Add source

examples/meta/continuations.pl

@@ -0,0 +1,22 @@
+% Based on
+% https://occasionallycogent.com/continuations_in_prolog/
+
+test(Cont, Term) :-
+    writeln("Inside test"),
+    reset(test_, Term, Cont),
+    writeln("After reset").
+
+test_ :-
+    writeln("Entering reset"),
+    shift(Y),
+    X is 1 + (2 * Y),
+    write("In test X = "), write(X), writeln("; done").
+
+main :-
+    test(Cont, Term),
+    \+ \+ ( writeln("Calling Cont(2)"),
+            Term = 2, call(Cont)),
+    \+ \+ ( writeln("Calling Cont(4)"),
+            Term = 4, call(Cont)).
+
+:- initialization(main).

src/interpreter/Preprocessor.kt

@@ -3,6 +3,7 @@ package interpreter
 import io.Logger
 import prolog.ast.arithmetic.Expression
 import prolog.ast.arithmetic.Integer
+import prolog.ast.lists.List as PList
 import prolog.ast.logic.Clause
 import prolog.ast.logic.Fact
 import prolog.ast.logic.LogicOperand
@@ -50,9 +51,9 @@ open class Preprocessor {
     protected open fun preprocess(term: Term, nested: Boolean = false): Term {
         // TODO Remove hardcoding by storing the functors as constants in operators?
 
-        val prepped = when {
-            term == Variable("_") -> AnonymousVariable.create()
-            term is Atom || term is Structure -> {
+        val prepped = when (term) {
+            Variable("_") -> AnonymousVariable.create()
+            is Atom, is Structure -> {
                 // Preprocess the arguments first to recognize builtins
                 val args = if (term is Structure) {
                     term.arguments.map { preprocess(it, nested = true) }
@@ -144,11 +145,23 @@ open class Preprocessor {
                         }
                     }
 
+                    // Delimited Continuations
+                    Functor.of("reset/3") -> Reset(args[0] as Goal, args[1], args[2])
+                    Functor.of("shift/1") -> Shift(args[0])
+
                     // IO
                     Functor.of("write/1") -> Write(args[0])
                     Functor.of("nl/0") -> Nl
+                    Functor.of("writeln/1") -> WriteLn(args[0])
                     Functor.of("read/1") -> Read(args[0])
 
+                    // Lists
+                    Functor.of("member/2") -> Member(args[0], args[1] as PList)
+
+                    // Meta
+                    Functor.of("call/1") -> Call(args[0])
+                    Functor.of("ignore/1") -> Ignore(args[0] as Goal)
+
                     // Other
                     Functor.of("initialization/1") -> Initialization(args[0] as Goal)
                     Functor.of("forall/2") -> ForAll(args[0] as LogicOperand, args[1] as Goal)

src/parser/grammars/TermsGrammar.kt

@@ -3,9 +3,10 @@ package parser.grammars
 import com.github.h0tk3y.betterParse.combinators.*
 import com.github.h0tk3y.betterParse.grammar.parser
 import com.github.h0tk3y.betterParse.parser.Parser
+import com.github.h0tk3y.betterParse.utils.Tuple2
 import prolog.ast.arithmetic.Float
 import prolog.ast.arithmetic.Integer
-import prolog.ast.lists.List
+import prolog.ast.lists.List as PList
 import prolog.ast.terms.*
 import prolog.builtins.Dynamic
 import prolog.ast.lists.List.Empty
@@ -96,14 +97,19 @@ open class TermsGrammar : Tokens() {
         if (t2 == null) t1 else CompoundTerm(Atom(t2!!.t1), listOf(t1, t2!!.t2))
     }
 
+    protected val not: Parser<Term> by (notOp * parser(::term900)) use {
+        CompoundTerm(Atom(t1.text), listOf(t2))
+    }
+    protected val term900: Parser<Term> by (not or term700)
+
     protected val op1000: Parser<String> by (comma) use { text }
-    protected val term1000: Parser<Term> by (term700 * zeroOrMore(op1000 * term700)) use {
-        t2.fold(t1) { acc, (op, term) -> CompoundTerm(Atom(op), listOf(acc, term)) }
+    protected val term1000: Parser<Term> by (term900 * zeroOrMore(op1000 * term900)) use {
+        constructRightAssociative(t1, t2)
     }
 
     protected val op1100: Parser<String> by (semicolon) use { text }
     protected val term1100: Parser<Term> by (term1000 * zeroOrMore(op1100 * term1000)) use {
-        t2.fold(t1) { acc, (op, term) -> CompoundTerm(Atom(op), listOf(acc, term)) }
+        constructRightAssociative(t1, t2)
     }
 
     protected val dynamic: Parser<Term> by (dynamicOp * functor) use {
@@ -117,12 +123,12 @@ open class TermsGrammar : Tokens() {
     }
 
     // Lists
-    protected val list: Parser<List> by (-leftBracket * separated(
+    protected val list: Parser<PList> by (-leftBracket * separated(
         parser(::termNoConjunction),
         comma,
         acceptZero = true
     ) * -rightBracket) use {
-        var list: List = Empty
+        var list: PList = Empty
         // Construct the list in reverse order
         for (term in this.terms.reversed()) {
             list = Cons(term, list)
@@ -139,4 +145,12 @@ open class TermsGrammar : Tokens() {
     protected val body: Parser<Body> by term use { this as Body }
 
     override val rootParser: Parser<Any> by term
+
+    fun constructRightAssociative(left: Term, pairs: List<Tuple2<String, Term>>): Term {
+        if (pairs.isEmpty()) return left
+
+        val (name, next) = pairs.first()
+        val remainingPairs = pairs.drop(1)
+        return CompoundTerm(Atom(name), listOf(left, constructRightAssociative(next, remainingPairs)))
+    }
 }

src/parser/grammars/Tokens.kt

@@ -21,6 +21,8 @@ abstract class Tokens : Grammar<Any>() {
     protected val semicolon: Token by literalToken(";")
     // 1000
     protected val comma: Token by literalToken(",")
+    // 900
+    protected val notOp: Token by literalToken("\\+")
     // 700
     protected val univOp: Token by literalToken("=..")
     protected val notEquivalent: Token by literalToken("\\==")

src/prolog/builtins/controlOperators.kt

@@ -9,6 +9,7 @@ import prolog.ast.terms.Atom
 import prolog.ast.terms.Body
 import prolog.ast.terms.Goal
 import prolog.flags.AppliedCut
+import prolog.flags.AppliedShift
 import prolog.logic.applySubstitution
 import prolog.logic.numbervars
 
@@ -17,7 +18,7 @@ import prolog.logic.numbervars
  */
 object Fail : Atom("fail"), Body {
     override fun satisfy(subs: Substitutions): Answers = emptySequence()
-    override fun applySubstitution(subs: Substitutions): Fail = Fail
+    override fun applySubstitution(subs: Substitutions): Fail = this
 }
 
 /**
@@ -30,7 +31,7 @@ typealias False = Fail
  */
 object True : Atom("true"), Body {
     override fun satisfy(subs: Substitutions): Answers = sequenceOf(Result.success(emptyMap()))
-    override fun applySubstitution(subs: Substitutions): True = True
+    override fun applySubstitution(subs: Substitutions): True = this
 }
 
 // TODO Repeat/0
@@ -40,62 +41,72 @@ class Cut() : Atom("!") {
         return sequenceOf(Result.failure(AppliedCut(emptyMap())))
     }
 
-    override fun applySubstitution(subs: Substitutions): Cut = Cut()
+    override fun applySubstitution(subs: Substitutions): Cut = this
 }
 
 /**
  * Conjunction (and). True if both Goal1 and Goal2 are true.
  */
-class Conjunction(val left: LogicOperand, private val right: LogicOperand) :
+open class Conjunction(val left: LogicOperand, private val right: LogicOperand) :
     LogicOperator(Atom(","), left, right) {
     override fun satisfy(subs: Substitutions): Answers = sequence {
+        fun satisfyRight(leftSubs: Substitutions): Answers = sequence {
+            right.satisfy(subs + leftSubs).forEach { right ->
+                right.fold(
+                    // If the right part succeeds, yield the result with the left substitutions
+                    onSuccess = { rightSubs ->
+                        yield(Result.success(leftSubs + rightSubs))
+                    },
+                    onFailure = { exception ->
+                        // If the right part fails, check if it's a cut
+                        if (exception is AppliedCut) {
+                            // If it's a cut, yield the result with the left substitutions
+                            val newSubs = if (exception.subs != null) leftSubs + exception.subs else null
+                            yield(Result.failure(AppliedCut(newSubs)))
+                            return@sequence
+                        }
+
+                        // If it's not a cut, yield the failure
+                        yield(Result.failure(exception))
+                    }
+                )
+            }
+        }
+
+        fun findNextCutSolution(appliedCut: AppliedCut): Answers = sequence {
+            val leftSubs = appliedCut.subs
+            right.satisfy(subs + (appliedCut.subs!!)).firstOrNull()?.map { rightSubs ->
+                // If the right part succeeds, yield the result with the left substitutions
+                yield(Result.success(leftSubs + rightSubs))
+                return@sequence
+            } ?: yield(Result.failure(AppliedCut()))
+        }
+
         // Satisfy the left part first, which either succeeds or fails
         left.satisfy(subs).forEach { left ->
             left.fold(
                 // If it succeeds, satisfy the right part with the updated substitutions and return all results
-                onSuccess = { leftSubs ->
-                    right.satisfy(subs + leftSubs).forEach { right ->
-                        right.fold(
-                            // If the right part succeeds, yield the result with the left substitutions
-                            onSuccess = { rightSubs ->
-                                yield(Result.success(leftSubs + rightSubs))
-                            },
-                            // If the right part fails, check if it's a cut
-                            onFailure = { exception ->
-                                if (exception is AppliedCut) {
-                                    if (exception.subs != null) {
-                                        // If it's a cut, yield the result with the left substitutions
-                                        yield(Result.failure(AppliedCut(leftSubs + exception.subs)))
-                                    } else {
-                                        yield(Result.failure(AppliedCut()))
-                                    }
-                                    return@sequence
-                                }
-
-                                // If it's not a cut, yield the failure
-                                yield(Result.failure(exception))
-                            }
-                        )
-                    }
-                },
+                onSuccess = { leftSubs -> yieldAll(satisfyRight(leftSubs)) },
                 // If it fails, check these conditions:
                 onFailure = { exception ->
-                    // 1. If the left part is a cut, satisfy the right part ONCE, and stop searching for more solutions
-                    if (exception is AppliedCut) {
-                        right.satisfy(subs + (exception.subs!!)).firstOrNull()?.fold(
-                            onSuccess = {
-                                // If the right part succeeds, yield the result with the left substitutions
-                                yield(Result.success(exception.subs + it))
-                                return@sequence
-                            },
-                            onFailure = {
-                                // If the right part fails, yield the failure
-                                yield(Result.failure(it))
+                    when (exception) {
+                        // 1. If the left part is a cut, satisfy the right part ONCE, and stop searching for more solutions
+                        is AppliedCut -> yieldAll(findNextCutSolution(exception))
+
+                        // 2. If the left part is a shift, we need to check if the right part can be satisfied
+                        is AppliedShift -> {
+                            if (exception.cont == null) {
+                                // Pass the right of our disjunction as the continuation
+                                val newShift = AppliedShift(exception.subs, exception.ball, right as Goal)
+                                yield(Result.failure(newShift))
+                            } else {
+                                // Satisfy the right part with the updated substitutions
+                                yieldAll(satisfyRight(exception.subs))
                             }
-                        ) ?: yield(Result.failure(AppliedCut()))
-                    } else {
-                        // 2. Any other failure should be returned as is
-                        yield(Result.failure(exception))
+                        }
+
+                        // 3. Any other failure should be returned as is
+                        else -> yield(Result.failure(exception))
                     }
                 }
             )
@@ -114,7 +125,20 @@ class Conjunction(val left: LogicOperand, private val right: LogicOperand) :
 open class Disjunction(private val left: LogicOperand, private val right: LogicOperand) :
     LogicOperator(Atom(";"), left, right) {
     override fun satisfy(subs: Substitutions): Answers = sequence {
-        yieldAll(left.satisfy(subs))
+        left.satisfy(subs).forEach { left ->
+            left.fold(
+                onSuccess = { leftSubs ->
+                    yield(Result.success(leftSubs))
+                },
+                onFailure = { failure ->
+                    if (failure is AppliedCut) {
+                        val leftSubs = failure.subs
+                        yield(Result.failure(AppliedCut(leftSubs)))
+                        return@sequence
+                    }
+                }
+            )
+        }
         yieldAll(right.satisfy(subs))
     }
 

src/prolog/builtins/delimitedContinuationsOperators.kt

@@ -0,0 +1,75 @@
+package prolog.builtins
+
+import prolog.Answers
+import prolog.Substitutions
+import prolog.ast.arithmetic.Integer
+import prolog.ast.terms.Atom
+import prolog.ast.terms.Goal
+import prolog.ast.terms.Structure
+import prolog.ast.terms.Term
+import prolog.flags.AppliedShift
+import prolog.logic.applySubstitution
+import prolog.logic.unifyLazy
+
+/**
+ * These classes provide support for delimited continuations in Prolog.
+ * More specifically, they implement the reset/3 and shift/1 operators.
+ *
+ * See also [SWI-Prolog 4.9 Delimited Continuations](https://www.swi-prolog.org/pldoc/man?section=delcont)
+ */
+
+/**
+ * Call [Goal]. If Goal calls [Shift], and the arguments of Shift can be unified with Ball, Shift causes Reset to
+ * return, unifying Cont with a Goal that represents the continuation after shift.
+ */
+class Reset(private val goal: Goal, private val ball: Term, private val cont: Term) :
+    Structure(Atom("reset"), listOf(goal, ball, cont)) {
+    override fun satisfy(subs: Substitutions): Answers = sequence {
+        goal.satisfy(subs).forEach { goalResult ->
+            goalResult.fold(
+                // If Goal succeeds, then reset/3 also succeeds and binds Cont and Term1 to 0.
+                onSuccess = { goalSubs ->
+                    unifyLazy(cont, Integer(0), goalSubs).forEach { contResult ->
+                        contResult.map { contSubs ->
+                            yield(Result.success(goalSubs + contSubs))
+                        }
+                    }
+                },
+                onFailure = { failure ->
+                    // If at some point Goal calls shift(Term2), then its further execution is suspended.
+                    if (failure is AppliedShift) {
+                        require(failure.cont != null) { "Shift must have a continuation" }
+                        // Reset/3 succeeds immediately, binding Term1 to Term2 and Cont to the remainder of Goal.
+                        val shiftSubs = failure.subs
+                        val appliedBall = applySubstitution(failure.ball, shiftSubs)
+                        val appliedCont = applySubstitution(failure.cont, shiftSubs)
+                        Conjunction(Unify(ball, appliedBall), Unify(appliedCont, cont))
+                            .satisfy(shiftSubs)
+                            .forEach { conResult ->
+                                conResult.map { conSubs ->
+                                    yield(Result.success(shiftSubs + conSubs))
+                                }
+                            }
+                    } else {
+                        // If Goal fails, then reset also fails.
+                        yield(Result.failure(failure))
+                    }
+                }
+            )
+        }
+    }
+}
+
+/**
+ * Variables that have been bound during the procedure called by reset/3 stay bound after a shift/1:
+ */
+class Shift(private val ball: Term) : Structure(Atom("shift"), listOf(ball)) {
+    override fun satisfy(subs: Substitutions): Answers = sequence {
+        val shift = AppliedShift(
+            subs = subs,
+            ball = ball,
+            cont = null
+        )
+        yield(Result.failure(shift))
+    }
+}

src/prolog/builtins/ioOperators.kt

@@ -17,6 +17,8 @@ import prolog.logic.unifyLazy
  * Write [Term] to the current output, using brackets and operators where appropriate.
  */
 class Write(private val term: Term) : Operator(Atom("write"), null, term), Satisfiable {
+    constructor(message: String) : this(Atom(message))
+
     override fun satisfy(subs: Substitutions): Answers {
         val t = applySubstitution(term, subs)
 
@@ -45,6 +47,12 @@ object Nl : Atom("nl"), Satisfiable {
     override fun applySubstitution(subs: Substitutions): Nl = this
 }
 
+class WriteLn(private val term: Term) : Conjunction(Write(term), Nl) {
+    constructor(message: String) : this(Atom(message))
+    override fun applySubstitution(subs: Substitutions): WriteLn = WriteLn(applySubstitution(term, subs))
+    override fun toString(): String = "writeln($term)"
+}
+
 /**
  * Read the next Prolog term from the current input stream and unify it with [Term].
  *

src/prolog/builtins/listOperators.kt

@@ -8,20 +8,23 @@ import prolog.ast.lists.List.Empty
 import prolog.ast.terms.Atom
 import prolog.ast.terms.Operator
 import prolog.ast.terms.Term
+import prolog.logic.applySubstitution
 
 class Member(private val element: Term, private val list: List) : Operator(Atom("member"), element, list) {
-    private var solution: Operator = if (list is Empty) {
-        Unify(element, list)
-    } else if (list is Cons) {
-        Disjunction(
+    private var solution: Operator = when (list) {
+        is Empty -> Disjunction(Fail, Fail)
+        is Cons -> Disjunction(
             Unify(element, list.head),
             Member(element, list.tail)
         )
-    } else {
-        throw IllegalArgumentException("Invalid list type: ${list::class.simpleName}")
     }
 
     override fun satisfy(subs: Substitutions): Answers = solution.satisfy(subs)
 
+    override fun applySubstitution(subs: Substitutions): Member = Member(
+        applySubstitution(element, subs),
+        applySubstitution(list, subs) as List
+    )
+
     override fun toString(): String = "$element ∈ $list"
 }

src/prolog/builtins/metaOperators.kt

@@ -0,0 +1,44 @@
+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.ast.terms.Term
+import prolog.flags.AppliedCut
+import prolog.logic.applySubstitution
+
+class Call(private val goal: Term) : Operator(Atom("call"), null, goal) {
+    override fun satisfy(subs: Substitutions): Answers {
+        val appliedGoal = applySubstitution(goal, subs) as Goal
+        return appliedGoal.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))
+                    }
+                }
+            )
+        }
+    }
+}

src/prolog/flags/AppliedShift.kt

@@ -0,0 +1,14 @@
+package prolog.flags
+
+import prolog.Substitutions
+import prolog.ast.terms.Goal
+import prolog.ast.terms.Term
+
+/**
+ * An exception that indicates that a shift has been applied in the Prolog engine.
+ */
+data class AppliedShift(
+    val subs: Substitutions,
+    val ball: Term,
+    val cont: Term? = null,
+) : Throwable()

src/prolog/logic/unification.kt

@@ -7,12 +7,11 @@ import prolog.ast.arithmetic.Expression
 import prolog.ast.arithmetic.Float
 import prolog.ast.arithmetic.Integer
 import prolog.ast.arithmetic.Number
-import prolog.ast.lists.List
 import prolog.ast.lists.List.Cons
 import prolog.ast.lists.List.Empty
 import prolog.ast.logic.Fact
-import prolog.ast.logic.LogicOperator
 import prolog.ast.terms.*
+import prolog.ast.lists.List as PList
 
 // Apply substitutions to a term
 fun applySubstitution(term: Term, subs: Substitutions): Term = when {
@@ -29,16 +28,8 @@ fun applySubstitution(term: Term, subs: Substitutions): Term = when {
     else -> term
 }
 
-//TODO Combine with the other applySubstitution function
-fun applySubstitution(expr: Expression, subs: Substitutions): Expression = when {
-    variable(expr, emptyMap()) -> applySubstitution(expr as Term, subs) as Expression
-    atomic(expr, subs) -> expr
-    expr is LogicOperator -> {
-        expr.arguments = expr.arguments.map { applySubstitution(it, subs) }
-        expr
-    }
-
-    else -> expr
+fun applySubstitution(expr: Expression, subs: Substitutions): Expression {
+    return applySubstitution(expr as Term, subs) as Expression
 }
 
 // Check if a variable occurs in a term
@@ -90,7 +81,7 @@ fun unifyLazy(term1: Term, term2: Term, subs: Substitutions): Answers = sequence
             }
         }
 
-        t1 is List && t2 is List -> {
+        t1 is PList && t2 is PList -> {
             if (equivalent(t1, t2, subs)) {
                 yield(Result.success(emptyMap()))
             } else if (t1.size == t2.size) {
@@ -120,8 +111,8 @@ fun unifyLazy(term1: Term, term2: Term, subs: Substitutions): Answers = sequence
 }
 
 private fun unifyArgs(
-    args1: kotlin.collections.List<Argument>,
-    args2: kotlin.collections.List<Argument>,
+    args1: List<Argument>,
+    args2: List<Argument>,
     subs: Substitutions
 ): Answers = sequence {
     // Using the current subs, unify the first argument of each list
@@ -171,7 +162,7 @@ fun equivalent(term1: Term, term2: Term, subs: Substitutions): Boolean {
         term1 is Variable && term2 is Variable -> term1 == term2
         term1 is Variable -> term1 in subs && equivalent(subs[term1]!!, term2, subs)
         term2 is Variable -> term2 in subs && equivalent(subs[term2]!!, term1, subs)
-        term1 is List && term2 is List -> {
+        term1 is PList && term2 is PList -> {
             if (term1.isEmpty() && term2.isEmpty()) {
                 true
             } else if (term1.isEmpty() || term2.isEmpty()) {

src/repl/Repl.kt

@@ -6,6 +6,7 @@ import io.Terminal
 import parser.ReplParser
 import prolog.Answer
 import prolog.Answers
+import prolog.flags.AppliedCut
 
 class Repl {
     private val io = Terminal()
@@ -101,8 +102,15 @@ class Repl {
                 }
                 return subs.entries.joinToString(",\n") { "${it.key} = ${it.value}" }
             },
-            onFailure = {
-                return "ERROR: ${it.message}"
+            onFailure = { failure ->
+                if (failure is AppliedCut) {
+                    if (failure.subs != null) {
+                        return prettyPrint(Result.success(failure.subs))
+                    }
+                    return "false."
+                }
+
+                return "ERROR: ${failure.message}"
             }
         )
     }

src/tool/mapEach.kt

@@ -0,0 +1,3 @@
+package tool
+
+fun <T, R> Sequence<T>.mapEach(transform: (T) -> R): Sequence<R> = map(transform)

tests/e2e/Examples.kt

@@ -27,7 +27,7 @@ class Examples {
 
     @Test
     fun debugHelper() {
-        loader.load("examples/basics/arithmetics.pl")
+        loader.load("examples/meta/continuations.pl")
     }
 
     @ParameterizedTest
@@ -65,6 +65,7 @@ class Examples {
     )
 
     fun meta() = listOf(
+        Arguments.of("continuations.pl", "Inside test\nEntering reset\nAfter reset\nCalling Cont(2)\nIn test X = 5; done\nCalling Cont(4)\nIn test X = 9; done\n"),
         Arguments.of("mib_voorbeelden.pl", "b\nf(b)\nf(g(a,a),h(c,d),i(e,f))\nf(g(a,a),h(c,c),i(e,f))\nf(g(a,a),h(c,c),i(e,e))\n")
     )
 

tests/parser/grammars/LogicGrammarTests.kt

@@ -126,10 +126,11 @@ class LogicGrammarTests {
         val rule = result[0] as Rule
         assertEquals(Functor.of("guest/2"), rule.head.functor, "Expected functor 'guest/2'")
         assertEquals(Functor.of(",/2"), (rule.body as CompoundTerm).functor, "Expected functor ',/2'")
-        val l1 = (rule.body as CompoundTerm).arguments[0] as CompoundTerm
-        assertEquals(Functor.of(",/2"), l1.functor, "Expected functor ',/2'")
-        val l2 = l1.arguments[0] as CompoundTerm
-        assertEquals(Functor.of("invited/2"), l2.functor, "Expected functor 'invited/2'")
+        val l0 = (rule.body as CompoundTerm)
+        val l1 = l0.arguments[0] as CompoundTerm
+        assertEquals(Functor.of("invited/2"), l1.functor, "Expected functor 'invited/2'")
+        val l2 = l0.arguments[1] as CompoundTerm
+        assertEquals(Functor.of(",/2"), l2.functor, "Expected functor ',/2'")
     }
 
     @Test

tests/prolog/builtins/DelimitedContinuationsOperatorsTests.kt

@@ -0,0 +1,75 @@
+package prolog.builtins
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+import prolog.ast.Database.Program
+import prolog.ast.arithmetic.Integer
+import prolog.ast.lists.List.Cons
+import prolog.ast.lists.List.Empty
+import prolog.ast.logic.Rule
+import prolog.ast.terms.Atom
+import prolog.ast.terms.Structure
+import prolog.ast.terms.Variable
+import java.io.ByteArrayOutputStream
+import java.io.PrintStream
+
+class DelimitedContinuationsOperatorsTests {
+    @BeforeEach
+    fun setup() {
+        Program.reset()
+    }
+
+    @Test
+    fun `example member`() {
+        val member = Member(Variable("X"), Cons(Integer(1), Cons(Integer(2), Cons(Integer(3), Empty))))
+        val reset = Reset(member, Variable("Ball"), Variable("Cont"))
+
+        val result = reset.satisfy(emptyMap()).toList()
+
+        assertEquals(3, result.size, "Expected 3 results")
+
+        assertTrue(result[0].isSuccess, "Expected success for first result")
+        val subs0 = result[0].getOrNull()!!
+        assertEquals(2, subs0.size, "Expected 2 substitutions for first result")
+        assertEquals(Integer(1), subs0[Variable("X")])
+        assertEquals(Integer(0), subs0[Variable("Cont")])
+
+        assertTrue(result[1].isSuccess, "Expected success for second result")
+        val subs1 = result[1].getOrNull()!!
+        assertEquals(2, subs1.size, "Expected 2 substitutions for second result")
+        assertEquals(Integer(2), subs1[Variable("X")])
+        assertEquals(Integer(0), subs1[Variable("Cont")])
+
+        assertTrue(result[2].isSuccess, "Expected success for third result")
+        val subs2 = result[2].getOrNull()!!
+        assertEquals(2, subs2.size, "Expected 2 substitutions for third result")
+        assertEquals(Integer(3), subs2[Variable("X")])
+        assertEquals(Integer(0), subs2[Variable("Cont")])
+    }
+
+    @Test
+    fun `example with output`() {
+        val reset = Reset(Atom("test_"), Variable("Term"), Variable("Cont"))
+        val isOp = Is(Variable("X"), Add(Integer(1), Multiply(Integer(2), Variable("Y"))))
+        Program.load(
+            listOf(
+                Rule(
+                    Structure(Atom("test"), listOf(Variable("Cont"), Variable("Term"))),
+                    Conjunction(WriteLn("Inside test"), Conjunction(reset, WriteLn("After reset")))
+                ),
+                Rule(
+                    Atom("test_"),
+                    Conjunction(WriteLn("Entering reset"), Conjunction(Shift(Variable("Y")), Conjunction(isOp, Conjunction(Write("In test X = "), Conjunction(Write(Variable("X")), WriteLn("; done"))))))
+                )
+            )
+        )
+
+        val notNot2 = Not(Not(Conjunction(WriteLn("calling Cont(2)"), Conjunction(Unify(Variable("Term"), Integer(2)), Call(Variable("Cont"))))))
+        val notNot4 = Not(Not(Conjunction(WriteLn("calling Cont(4)"), Conjunction(Unify(Variable("Term"), Integer(4)), Call(Variable("Cont"))))))
+        val query = Conjunction(Structure(Atom("test"), listOf(Variable("Cont"), Variable("Term"))), Conjunction(notNot2, notNot4))
+
+        val result = query.satisfy(emptyMap()).toList()
+    }
+}

tests/prolog/builtins/MetaOperatorsTests.kt

@@ -0,0 +1,34 @@
+package prolog.builtins
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+import prolog.ast.arithmetic.Integer
+import prolog.ast.lists.List.Cons
+import prolog.ast.lists.List.Empty
+import prolog.ast.terms.Variable
+
+class MetaOperatorsTests {
+    @Test
+    fun `ignore of failing goal succeeds`() {
+        val goal = Member(Integer(4), Cons(Integer(1), Cons(Integer(2), Cons(Integer(3), Empty))))
+
+        val result = Ignore(goal).satisfy(emptyMap()).toList()
+
+        assertEquals(1, result.size, "Should return one result")
+        assertTrue(result[0].isSuccess, "Result should be successful")
+        assertTrue(result[0].getOrNull()!!.isEmpty(), "Expected empty substitutions")
+    }
+
+    @Test
+    fun `ignore of succeeding goal returns first solution`() {
+        val goal = Member(Variable("X"), Cons(Integer(1), Cons(Integer(2), Cons(Integer(3), Empty))))
+
+        val result = Ignore(goal).satisfy(emptyMap()).toList()
+
+        assertEquals(1, result.size, "Should return one result")
+        assertTrue(result[0].isSuccess, "Result should be successful")
+        val subs = result[0].getOrNull()!!
+        assertEquals(Integer(1), subs[Variable("X")], "Expected first solution to be 1")
+    }
+}