E.g. if a structure is declared by specifying the prototype
book(author, title, year, publisher)then subsequently book/4-terms can be written as follows:
book{}
book{title:'tom sawyer'}
book{year:1886, title:'tom sawyer'}
which will be completely equivalent to the usual
book(_, _, _, _) book(_, 'tom sawyer', _, _) book(_, 'tom sawyer', 1886, _)The advantage is that the order and position of the fields or the arity of the whole structure do not have to be known and can be changed by just changing the initial declaration.
The argument index of a field in a structure can be obtained using a term of the form
FieldName of StructNameso instead of arg(3,B,Y) one can write
arg(year of book, B, Y)
The arity of the structure can be obtained using a term of the following form
property(arity) of StructName
instead of having to explicitly give the arity of the structure. So,
property(arity) of book
would be equivalent to the integer 4. Similarly, a Name/Arity
specification can be obtained by writing
property(functor) of StructName
so book/4 can alternatively be written as
property(functor) of book
Structures can also be declared to contain other structures, e.g.
:- local struct(film(based_on:book,director,year)).This allows the fields of book to be accessed as if they were fields of film. Note that the declaration of the year-field in the film-structure hides the year-field in the book structure.
% A simple structure:
[eclipse 1]: local struct(person(name,address,age)).
yes.
[eclipse 2]: John = person{age:30, name:john},
John = person{age:A},
arg(name of person, John, N).
John = person(john, _146, 30)
A = 30
N = john
yes.
[eclipse 3]: N is (property(arity) of person) + 1.
N = 4
yes.
[eclipse 4]: PersonStructure = (property(functor) of person).
PersonStructure = person / 3
yes.
% Example for structure inheritance:
[eclipse 4]: local struct(employee(p:person,salary)).
yes.
[eclipse 5]: Emp = employee{name:john,salary:2000}.
Emp = employee(person(john, _105, _106), 2000)
yes.
[eclipse 6]: Emp = employee{name:john, salary:2000},
Emp = employee{p:Person, salary:S},
arg(name of employee, Emp, N).
Person = person(john, _169, _170)
S = 2000
Emp = employee(person(john, _169, _170), 2000)
N = john
yes.
% Subscript syntax can be used with structures:
[eclipse 7]: Emp = employee{name:john, salary:2000},
Cost is 5 * Emp[salary of employee].
Cost = 10000
Emp = employee(person(john, _137, _138), 2000)
yes.