add de facto standard forall/2 predicate as a built-in predicate

README.md

@@ -44,7 +44,7 @@ Extend Scryer Prolog to include the following, among other features:
          - [x] `call_residue_vars/2`
 - [x] `if_` and related predicates, following the developments of the
       paper "Indexing `dif/2`".
-- [x] All-solutions predicates (`findall/{3,4}`, `bagof/3`, `setof/3`).
+- [x] All-solutions predicates (`findall/{3,4}`, `bagof/3`, `setof/3`, `forall/2`).
 - [x] Clause creation and destruction (`asserta/1`, `assertz/1`,
       `retract/1`, `abolish/1`) with logical update semantics.
 - [x] Backtrackable and non-backtrackable global variables via `bb_get/2`
@@ -182,6 +182,7 @@ The following predicates are built-in to Scryer.
 * `false/0`
 * `findall/{3,4}`
 * `float/1`
+* `forall/2`
 * `freeze/2`
 * `functor/3`
 * `gen_int/1`

src/prolog/lib/builtins.pl

@@ -10,7 +10,7 @@
 	bb_get/2, bb_put/2, call_cleanup/2,
 	call_with_inference_limit/3, catch/3, clause/2,
 	current_predicate/1, current_prolog_flag/2, expand_goal/2,
-	expand_term/2, findall/3, findall/4, halt/0, once/1, op/3,
+	expand_term/2, findall/3, findall/4, forall/2, halt/0, once/1, op/3,
 	repeat/0, retract/1, set_prolog_flag/2, setof/3,
 	setup_call_cleanup/3, term_variables/2, throw/1, true/0,
 	false/0, write/1, write_canonical/1, writeq/1, write_term/2]).
@@ -405,6 +405,9 @@ findall(Template, Goal, Solutions0, Solutions1) :-
 				      Error,
 				      ( truncate_lh_to(LhLength), throw(Error) ))).
 
+forall(Generate, Test) :-
+    \+ (Generate, \+ Test).
+
 set_difference([X|Xs], [Y|Ys], Zs) :-
     X == Y, !, set_difference(Xs, [Y|Ys], Zs).
 set_difference([X|Xs], [Y|Ys], [X|Zs]) :-

src/tests.rs

@@ -1802,6 +1802,13 @@ fn test_queries_on_builtins()
                            [["S = [_240, 1, 2]", "X = _0", "Y = _5"]]);
     assert_prolog_success!(&mut wam, "?- setof(X, (exists(U,V) ^ member(X, [V,U,f(U),f(V)])), [a,b,f(b),f(a)]).");
         
+    assert_prolog_failure!(&mut wam, "?- forall(true, fail).");
+    assert_prolog_success!(&mut wam, "?- forall(fail, true)");
+    assert_prolog_success!(&mut wam, "?- catch(forall(_, true), error(instantiation_error, _), true).");
+    assert_prolog_success!(&mut wam, "?- catch(forall(true, _), error(instantiation_error, _), true).");
+    assert_prolog_success!(&mut wam, "?- catch(forall(1, true), error(type_error(callable, 1), _), true).");
+    assert_prolog_success!(&mut wam, "?- catch(forall(true, 1), error(type_error(callable, 1), _), true).");
+
     submit(&mut wam, "
 :- dynamic(cat/0).
 cat.