5.4.1.1 User defined functions on dicts

The tag of a dict associates the dict to a module. If the dot notation uses a compound term, this calls the goal below.

<module>:<name>(Arg1, ..., +Dict, -Value)

Functions are normal Prolog predicates. The dict infrastructure provides a more convenient syntax for representing the head of such predicates without worrying about the argument calling conventions. The code below defines a function multiply(Times) on a point that creates a new point by multiplying both coordinates. and len^{181as length would result in a predicate length/2, this name cannot be used. This might change in future versions.} to compute the length from the origin. The . and := operators are used to abstract the location of the predicate arguments. It is allowed to define multiple a function with multiple clauses, providing overloading and non-determinism.

:- module(point, []).

M.multiply(F) := point{x:X, y:Y} :-
        X is M.x*F,
        Y is M.y*F.

M.len() := Len :-
        Len is sqrt(M.x**2 + M.y**2).

After these definitions, we can evaluate the following functions:

?- X = point{x:1, y:2}.multiply(2).
X = point{x:2, y:4}.

?- X = point{x:1, y:2}.multiply(2).len().
X = 4.47213595499958.