a type name is generated using the names of associated types and the dollar sign character (‘$’), and possibly a number. Therefore, users should not use ‘$’ in their type names.