% ECRC Thom Fruehwirth 931214
% Sound if_then_else, negation and check, lazy conjunction and disjunction

handler control.

option(already_in_store, off). 
option(already_in_heads, off).
option(check_guard_bindings, on). 	% essential

operator(1180,fx,chr_if).
operator(900,fy,chr_not).
operator(900,fy,chr_chk).

operator(1000,xfy,and).
operator(1100,xfy,or).


constraints (chr_if)/1,(chr_not)/1,(chr_chk)/1. 

% Condition C must be without side-effects

if_then @ (chr_if C then A) <=> eval_condition(C,R) | 
	(R==true -> A).
if_then_else @ (chr_if C then A else B) <=> eval_condition(C,R) | 
	(R==true -> A ; B).

not_not @ chr_not chr_not C <=> 
	chr_chk C.
(not) @ chr_not C <=> eval_condition(C,R) | 
	R==fail. 		% if C then fail else true.

chk @ chr_chk C <=> eval_condition(C,R) | 
	R==true. 		% C succeeds without delay or touching vars

	eval_condition(C,R):-	% hackers delight, C without side-effects
		nonvar(C),
		(copy_term(C,C1),call(C1) -> C=C1,R=true ; R=fail).


constraints (then)/2,(or)/2.

(then) @ (A then B) <=> eval_condition(A,R) |	% lazy deterministic conjunction
	(R==true -> B).

label_with A or B if nonvar(A),nonvar(B). % delay disjunction till labeling time
A or B :- A ; B.


% end of handler control