term_variables.chr

:- module(term_variables, [empty/1, one/2, repeated/2, multi/4, nested/2]).
:- use_module(prelude).
:- use_module(library(lists)).
:- chr_constraint empty/1, one/2, repeated/2, multi/4, nested/2.
% Ground term: result is empty list.
empty(R) <=>
    R is term_variables(quote(p(1, foo, [a, b]))).

% One variable: result has length 1, head is the variable.
one(X, R) <=>
    Vs is term_variables(quote(p(1, X, foo))),
    L is length(Vs),
    R = result(L, Vs).

% Repeated variable in term: deduplicated to one.
repeated(X, R) <=>
    Vs is term_variables(quote(p(X, X, X))),
    L is length(Vs),
    R = L.

% Three distinct variables: length 3 (atoms in the term are excluded).
multi(X, Y, Z, R) <=>
    Vs is term_variables(quote(p(X, Y, Z, foo))),
    L is length(Vs),
    R = L.

% Nested compound containing a variable.
nested(X, R) <=>
    Vs is term_variables(quote(p(q(r(X, X))))),
    L is length(Vs),
    R = L.