Call & Ignore

src/prolog/builtins/metaOperators.kt

@@ -0,0 +1,39 @@
+package prolog.builtins
+
+import prolog.Answers
+import prolog.Substitutions
+import prolog.ast.terms.Atom
+import prolog.ast.terms.Goal
+import prolog.ast.terms.Operator
+import prolog.flags.AppliedCut
+
+class Call(private val goal: Goal) : Operator(Atom("call"), null, goal) {
+    override fun satisfy(subs: Substitutions): Answers = goal.satisfy(subs)
+}
+
+/**
+ * Calls [Goal] once, but succeeds, regardless of whether Goal succeeded or not.
+ */
+class Ignore(goal: Goal) : Operator(Atom("ignore"), null, goal) {
+    private val disjunction = Disjunction(
+        Conjunction(Call(goal), Cut()),
+        True
+    )
+
+    override fun satisfy(subs: Substitutions): Answers = sequence {
+        disjunction.satisfy(subs).forEach { result ->
+            result.fold(
+                onSuccess = { newSubs ->
+                    yield(Result.success(newSubs))
+                },
+                onFailure = { failure ->
+                    if (failure is AppliedCut && failure.subs != null) {
+                        yield(Result.success(failure.subs))
+                    } else {
+                        yield(Result.failure(failure))
+                    }
+                }
+            )
+        }
+    }
+}