Checkpoint

src/prolog/builtins/control.kt

@@ -1,11 +1,16 @@
 package prolog.builtins
 
+import prolog.components.expressions.Operand
+import prolog.components.expressions.Operator
 import prolog.components.terms.Atom
+import prolog.components.terms.Body
 
 /**
  * Always fail.
  */
-class Fail: Atom("fail")
+class Fail: Atom("fail"), Body {
+    override fun prove(): Boolean = false
+}
 
 /**
  * Same as fail, but the name has a more declarative connotation.
@@ -15,6 +20,32 @@ typealias False = Fail
 /**
  * Always succeed.
  */
-class True: Atom("true")
+class True: Atom("true"), Body {
+    override fun prove(): Boolean = true
+}
 
 // TODO Repeat/0
+
+// TODO !/0 (Cut)
+
+/**
+ * Conjunction (and). True if both Goal1 and Goal2 are true.
+ */
+class Conjunction(leftOperand: Operand, rightOperand: Operand) : Operator(Atom(","), leftOperand, rightOperand) {
+    override fun prove(): Boolean {
+        val left = leftOperand?.prove() ?: true
+        val right = rightOperand.prove()
+        return left && right
+    }
+}
+
+/**
+ * Disjunction (or). True if either Goal1 or Goal2 succeeds.
+ */
+class Disjunction(leftOperand: Operand, rightOperand: Operand) : Operator(Atom(";"), leftOperand, rightOperand) {
+    override fun prove(): Boolean {
+        val left = leftOperand?.prove() ?: false
+        val right = rightOperand.prove()
+        return left || right
+    }
+}