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17.9  Examples of Using Attributed Variables

17.9.1  Variables with Enumerated Domains

As an example, let us implement variables of enumerable types using attributes. We choose to represent these variable as attributed variables whose attribute is a enum/1 structure with a list holding the values the variable may take, e.g.,


We have to specify now what should happen when such a variable is bound. This is done by writing a handler for the unify operation. The predicate unify_enum/2 defined below is this handler. Its first argument is the value that the attributed variable has been bound to, the second is the attribute that the bound attributed variable had (keep in mind that the system has already bound the attributed variable to the new value). We distinguish two cases:

[eclipse 2]: module(enum).
warning: creating a new module in module(enum)
[enum 3]: [user].
:- meta_attribute(enum, [unify:unify_enum/2, print:print_enum/2]).
:- import setarg/3 from sepia_kernel.

% unify_enum(+Term, Attribute)
unify_enum(_, Attr) :-
    /*** ANY + VAR ***/
    var(Attr).                 % Ignore if no attribute for this extension
unify_enum(Term, Attr) :-
    unify_term_enum(Term, Attr).

unify_term_enum(Value, enum(ListY)) :-
    nonvar(Value),             % The attributed variable was instantiated
    /*** NONVAR + META ***/
    memberchk(Value, ListY).
unify_term_enum(Y{AttrY}, AttrX) :-
    unify_enum_enum(Y, AttrX, AttrY).

unify_enum_enum(_, AttrX, AttrY) :-
    var(AttrY),                         % no attribute for this extension
    /*** VAR + META ***/
    AttrX = AttrY.                      % share the attribute
unify_enum_enum(Y, enum(ListX), AttrY) :-
    /*** META + META ***/
    AttrY = enum(ListY),
     intersection(ListX, ListY, ListXY),
     ( ListXY = [Val] ->
            Y = Val
            ListXY \= [],
            setarg(1, AttrY, ListXY)

print_enum(_{enum(List)}, Attr) :-
    Attr = List.
 user       compiled traceable 1188 bytes in 0.03 seconds

[enum 4]: A{enum([yellow, blue, white, green])}
                = B{enum([orange, blue, red, yellow])}.

A = B = A{[blue, yellow]}
[enum 5]: A{enum([yellow, blue, white, green])}
                = B{enum([orange, blue, red, black])}.

A = B = blue
[enum 6]: A{enum([yellow, blue, white, green])} = white.

A = white
[enum 7]: A{enum([yellow, blue, white, green])} = red.

no (more) solution.

Some further remarks on this code: The second clause of unify_term_enum/2 is a matching clause, as indicated by the -?-> guard. A matching clause is the only way to decompose an attributed variable. Note that this clause matches only calls that have an attributed variable with nonempty enum attribute on the first argument position.

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