5.4.1 Functions on dicts

The infix operator dot (op(100, yfx, .) is used to extract values and evaluate functions on dicts. Functions are recognised if they appear in the argument of a goal in the source text, possibly nested in a term. The keys act as field selector, which is illustrated in this example.

?- X = point{x:1,y:2}.x.
X = 1.

?- Pt = point{x:1,y:2}, write(Pt.y).
2
Pt = point{x:1,y:2}.

?- X = point{x:1,y:2}.C.
X = 1,
C = x ;
X = 2,
C = y.

The compiler translates a goal that contains ./2 terms in its arguments into a conjunction of calls to ./3 defined in the system module. Terms functor.2 that appears in the head are replaced with a variable and calls to ./3 are inserted at the start of the body. Below are two examples, where the first extracts the x key from a dict and the second extends a dict containing an address with the postal code, given a find_postal_code/4 predicate.

dict_x(X, X.x).

add_postal_code(Dict, Dict.put(postal_code, Code)) :-
        find_postal_code(Dict.city,
                         Dict.street,
                         Dict.house_number,
                         Code).

Note that expansion of ./2 terms implies that such terms cannot be created by writing them explicitly in your source code. Such terms can still be created with functor/3, =../2, compound_name_arity/3 and compound_name_arguments/3.^{149Traditional code is unlikely to use ./2 terms because they were practically reserved for usage in lists. We do not provide a quoting mechanism as found in functional languages because it would only be needed to quote ./2 terms, such terms are rare and term manipulation provides an escape route.}

.(+Dict, +Function, -Result)

This predicate is called to evaluate ./2 terms found in the arguments of a goal. This predicate evaluates the field extraction described above, which is mapped to get_dict_ex/3. If Function is a compound term, it checks for the predefined functions on dicts described in section 5.4.1.2 or executes a user defined function as described in section 5.4.1.1.

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()^{150as 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.

5.4.1.2 Predefined functions on dicts

Dicts currently define the following reserved functions: