evaluated_tell_args.chr

:- module(evaluated_tell_args, [direct/2, via_body/2, quoted/2, plus/2, type(pair_t/0)]).
:- use_module(prelude).

:- chr_type pair_t ---> pair(any, any).

:- function plus/2.
plus(X, Y) -> X + Y.

:- chr_constraint
    direct/2,
    via_body/2,
    relay/2,
    quoted/2.

% A goal of the form 'direct(EXPR, R)' tests that EXPR is evaluated
% before the constraint is told. R is unified with the (evaluated)
% first argument.
direct(N, R) <=> R = N.

% A goal of the form 'via_body(EXPR, R)' tests that EXPR is evaluated
% before the rule body's tell. The body retells 'relay(EXPR + 10, R)';
% the relay rule then unifies R with the (evaluated) value.
via_body(N, R) <=> relay(N + 10, R).
relay(X, R)    <=> R = X.

% 'quoted(EXPR, R)' uses quote/1 to opt out of evaluation: EXPR stays as
% a data term. This pins the existing quoting escape hatch and ensures
% the opt-out keeps working after the semantic change.
quoted(N, R) <=> R = N.