Convenient monotonic if

if.prolog

:- use_module(library(reif)).

% A convenient version that can "understand" the non-monotonic predicates
iff_(Cond, Success, Failure) :-
    functor(Cond, _, Arity),
    if_(
        Arity = 3,
        if_(Cond, Success, Failure),
        if_([T]>> (call(Cond) -> T = true; T = false), Success, Failure)
    ).

% ?- iff_(ground(X), Result = true, Result = false).
% Result = false.
%
% ?- X = foo, iff_(ground(X), Result = true, Result = false).
% X = foo,
% Result = true.

% ?- member(1, [1,X]). % checks membership, but also eagerly unifies..
% true ;
% X = 1.
%
% ?- assert((
%   memberd(X, [E|Es]) :-
%     iff_( X = E
%         , true
%         , memberd(X, Es) )
% )).
% true.
%
% ?- memberd(1, [1,X]). % fully deterministic
% true.