Checkpoint

src/interpreter/Preprocessor.kt

@@ -59,6 +59,7 @@ open class Preprocessor {
             Structure(Atom("!"), emptyList()) -> Cut()
             Atom("inf") -> Integer(Int.MAX_VALUE)
             Atom("nl") -> Nl
+            Variable("_") -> AnonymousVariable.create()
             is Structure -> {
                 // Preprocess the arguments first to recognize builtins
                 val args = term.arguments.map { preprocess(it, nested = true) }
@@ -86,6 +87,10 @@ open class Preprocessor {
                         Not(args[0] as Goal)
                     }
 
+                    term.functor == "==/2" -> {
+                        Equivalent(args[0], args[1])
+                    }
+
                     term.functor == "=\\=/2" && args.all { it is Expression } -> {
                         EvaluatesToDifferent(args[0] as Expression, args[1] as Expression)
                     }
@@ -133,7 +138,10 @@ open class Preprocessor {
                     term.functor == "read/1" -> Read(args[0])
                     term.functor == "initialization/1" -> Initialization(args[0] as Goal)
 
-                    else -> term
+                    else -> {
+                        term.arguments = args
+                        term
+                    }
                 }
             }
 

src/parser/grammars/TermsGrammar.kt

@@ -36,8 +36,8 @@ import prolog.ast.terms.*
 open class TermsGrammar : Tokens() {
 
     // Basic named terms
-    protected val variable: Parser<Variable> by variableToken use { Variable(text) }
-    protected val simpleAtom: Parser<Atom> by nameToken use { Atom(text) }
+    protected val variable: Parser<Variable> by (variableToken or anonymousVariableToken) use { Variable(text) }
+    protected val simpleAtom: Parser<Atom> by (nameToken or exclamation) use { Atom(text) }
     protected val quotedAtom: Parser<Atom> by (dummy
             or ticked
             or doubleTicked
@@ -85,7 +85,7 @@ open class TermsGrammar : Tokens() {
     }
 
     // Level 700 - comparison operators
-    protected val op700: Parser<String> by (equals or notEquals) use { text }
+    protected val op700: Parser<String> by (equivalent or equals or notEquals or isOp) use { text }
     protected val term700: Parser<Term> by (term500 * optional(op700 * term500)) use {
         if (t2 == null) t1 else CompoundTerm(Atom(t2!!.t1), listOf(t1, t2!!.t2))
     }

src/parser/grammars/Tokens.kt

@@ -8,28 +8,32 @@ import com.github.h0tk3y.betterParse.lexer.regexToken
 import com.github.h0tk3y.betterParse.lexer.token
 
 abstract class Tokens : Grammar<Any>() {
-    // Prolog tokens
-    protected val nameToken: Token by regexToken("[a-z][a-zA-Z0-9_]*")
-    protected val variableToken: Token by regexToken("[A-Z][a-zA-Z0-9_]*")
-
-    // Arithmetic tokens
-    protected val floatToken: Token by regexToken("-?[1-9][0-9]*\\.[0-9]+")
-    protected val integerToken: Token by regexToken("-?([1-9][0-9]*|0)")
-
     // Special tokens
     protected val neck by literalToken(":-")
     protected val leftParenthesis: Token by literalToken("(")
     protected val rightParenthesis: Token by literalToken(")")
     protected val comma: Token by literalToken(",")
     protected val semicolon: Token by literalToken(";")
+    protected val equivalent: Token by literalToken("==")
     protected val equals: Token by literalToken("=")
     protected val notEquals: Token by literalToken("\\=")
     protected val plus: Token by literalToken("+")
     protected val minus: Token by literalToken("-")
     protected val multiply: Token by literalToken("*")
     protected val divide: Token by literalToken("/")
+    protected val exclamation: Token by literalToken("!")
+    protected val isOp: Token by literalToken("is")
     protected val dot by literalToken(".")
 
+    // Prolog tokens
+    protected val nameToken: Token by regexToken("[a-z][a-zA-Z0-9_]*")
+    protected val variableToken: Token by regexToken("[A-Z][a-zA-Z0-9_]*")
+    protected val anonymousVariableToken: Token by literalToken("_")
+
+    // Arithmetic tokens
+    protected val floatToken: Token by regexToken("-?[1-9][0-9]*\\.[0-9]+")
+    protected val integerToken: Token by regexToken("-?([1-9][0-9]*|0)")
+
     // Ignored tokens
     protected val whitespace: Token by regexToken("\\s+", ignore = true)
     protected val singleLineComment: Token by regexToken("%[^\\n]*", ignore = true)
@@ -41,4 +45,4 @@ abstract class Tokens : Grammar<Any>() {
 
     // Helper
     protected val dummy by token { _, _ -> -1 } use { throw IllegalStateException("This parser should not be used") }
-}
+}

src/prolog/ast/terms/AnonymousVariable.kt

@@ -0,0 +1,17 @@
+package prolog.ast.terms
+
+import io.Logger
+
+class AnonymousVariable(id: Int) : Variable("_$id") {
+    companion object {
+        private var counter = 0
+        fun create(): AnonymousVariable {
+            val id = counter
+            counter++
+            Logger.debug("Creating anonymous variable: _${id}")
+            return AnonymousVariable(id)
+        }
+    }
+
+    override fun toString(): String = "_"
+}

src/prolog/ast/terms/Variable.kt

@@ -6,7 +6,7 @@ import prolog.ast.arithmetic.Expression
 import prolog.ast.arithmetic.Simplification
 import prolog.ast.logic.LogicOperand
 
-data class Variable(val name: String) : Term, Body, Expression, LogicOperand() {
+open class Variable(val name: String) : Term, Body, Expression, LogicOperand() {
     override fun simplify(subs: Substitutions): Simplification {
         // If the variable is bound, return the value of the binding
         // If the variable is not bound, return the variable itself
@@ -28,5 +28,15 @@ data class Variable(val name: String) : Term, Body, Expression, LogicOperand() {
         return sequenceOf(Result.failure(IllegalArgumentException("Unbound variable: $this")))
     }
 
+    override fun equals(other: Any?): Boolean {
+        if (this === other) return true
+        if (other == null || other !is Variable) return false
+        return name == other.name
+    }
+
+    override fun hashCode(): Int {
+        return name.hashCode()
+    }
+
     override fun toString(): String = name
 }

src/prolog/builtins/controlOperators.kt

@@ -3,10 +3,10 @@ package prolog.builtins
 import prolog.Answers
 import prolog.Substitutions
 import prolog.ast.logic.LogicOperand
+import prolog.ast.logic.LogicOperator
 import prolog.ast.terms.Atom
 import prolog.ast.terms.Body
 import prolog.ast.terms.Goal
-import prolog.ast.logic.LogicOperator
 import prolog.flags.AppliedCut
 
 /**

src/prolog/builtins/io.kt

@@ -18,12 +18,7 @@ import prolog.logic.unifyLazy
  */
 class Write(private val term: Term) : Operator(Atom("write"), null, term), Satisfiable {
     override fun satisfy(subs: Substitutions): Answers {
-        var t = term
-        var temp = applySubstitution(t, subs)
-        while (t != temp) {
-            t = temp
-            temp = applySubstitution(t, subs)
-        }
+        val t = applySubstitution(term, subs)
 
         Terminal().say(t.toString())
 

src/prolog/logic/arithmetic.kt

@@ -58,7 +58,7 @@ fun succ(term1: Expression, term2: Expression, subs: Substitutions): Answers {
             it.fold(
                 onSuccess = { result ->
                     val t1 = applySubstitution(term1, result)
-                    if (t1 in result) {
+                    if (t1 in result || t1 in result.values) {
                         val e1 = t1.simplify(result)
                         if (e1.to is Integer && e1.to.value < 0) {
                             return@sequence

src/prolog/logic/unification.kt

@@ -13,7 +13,15 @@ import prolog.ast.arithmetic.Float
 
 // Apply substitutions to a term
 fun applySubstitution(term: Term, subs: Substitutions): Term = when {
-    variable(term, emptyMap()) -> subs[(term as Variable)] ?: term
+    variable(term, emptyMap()) -> {
+        var result = subs[(term as Variable)]
+
+        while (result != null && result is Variable && result in subs) {
+            result = subs[result]
+        }
+
+        result ?: term
+    }
     atomic(term, subs) -> term
     compound(term, subs) -> {
         val structure = term as Structure
@@ -25,7 +33,7 @@ fun applySubstitution(term: Term, subs: Substitutions): Term = when {
 
 //TODO Combine with the other applySubstitution function
 fun applySubstitution(expr: Expression, subs: Substitutions): Expression = when {
-    variable(expr, subs) -> applySubstitution(expr as Term, subs) as Expression
+    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) }

tests/e2e/Examples.kt

@@ -4,43 +4,43 @@ import interpreter.FileLoader
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.TestInstance
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.Arguments
+import org.junit.jupiter.params.provider.MethodSource
+import org.junit.jupiter.params.provider.ValueSource
 import prolog.Program
 import java.io.ByteArrayOutputStream
 import java.io.PrintStream
 
+@TestInstance(TestInstance.Lifecycle.PER_CLASS)
 class Examples {
     private val loader = FileLoader()
+    private lateinit var outStream: ByteArrayOutputStream
 
     @BeforeEach
     fun setup() {
         Program.reset()
-    }
 
-    @Test
-    fun fraternity() {
-        val inputFile = "examples/basics/fraternity.pl"
-
-        loader.load(inputFile)
-    }
-
-    @Test
-    fun unification() {
-        val inputFile = "examples/basics/unification.pl"
-
-        loader.load(inputFile)
-    }
-
-    @Test
-    fun write() {
-        val expected = "gpl zegt: dag(wereld)\n"
-
-        val outStream = ByteArrayOutputStream()
+        outStream = ByteArrayOutputStream()
         System.setOut(PrintStream(outStream))
+    }
 
-        val inputFile = "examples/basics/write.pl"
-
-        // Compare the stdio output with the expected output
+    @ParameterizedTest
+    @MethodSource("expectations")
+    fun test(inputFile: String, expected: String) {
         loader.load(inputFile)
         assertEquals(expected, outStream.toString())
     }
+
+    fun expectations() = listOf(
+        Arguments.of("examples/basics/arithmetics.pl", "gimli is a level 4 fighter with 35 hitpoints.\nlegolas is a level 5 ranger with 30 hitpoints.\ngandalf is a level 10 wizard with 25 hitpoints.\nfrodo is a level 2 rogue with 20 hitpoints.\nlegolas threw gimli, and gimli took 5 damage.\ngimli is a level 4 fighter with 30 hitpoints.\ngandalf casts aid.\ngimli is a level 4 fighter with 35 hitpoints.\nlegolas leveled up.\nlegolas is a level 6 ranger with 30 hitpoints"),
+        Arguments.of("examples/basics/backtracking.pl", "0\ns(0)\ns(s(0))\ns(s(s(0)))\n"),
+        Arguments.of("examples/basics/cut.pl", "0\n"),
+        Arguments.of("examples/basics/disjunction.pl", "Alice likes Italian food.\nBob likes Italian food.\n"),
+        Arguments.of("examples/basics/equality.pl", "X == Y failed\nX = Y succeeded\nX == Y succeeded\nX = Y succeeded\nX == Y succeeded\n"),
+        Arguments.of("examples/basics/fraternity.pl", "Citizen robespierre is eligible for the event.\nCitizen danton is eligible for the event.\nCitizen camus is eligible for the event.\n"),
+        Arguments.of("examples/basics/unification.pl", "While alice got an A, carol got an A, but bob did not get an A, dave did not get an A, unfortunately.\n"),
+        Arguments.of("examples/basics/write.pl", "gpl zegt: dag(wereld)\n"),
+    )
 }

tests/interpreter/OpenPreprocessor.kt

@@ -0,0 +1,9 @@
+package interpreter
+
+import prolog.ast.terms.Term
+
+class OpenPreprocessor : Preprocessor() {
+    public override fun preprocess(term: Term, nested: Boolean): Term {
+        return super.preprocess(term, nested)
+    }
+}

tests/interpreter/ParserPreprocessorIntegrationTests.kt

@@ -0,0 +1,93 @@
+package interpreter
+
+import com.github.h0tk3y.betterParse.grammar.parseToEnd
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Nested
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.ValueSource
+import parser.grammars.TermsGrammar
+import prolog.Program
+import prolog.ast.arithmetic.Float
+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.ast.terms.Variable
+import prolog.builtins.Is
+import prolog.builtins.Subtract
+
+class ParserPreprocessorIntegrationTests {
+    @Nested
+    class `Arithmetic`() {
+        val parser = TermsGrammar()
+        val preprocessor = OpenPreprocessor()
+
+        @ParameterizedTest
+        @ValueSource(strings = ["-1", "-1.0", "-1.5"])
+        fun `can parse negative numbers`(input: String) {
+            val number = if (input.contains('.')) {
+                Float(input.substring(1).toFloat())
+            } else {
+                Integer(input.substring(1).toInt())
+            }
+            val negativeNumber = if (input.contains('.')) {
+                Float(input.toFloat())
+            } else {
+                Integer(input.toInt())
+            }
+
+            // Check if parser returns the same result
+
+            val parsed = parser.parseToEnd("X is $input") as Term
+
+            assertEquals(
+                Structure(Atom("is"), listOf(
+                    Variable("X"),
+                    Structure(Atom("-"), listOf(number)),
+                )),
+                parsed
+            )
+
+            // Check if preprocessor returns the same result
+
+            val prepped = preprocessor.preprocess(parsed)
+
+            val expected = Is(
+                Variable("X"),
+                Subtract(Integer(0), number)
+            )
+
+            assertEquals(expected, prepped)
+            assertEquals(expected.toString(), prepped.toString())
+
+            // Check if evaluation is correct
+
+            val solutions = (prepped as Is).satisfy(emptyMap()).toList()
+
+            assertEquals(1, solutions.size)
+            assertEquals(negativeNumber, solutions[0].getOrNull()!![Variable("X")])
+        }
+
+        @ParameterizedTest
+        @ValueSource(strings = ["X is 1 - 2", "X is 1-2"])
+        fun `can add negative numbers`(input: String) {
+            val result = parser.parseToEnd(input) as Term
+
+            assertEquals(
+                Structure(Atom("is"), listOf(Variable("X"), Structure(Atom("-"), listOf(Integer(1), Integer(2))))),
+                result
+            )
+
+            val prepped = preprocessor.preprocess(result)
+
+            val expected = Is(
+                Variable("X"),
+                Subtract(Integer(1), Integer(2))
+            )
+
+            assertEquals(expected, prepped)
+            assertEquals(expected.toString(), prepped.toString())
+        }
+    }
+}

tests/interpreter/PreprocessorTests.kt

@@ -1,32 +1,80 @@
 package interpreter
 
-import org.junit.jupiter.api.Assertions.assertEquals
+import com.github.h0tk3y.betterParse.grammar.parseToEnd
+import org.junit.jupiter.api.Assertions.*
 import org.junit.jupiter.api.Nested
 import org.junit.jupiter.api.Test
+import parser.grammars.TermsGrammar
 import prolog.ast.arithmetic.Integer
-import prolog.ast.terms.Atom
-import prolog.ast.terms.CompoundTerm
-import prolog.ast.terms.Term
-import prolog.ast.terms.Variable
+import prolog.ast.terms.*
 import prolog.builtins.*
 
 class PreprocessorTests {
-    class OpenPreprocessor : Preprocessor() {
-        public override fun preprocess(term: Term, nested: Boolean): Term {
-            return super.preprocess(term, nested)
-        }
-    }
+    val preprocessor = OpenPreprocessor()
 
     companion object {
+        val preprocessor = OpenPreprocessor()
+
         fun test(tests: Map<Term, Term>) {
             for ((input, expected) in tests) {
-                val result = OpenPreprocessor().preprocess(input)
+                val result = preprocessor.preprocess(input)
                 assertEquals(expected, result, "Expected preprocessed")
                 assertEquals(expected::class, result::class, "Expected same class")
             }
         }
     }
 
+    @Test
+    fun `can preprocess anonymous variable`() {
+        val input = Variable("_")
+
+        val result = preprocessor.preprocess(input)
+
+        assertInstanceOf(AnonymousVariable::class.java, result, "Expected anonymous variable")
+        assertTrue((result as Variable).name.matches("_\\d+".toRegex()), "Expected anonymous variable name")
+    }
+
+    @Test
+    fun `multiple anonymous variables should be unique`() {
+        val input = CompoundTerm(Atom("foo"), listOf(Variable("_"), Variable("_")))
+
+        val result = preprocessor.preprocess(input)
+
+        assertInstanceOf(CompoundTerm::class.java, result, "Expected compound term")
+        assertEquals(2, (result as CompoundTerm).arguments.size, "Expected two terms")
+        for (argument in result.arguments) {
+            assertTrue(
+                (argument as Variable).name.matches("_\\d+".toRegex()),
+                "Expected anonymous variable name, but got ${argument.name}"
+            )
+        }
+        val first = result.arguments[0] as Variable
+        val second = result.arguments[1] as Variable
+        assertNotEquals(first.name, second.name, "Expected different anonymous variable names")
+    }
+
+    @Test
+    fun `can preprocess nested anonymous variables`() {
+        val input = TermsGrammar().parseToEnd("name(character(Name, _, _, _))") as Term
+
+        val result = preprocessor.preprocess(input)
+
+        assertInstanceOf(CompoundTerm::class.java, result, "Expected compound term")
+        assertEquals(1, (result as CompoundTerm).arguments.size, "Expected one term")
+        assertInstanceOf(CompoundTerm::class.java, result.arguments[0], "Expected compound term")
+        val inner = result.arguments[0] as CompoundTerm
+        assertEquals(4, inner.arguments.size, "Expected four terms")
+        for (argument in inner.arguments) {
+            if ((argument as Variable).name != "Name") {
+                assertTrue(
+                    (argument as Variable).name.matches("_\\d+".toRegex()),
+                    "Expected anonymous variable name, but got ${argument.name}"
+                )
+            }
+        }
+
+    }
+
     @Nested
     class `Arithmetic operators` {
         @Test
@@ -432,5 +480,22 @@ class PreprocessorTests {
                 )
             )
         }
+
+        @Test
+        fun `is`() {
+            test(
+                mapOf(
+                    CompoundTerm(Atom("is"), listOf(Variable("T"), Integer(1))) to Is(Variable("T"), Integer(1)),
+                    CompoundTerm(Atom("is"), listOf(Variable("T"), Add(Variable("HP"), Integer(5)))) to Is(
+                        Variable("T"),
+                        Add(Variable("HP"), Integer(5))
+                    ),
+                    CompoundTerm(Atom("is"), listOf(Variable("T"), Subtract(Variable("HP"), Integer(5)))) to Is(
+                        Variable("T"),
+                        Subtract(Variable("HP"), Integer(5))
+                    ),
+                )
+            )
+        }
     }
 }

tests/parser/grammars/LogicGrammarTests.kt

@@ -133,6 +133,21 @@ class LogicGrammarTests {
         assertEquals("invited/2", l2.functor, "Expected functor 'invited/2'")
     }
 
+    @Test
+    fun `parse check_identical(X, Y)`() {
+        var input = "check_identical(X, Y) :- X == Y."
+
+        assertDoesNotThrow {
+            val result = parser.parseToEnd(input)
+        }
+
+        input = "check_identical(X, Y) :- X = Y, !, write('X == Y succeeded'), nl."
+
+        assertDoesNotThrow {
+            val result = parser.parseToEnd(input)
+        }
+    }
+
     @Test
     fun `parse constraints`() {
         val input = ":- a."

tests/parser/grammars/TermsGrammarTests.kt

@@ -17,6 +17,7 @@ import prolog.ast.terms.Atom
 import prolog.ast.terms.Structure
 import prolog.ast.terms.Term
 import prolog.ast.terms.Variable
+import prolog.builtins.Is
 import prolog.logic.equivalent
 
 class TermsGrammarTests {
@@ -60,7 +61,16 @@ class TermsGrammarTests {
     fun `parse variable`(name: String) {
         val result = parser.parseToEnd(name)
 
-        assertEquals(Variable(name), result, "Expected atom '$name'")
+        assertEquals(Variable(name), result)
+    }
+
+    @Test
+    fun `parse anonymous variable`() {
+        val input = "_"
+
+        val result = parser.parseToEnd(input)
+
+        assertEquals(Variable("_"), result, "Expected anonymous variable")
     }
 
     @Test
@@ -100,24 +110,33 @@ class TermsGrammarTests {
 
         val result = parser.parseToEnd(input)
 
-        assertTrue(
-            equivalent(Structure(Atom("f"), listOf(Atom("a"), Variable("X"))), result, emptyMap()),
+        assertEquals(
+            Structure(Atom("f"), listOf(Atom("a"), Variable("X"))),
+            result,
             "Expected atom 'f(a, X)'"
         )
     }
 
+    @Test
+    fun `parse compound term with var and int`() {
+        val input = "check_identical(A, 13)"
+        val result = parser.parseToEnd(input)
+        assertEquals(
+            Structure(Atom("check_identical"), listOf(Variable("A"), Integer(13))),
+            result,
+            "Expected atom 'check_identical(A, 13)'"
+        )
+    }
+
     @Test
     fun `parse nested compound term f(a, g(b))`() {
         val input = "f(a, g(b))"
 
         val result = parser.parseToEnd(input)
 
-        Assertions.assertTrue(
-            equivalent(
-                Structure(Atom("f"), listOf(Atom("a"), Structure(Atom("g"), listOf(Atom("b"))))),
-                result,
-                emptyMap()
-            ),
+        assertEquals(
+            Structure(Atom("f"), listOf(Atom("a"), Structure(Atom("g"), listOf(Atom("b"))))),
+            result,
             "Expected atom 'f(a, g(b))'"
         )
     }
@@ -128,24 +147,63 @@ class TermsGrammarTests {
 
         val result = parser.parseToEnd(input)
 
-        Assertions.assertTrue(
-            equivalent(
-                Structure(Atom("f"), listOf(Atom("a"), Structure(Atom("g"), listOf(Variable("X"))))),
-                result,
-                emptyMap()
-            ),
+        assertEquals(
+            Structure(Atom("f"), listOf(Atom("a"), Structure(Atom("g"), listOf(Variable("X"))))),
+            result,
             "Expected atom 'f(a, g(X))'"
         )
     }
 
+    @Test
+    fun `parse nested compound term with variables`() {
+        val input = "hit(character(Name, Class, Level, HP), character(Name, Class, Level, T))"
+
+        val result = parser.parseToEnd(input)
+
+        assertEquals(
+            Structure(
+                Atom("hit"),
+                listOf(
+                    Structure(Atom("character"), listOf(Variable("Name"), Variable("Class"), Variable("Level"), Variable("HP"))),
+                    Structure(Atom("character"), listOf(Variable("Name"), Variable("Class"), Variable("Level"), Variable("T")))
+                )
+            ),
+            result,
+            "Expected atom 'hit(character(Name, Class, Level, HP), character(Name, Class, Level, T))'"
+        )
+    }
+
+    @Test
+    fun `parse compound term with anonymous variables`() {
+        val input = "f(a, _, g(X))"
+
+        val result = parser.parseToEnd(input)
+
+        assertEquals(
+            Structure(Atom("f"), listOf(Atom("a"), Variable("_"), Structure(Atom("g"), listOf(Variable("X"))))),
+            result,
+            "Expected atom 'f(a, _, g(X))'"
+        )
+    }
+
     @ParameterizedTest
-    @ValueSource(ints = [-987654321, -543, -21, -1, 0, 1, 5, 12, 345, 123456789])
-    fun `parse integer`(number: Int) {
+    @ValueSource(ints = [0, 1, 5, 12, 345, 123456789])
+    fun `parse positive integer`(number: Int) {
         val input = number.toString()
 
         val result = parser.parseToEnd(input)
 
-        Assertions.assertEquals(Integer(number), result, "Expected integer '$number'")
+        assertEquals(Integer(number), result, "Expected integer '$number'")
+    }
+
+    @ParameterizedTest
+    @ValueSource(ints = [-987654321, -543, -21, -1])
+    fun `parse negative integer`(number: Int) {
+        val input = number.toString()
+
+        val result = parser.parseToEnd(input)
+
+        assertEquals(Structure(Atom("-"), listOf(Integer(0 - number))), result, "Expected integer '$number'")
     }
 
     @Test
@@ -154,10 +212,7 @@ class TermsGrammarTests {
 
         val result = parser.parseToEnd(input)
 
-        Assertions.assertTrue(
-            equivalent(Float(42.0f), result, emptyMap()),
-            "Expected float '42.0'"
-        )
+        assertEquals(Float(42.0f), result, "Expected float '42.0'")
     }
 
     @Test
@@ -166,7 +221,7 @@ class TermsGrammarTests {
 
         val result = parser.parseToEnd(input)
 
-        assertEquals(Float(-42.0f), result, "Expected float '-42.0'")
+        assertEquals(Structure(Atom("-"), listOf(Float(42.0f))), result, "Expected float '-42.0'")
     }
 
     @ParameterizedTest
@@ -176,7 +231,7 @@ class TermsGrammarTests {
     }
 
     @Nested
-    class `Operator precedence` {
+    class `Operators and precedence` {
         private lateinit var parser: Grammar<Term>
 
         @BeforeEach
@@ -184,6 +239,74 @@ class TermsGrammarTests {
             parser = TermsGrammar() as Grammar<Term>
         }
 
+        @Test
+        fun `can parse equivalent`() {
+            val input = "X == Y"
+
+            val result = parser.parseToEnd(input)
+
+            assertEquals(
+                Structure(Atom("=="), listOf(Variable("X"), Variable("Y"))),
+                result,
+                "Expected equivalent operator"
+            )
+        }
+
+        @Test
+        fun `can parse cut`() {
+            val input = "!"
+            val result = parser.parseToEnd(input)
+            assertEquals(Atom("!"), result, "Expected cut operator")
+        }
+
+        @Test
+        fun `can parse 'is'`() {
+            val input = "T is 1"
+            val result = parser.parseToEnd(input)
+            assertEquals(
+                Structure(Atom("is"), listOf(Variable("T"), Integer(1))),
+                result
+            )
+        }
+
+        @Test
+        fun `can parse 'is' with addition`() {
+            val input = "T is 1 + 2"
+            val result = parser.parseToEnd(input)
+            assertEquals(
+                Structure(Atom("is"), listOf(Variable("T"), Structure(Atom("+"), listOf(Integer(1), Integer(2))))),
+                result
+            )
+        }
+
+        @ParameterizedTest
+        @ValueSource(strings = ["+", "-", "*", "/"])
+        fun `can parse with spaces`(operator: String) {
+            val input = "1 $operator 2"
+
+            val result = parser.parseToEnd(input)
+
+            assertEquals(
+                Structure(Atom(operator), listOf(Integer(1), Integer(2))),
+                result,
+                "Expected operator '$operator'"
+            )
+        }
+
+        @ParameterizedTest
+        @ValueSource(strings = ["+", "-", "*", "/"])
+        fun `can parse without spaces`(operator: String) {
+            val input = "1${operator}2"
+
+            val result = parser.parseToEnd(input)
+
+            assertEquals(
+                Structure(Atom(operator), listOf(Integer(1), Integer(2))),
+                result,
+                "Expected operator '$operator' without spaces"
+            )
+        }
+
         @Test
         fun `parse addition and multiplication`() {
             val input = "1 + 2 * 3"

tests/prolog/logic/ArithmeticTests.kt

@@ -583,10 +583,23 @@ class ArithmeticTests {
         assertTrue(equivalent(result[0].getOrThrow()[t3]!!, Float(6.0f), result[0].getOrNull()!!), "X should be equal to 6.0")
     }
 
+    @Test
+    fun `addition with negative`() {
+        val t1 = Integer(1)
+        val t2 = Integer(-1)
+        val t3 = Integer(0)
+
+        val result = plus(t1, t2, t3, emptyMap()).toList()
+
+        assertEquals(1, result.size, "There should be one solution")
+        assertTrue(result[0].isSuccess, "Expected success")
+        assertTrue(result[0].getOrNull()!!.isEmpty(), "1 + -1 should already be equal to 0")
+    }
+
     @RepeatedTest(100)
     fun `random test for mul`() {
-        val t1 = Integer((0..1000).random())
-        val t2 = Integer((0..1000).random())
+        val t1 = Integer((-1000..1000).random())
+        val t2 = Integer((-1000..1000).random())
         val t3 = Variable("X")
 
         val result = mul(t1, t2, t3, emptyMap()).toList()

tests/prolog/logic/UnificationTests.kt

@@ -0,0 +1,370 @@
+package prolog.logic
+
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+import prolog.Substitutions
+import prolog.ast.arithmetic.Integer
+import prolog.ast.terms.Atom
+import prolog.ast.terms.Structure
+import prolog.ast.terms.Variable
+import prolog.builtins.Add
+import org.junit.jupiter.api.Disabled
+import org.junit.jupiter.api.Nested
+
+/*
+ * Based on: https://en.wikipedia.org/wiki/Unification_%28computer_science%29#Examples_of_syntactic_unification_of_first-order_terms
+ */
+class UnificationTests {
+    @Nested
+    class `unify` {
+        @Test
+        fun identical_atoms_unify() {
+            val atom1 = Atom("a")
+            val atom2 = Atom("a")
+
+            val result = unify(atom1, atom2)
+
+            assertTrue(result.isSuccess, "Identical atoms should unify")
+            assertEquals(0, result.getOrNull()!!.size, "No substitutions should be made")
+        }
+
+        @Test
+        fun different_atoms_do_not_unify() {
+            val atom1 = Atom("a")
+            val atom2 = Atom("b")
+
+            val result = unify(atom1, atom2)
+
+            assertFalse(result.isSuccess, "Different atoms should not unify")
+        }
+
+        /**
+         * ?- X = X.
+         * true.
+         */
+        @Test
+        fun identical_variables_unify() {
+            val variable1 = Variable("X")
+            val variable2 = Variable("X")
+
+            val result = unify(variable1, variable2)
+
+            assertTrue(result.isSuccess, "Identical variables should unify")
+            assertEquals(0, result.getOrNull()!!.size, "No substitutions should be made")
+        }
+
+        @Test
+        fun variable_unifies_with_atom() {
+            val variable = Variable("X")
+            val atom = Atom("a")
+
+            val result = unify(atom, variable)
+
+            assertTrue(result.isSuccess, "Variable should unify with atom")
+            assertEquals(1, result.getOrNull()!!.size, "There should be one substitution")
+            assertEquals(atom, result.getOrNull()!![variable], "Variable should be substituted with atom")
+        }
+
+        @Test
+        fun variables_alias_when_unified() {
+            val variable1 = Variable("X")
+            val variable2 = Variable("Y")
+
+            val result = unify(variable1, variable2)
+
+            assertTrue(result.isSuccess)
+            assertEquals(1, result.getOrNull()!!.size)
+            assertEquals(variable2, result.getOrNull()!![variable1], "Variable 1 should alias to variable 2")
+        }
+
+        @Test
+        fun identical_compound_terms_unify() {
+            val structure1 = Structure(Atom("f"), listOf(Atom("a"), Atom("b")))
+            val structure2 = Structure(Atom("f"), listOf(Atom("a"), Atom("b")))
+
+            val result = unify(structure1, structure2)
+
+            assertTrue(result.isSuccess, "Identical compound terms should unify")
+            assertEquals(0, result.getOrNull()!!.size, "No substitutions should be made")
+        }
+
+        @Test
+        fun compound_terms_with_different_arguments_do_not_unify() {
+            val structure1 = Structure(Atom("f"), listOf(Atom("a"), Atom("b")))
+            val structure2 = Structure(Atom("f"), listOf(Atom("a"), Atom("c")))
+
+            val result = unify(structure1, structure2)
+
+            assertFalse(result.isSuccess, "Different compound terms should not unify")
+        }
+
+        @Test
+        fun compound_terms_with_different_functors_do_not_unify() {
+            val structure1 = Structure(Atom("f"), listOf(Atom("a"), Atom("b")))
+            val structure2 = Structure(Atom("g"), listOf(Atom("a"), Atom("b")))
+
+            val result = unify(structure1, structure2)
+
+            assertFalse(result.isSuccess, "Compound terms with different functors should not unify")
+        }
+
+        /**
+         * ?- X = f(a, b).
+         * X = f(a, b).
+         */
+        @Test
+        fun variable_unifies_with_compound_term() {
+            val variable = Variable("X")
+            val structure = Structure(Atom("f"), listOf(Atom("a"), Atom("b")))
+
+            val result = unify(variable, structure)
+
+            assertTrue(result.isSuccess, "Variable should unify with compound term")
+
+            val subs = result.getOrNull()!!
+
+            assertEquals(1, subs.size, "There should be one substitution")
+            assertTrue(subs.containsKey(variable), "Variable should be in the substitution map")
+            assertTrue(
+                equivalent(Structure(Atom("f"), listOf(Atom("a"), Atom("b"))), subs[variable]!!, subs),
+                "Variable should be substituted with compound term"
+            )
+        }
+
+        @Test
+        fun compound_term_with_variable_unifies_with_part() {
+            val variable = Variable("X")
+            val structure1 = Structure(Atom("f"), listOf(Atom("a"), variable))
+            val structure2 = Structure(Atom("f"), listOf(Atom("a"), Atom("b")))
+
+            val result = unify(structure1, structure2)
+
+            assertTrue(result.isSuccess, "Compound term with variable should unify with part")
+
+            val subs = result.getOrNull()!!
+
+            assertEquals(1, subs.size, "There should be one substitution")
+            assertTrue(subs.containsKey(variable), "Variable should be in the substitution map")
+            val equivalence = equivalent(Atom("b"), subs[variable]!!, subs)
+            assertTrue(equivalence, "Variable should be substituted with atom")
+        }
+
+        @Test
+        fun compound_terms_with_variable_arguments_lists_alias_variables() {
+            val variable1 = Variable("X")
+            val variable2 = Variable("Y")
+
+            val structure1 = Structure(Atom("f"), listOf(variable1))
+            val structure2 = Structure(Atom("f"), listOf(variable2))
+
+            val result = unify(structure1, structure2)
+
+            assertTrue(result.isSuccess, "Compound terms with variable arguments should unify")
+
+            val subs = result.getOrNull()!!
+
+            assertEquals(1, subs.size, "There should be one substitution")
+            assertTrue(subs.containsKey(variable1), "Variable 1 should be in the substitution map")
+            assertEquals(variable2, subs[variable1], "Variable 1 should alias to variable 2")
+        }
+
+        /**
+         * f(X) = f(Y, Z)
+         */
+        @Test
+        fun compound_terms_with_different_arity_do_not_unify() {
+            val structure1 = Structure(Atom("f"), listOf(Variable("X")))
+            val structure2 = Structure(Atom("f"), listOf(Variable("Y"), Variable("Z")))
+
+            val result = unify(structure1, structure2)
+
+            assertFalse(result.isSuccess, "Compound terms with different arity should not unify")
+        }
+
+        /**
+         * ?- f(g(X)) = f(Y).
+         * Y = g(X).
+         */
+        @Test
+        fun nested_compound_terms_with_variables_unify() {
+            val variable2 = Variable("Y")
+
+            val structure1 = Structure(Atom("f"), listOf(Structure(Atom("g"), listOf(Variable("X")))))
+            val structure2 = Structure(Atom("f"), listOf(variable2))
+
+            val result = unify(structure1, structure2)
+
+            assertTrue(result.isSuccess, "Nested compound terms with variables should unify")
+
+            val subs = result.getOrNull()!!
+
+            assertEquals(1, subs.size, "There should be one substitution")
+            assertTrue(subs.containsKey(variable2), "Variable 2 should be in the substitution map")
+            assertTrue(
+                equivalent(Structure(Atom("g"), listOf(Variable("X"))), subs[variable2]!!, subs),
+                "Variable should be substituted with compound term"
+            )
+        }
+
+        /**
+         * ?- f(g(X), X) = f(Y, a).
+         * X = a,
+         * Y = g(a).
+         */
+        @Test
+        fun compound_terms_with_more_variables() {
+            val variable1 = Variable("X")
+            val variable2 = Variable("Y")
+
+            val structure1 = Structure(Atom("f"), listOf(Structure(Atom("g"), listOf(variable1)), variable1))
+            val structure2 = Structure(Atom("f"), listOf(variable2, Atom("a")))
+
+            val result = unify(structure1, structure2)
+
+            assertTrue(result.isSuccess, "Compound terms with more variables should unify")
+
+            val subs = result.getOrNull()!!
+
+            assertEquals(2, subs.size, "There should be two substitutions")
+            assertTrue(subs.containsKey(variable1), "Variable 1 should be in the substitution map")
+            assertTrue(
+                equivalent(Atom("a"), subs[variable1]!!, subs),
+                "Variable 1 should be substituted with atom"
+            )
+            assertTrue(subs.containsKey(variable2), "Variable 2 should be in the substitution map")
+            assertTrue(
+                equivalent(Structure(Atom("g"), listOf(Atom("a"))), subs[variable2]!!, subs),
+                "Variable 2 should be substituted with compound term"
+
+            )
+        }
+
+        /**
+         * ?- X = f(X).
+         * X = f(f(X)).
+         */
+        @Test
+        @Disabled("If the occurs check is applied, this should fail")
+        fun recursive_unification() {
+            val variable1 = Variable("X")
+            val structure2 = Structure(Atom("f"), listOf(Variable("X")))
+
+            val result = unifyLazy(variable1, structure2, emptyMap()).toList()
+
+            assertEquals(1, result.size, "There should be one result")
+            assertTrue(result[0].isSuccess, "Recursive unification should succeed")
+
+            val subs = result[0].getOrNull()!!
+
+            assertEquals(1, subs.size, "There should be one substitution")
+            assertTrue(subs.containsKey(variable1), "Variable should be in the substitution map")
+            assertEquals(structure2, subs[variable1], "Variable should be substituted with compound term")
+        }
+
+        /**
+         * ?- X = bar, Y = bar, X = Y.
+         * X = Y, Y = bar.
+         */
+        @Test
+        fun multiple_unification() {
+            val variable1 = Variable("X")
+            val variable2 = Variable("Y")
+            val atom = Atom("bar")
+
+            val map: Substitutions = mapOf(
+                variable1 to atom,
+                variable2 to atom
+            )
+            val result = unifyLazy(variable1, variable2, map).toList()
+
+            assertEquals(1, result.size, "There should be one substitution")
+            assertTrue(result[0].isSuccess, "Multiple unification should succeed")
+            assertEquals(0, result[0].getOrNull()!!.size, "No (additional) substitutions should be made")
+        }
+
+        /**
+         * ?- a = a().
+         * false.
+         */
+        @Test
+        fun atom_with_different_arity() {
+            val atom1 = Atom("a")
+            val structure2 = Structure(Atom("a"), emptyList())
+
+            val result = unify(atom1, structure2)
+
+            assertFalse(result.isSuccess, "Atom with different arity should not unify")
+        }
+
+        @Test
+        fun identical_integers_unify() {
+            val int1 = Integer(1)
+            val int2 = Integer(1)
+
+            val result = unify(int1, int2)
+
+            assertTrue(result.isSuccess, "Identical integers should unify")
+            assertEquals(0, result.getOrNull()!!.size, "No substitutions should be made")
+        }
+
+        @Test
+        fun different_integers_do_not_unify() {
+            val int1 = Integer(1)
+            val int2 = Integer(2)
+
+            val result = unify(int1, int2)
+
+            assertFalse(result.isSuccess, "Different integers should not unify")
+        }
+
+
+        @Test
+        fun `1 + 2 does not unify with 3`() {
+            val expr1 = Add(Integer(1), Integer(2))
+            val expr2 = Integer(3)
+
+            val result = unify(expr1, expr2)
+
+            assertFalse(result.isSuccess, "1 + 2 should not unify with 3")
+        }
+    }
+
+    @Nested
+    class `applySubstitution` {
+        @Test
+        fun `apply substitution without sub`() {
+            val term = Variable("X")
+            val subs: Substitutions = emptyMap()
+
+            val result = applySubstitution(term, subs)
+
+            assertEquals(term, result)
+        }
+
+        @Test
+        fun `apply single substitution`() {
+            val sub = Variable("X") to Integer(5)
+            val subs: Substitutions = mapOf(sub)
+
+            val term = Variable("X")
+
+            val result = applySubstitution(term, subs)
+
+            assertEquals(Integer(5), result)
+        }
+
+        @Test
+        fun `apply chained substitution`() {
+            val sub1 = Variable("HP") to Variable("HP(19)")
+            val sub2 = Variable("HP(19)") to Integer(35)
+
+            val subs: Substitutions = mapOf(sub1, sub2)
+
+            val term = Variable("HP")
+
+            val result = applySubstitution(term, subs)
+
+            assertEquals(Integer(35), result)
+        }
+    }
+}

tests/prolog/logic/UnifyTests.kt

@@ -1,328 +0,0 @@
-package prolog.logic
-
-import org.junit.jupiter.api.Assertions.*
-import org.junit.jupiter.api.Test
-import prolog.Substitutions
-import prolog.ast.arithmetic.Integer
-import prolog.ast.terms.Atom
-import prolog.ast.terms.Structure
-import prolog.ast.terms.Variable
-import prolog.builtins.Add
-import org.junit.jupiter.api.Disabled
-import org.junit.jupiter.api.fail
-
-/*
- * Based on: https://en.wikipedia.org/wiki/Unification_%28computer_science%29#Examples_of_syntactic_unification_of_first-order_terms
- */
-class UnifyTests {
-    @Test
-    fun identical_atoms_unify() {
-        val atom1 = Atom("a")
-        val atom2 = Atom("a")
-
-        val result = unify(atom1, atom2)
-
-        assertTrue(result.isSuccess, "Identical atoms should unify")
-        assertEquals(0, result.getOrNull()!!.size, "No substitutions should be made")
-    }
-
-    @Test
-    fun different_atoms_do_not_unify() {
-        val atom1 = Atom("a")
-        val atom2 = Atom("b")
-
-        val result = unify(atom1, atom2)
-
-        assertFalse(result.isSuccess, "Different atoms should not unify")
-    }
-
-    /**
-     * ?- X = X.
-     * true.
-     */
-    @Test
-    fun identical_variables_unify() {
-        val variable1 = Variable("X")
-        val variable2 = Variable("X")
-
-        val result = unify(variable1, variable2)
-
-        assertTrue(result.isSuccess, "Identical variables should unify")
-        assertEquals(0, result.getOrNull()!!.size, "No substitutions should be made")
-    }
-
-    @Test
-    fun variable_unifies_with_atom() {
-        val variable = Variable("X")
-        val atom = Atom("a")
-
-        val result = unify(atom, variable)
-
-        assertTrue(result.isSuccess, "Variable should unify with atom")
-        assertEquals(1, result.getOrNull()!!.size, "There should be one substitution")
-        assertEquals(atom, result.getOrNull()!![variable], "Variable should be substituted with atom")
-    }
-
-    @Test
-    fun variables_alias_when_unified() {
-        val variable1 = Variable("X")
-        val variable2 = Variable("Y")
-
-        val result = unify(variable1, variable2)
-
-        assertTrue(result.isSuccess)
-        assertEquals(1, result.getOrNull()!!.size)
-        assertEquals(variable2, result.getOrNull()!![variable1], "Variable 1 should alias to variable 2")
-    }
-
-    @Test
-    fun identical_compound_terms_unify() {
-        val structure1 = Structure(Atom("f"), listOf(Atom("a"), Atom("b")))
-        val structure2 = Structure(Atom("f"), listOf(Atom("a"), Atom("b")))
-
-        val result = unify(structure1, structure2)
-
-        assertTrue(result.isSuccess, "Identical compound terms should unify")
-        assertEquals(0, result.getOrNull()!!.size, "No substitutions should be made")
-    }
-
-    @Test
-    fun compound_terms_with_different_arguments_do_not_unify() {
-        val structure1 = Structure(Atom("f"), listOf(Atom("a"), Atom("b")))
-        val structure2 = Structure(Atom("f"), listOf(Atom("a"), Atom("c")))
-
-        val result = unify(structure1, structure2)
-
-        assertFalse(result.isSuccess, "Different compound terms should not unify")
-    }
-
-    @Test
-    fun compound_terms_with_different_functors_do_not_unify() {
-        val structure1 = Structure(Atom("f"), listOf(Atom("a"), Atom("b")))
-        val structure2 = Structure(Atom("g"), listOf(Atom("a"), Atom("b")))
-
-        val result = unify(structure1, structure2)
-
-        assertFalse(result.isSuccess, "Compound terms with different functors should not unify")
-    }
-
-    /**
-     * ?- X = f(a, b).
-     * X = f(a, b).
-     */
-    @Test
-    fun variable_unifies_with_compound_term() {
-        val variable = Variable("X")
-        val structure = Structure(Atom("f"), listOf(Atom("a"), Atom("b")))
-
-        val result = unify(variable, structure)
-
-        assertTrue(result.isSuccess, "Variable should unify with compound term")
-
-        val subs = result.getOrNull()!!
-
-        assertEquals(1, subs.size, "There should be one substitution")
-        assertTrue(subs.containsKey(variable), "Variable should be in the substitution map")
-        assertTrue(
-            equivalent(Structure(Atom("f"), listOf(Atom("a"), Atom("b"))), subs[variable]!!, subs),
-            "Variable should be substituted with compound term"
-        )
-    }
-
-    @Test
-    fun compound_term_with_variable_unifies_with_part() {
-        val variable = Variable("X")
-        val structure1 = Structure(Atom("f"), listOf(Atom("a"), variable))
-        val structure2 = Structure(Atom("f"), listOf(Atom("a"), Atom("b")))
-
-        val result = unify(structure1, structure2)
-
-        assertTrue(result.isSuccess, "Compound term with variable should unify with part")
-
-        val subs = result.getOrNull()!!
-
-        assertEquals(1, subs.size, "There should be one substitution")
-        assertTrue(subs.containsKey(variable), "Variable should be in the substitution map")
-        val equivalence = equivalent(Atom("b"), subs[variable]!!, subs)
-        assertTrue(equivalence, "Variable should be substituted with atom")
-    }
-
-    @Test
-    fun compound_terms_with_variable_arguments_lists_alias_variables() {
-        val variable1 = Variable("X")
-        val variable2 = Variable("Y")
-
-        val structure1 = Structure(Atom("f"), listOf(variable1))
-        val structure2 = Structure(Atom("f"), listOf(variable2))
-
-        val result = unify(structure1, structure2)
-
-        assertTrue(result.isSuccess, "Compound terms with variable arguments should unify")
-
-        val subs = result.getOrNull()!!
-
-        assertEquals(1, subs.size, "There should be one substitution")
-        assertTrue(subs.containsKey(variable1), "Variable 1 should be in the substitution map")
-        assertEquals(variable2, subs[variable1], "Variable 1 should alias to variable 2")
-    }
-
-    /**
-     * f(X) = f(Y, Z)
-     */
-    @Test
-    fun compound_terms_with_different_arity_do_not_unify() {
-        val structure1 = Structure(Atom("f"), listOf(Variable("X")))
-        val structure2 = Structure(Atom("f"), listOf(Variable("Y"), Variable("Z")))
-
-        val result = unify(structure1, structure2)
-
-        assertFalse(result.isSuccess, "Compound terms with different arity should not unify")
-    }
-
-    /**
-     * ?- f(g(X)) = f(Y).
-     * Y = g(X).
-     */
-    @Test
-    fun nested_compound_terms_with_variables_unify() {
-        val variable2 = Variable("Y")
-
-        val structure1 = Structure(Atom("f"), listOf(Structure(Atom("g"), listOf(Variable("X")))))
-        val structure2 = Structure(Atom("f"), listOf(variable2))
-
-        val result = unify(structure1, structure2)
-
-        assertTrue(result.isSuccess, "Nested compound terms with variables should unify")
-
-        val subs = result.getOrNull()!!
-
-        assertEquals(1, subs.size, "There should be one substitution")
-        assertTrue(subs.containsKey(variable2), "Variable 2 should be in the substitution map")
-        assertTrue(
-            equivalent(Structure(Atom("g"), listOf(Variable("X"))), subs[variable2]!!, subs),
-            "Variable should be substituted with compound term"
-        )
-    }
-
-    /**
-     * ?- f(g(X), X) = f(Y, a).
-     * X = a,
-     * Y = g(a).
-     */
-    @Test
-    fun compound_terms_with_more_variables() {
-        val variable1 = Variable("X")
-        val variable2 = Variable("Y")
-
-        val structure1 = Structure(Atom("f"), listOf(Structure(Atom("g"), listOf(variable1)), variable1))
-        val structure2 = Structure(Atom("f"), listOf(variable2, Atom("a")))
-
-        val result = unify(structure1, structure2)
-
-        assertTrue(result.isSuccess, "Compound terms with more variables should unify")
-
-        val subs = result.getOrNull()!!
-
-        assertEquals(2, subs.size, "There should be two substitutions")
-        assertTrue(subs.containsKey(variable1), "Variable 1 should be in the substitution map")
-        assertTrue(
-            equivalent(Atom("a"), subs[variable1]!!, subs),
-            "Variable 1 should be substituted with atom"
-        )
-        assertTrue(subs.containsKey(variable2), "Variable 2 should be in the substitution map")
-        assertTrue(
-            equivalent(Structure(Atom("g"), listOf(Atom("a"))), subs[variable2]!!, subs),
-            "Variable 2 should be substituted with compound term"
-
-        )
-    }
-
-    /**
-     * ?- X = f(X).
-     * X = f(f(X)).
-     */
-    @Test
-    @Disabled("If the occurs check is applied, this should fail")
-    fun recursive_unification() {
-        val variable1 = Variable("X")
-        val structure2 = Structure(Atom("f"), listOf(Variable("X")))
-
-        val result = unifyLazy(variable1, structure2, emptyMap()).toList()
-
-        assertEquals(1, result.size, "There should be one result")
-        assertTrue(result[0].isSuccess, "Recursive unification should succeed")
-
-        val subs = result[0].getOrNull()!!
-
-        assertEquals(1, subs.size, "There should be one substitution")
-        assertTrue(subs.containsKey(variable1), "Variable should be in the substitution map")
-        assertEquals(structure2, subs[variable1], "Variable should be substituted with compound term")
-    }
-
-    /**
-     * ?- X = bar, Y = bar, X = Y.
-     * X = Y, Y = bar.
-     */
-    @Test
-    fun multiple_unification() {
-        val variable1 = Variable("X")
-        val variable2 = Variable("Y")
-        val atom = Atom("bar")
-
-        val map: Substitutions = mapOf(
-            variable1 to atom,
-            variable2 to atom
-        )
-        val result = unifyLazy(variable1, variable2, map).toList()
-
-        assertEquals(1, result.size, "There should be one substitution")
-        assertTrue(result[0].isSuccess, "Multiple unification should succeed")
-        assertEquals(0, result[0].getOrNull()!!.size, "No (additional) substitutions should be made")
-    }
-
-    /**
-     * ?- a = a().
-     * false.
-     */
-    @Test
-    fun atom_with_different_arity() {
-        val atom1 = Atom("a")
-        val structure2 = Structure(Atom("a"), emptyList())
-
-        val result = unify(atom1, structure2)
-
-        assertFalse(result.isSuccess, "Atom with different arity should not unify")
-    }
-
-    @Test
-    fun identical_integers_unify() {
-        val int1 = Integer(1)
-        val int2 = Integer(1)
-
-        val result = unify(int1, int2)
-
-        assertTrue(result.isSuccess, "Identical integers should unify")
-        assertEquals(0, result.getOrNull()!!.size, "No substitutions should be made")
-    }
-
-    @Test
-    fun different_integers_do_not_unify() {
-        val int1 = Integer(1)
-        val int2 = Integer(2)
-
-        val result = unify(int1, int2)
-
-        assertFalse(result.isSuccess, "Different integers should not unify")
-    }
-
-
-    @Test
-    fun `1 + 2 does not unify with 3`() {
-        val expr1 = Add(Integer(1), Integer(2))
-        val expr2 = Integer(3)
-
-        val result = unify(expr1, expr2)
-
-        assertFalse(result.isSuccess, "1 + 2 should not unify with 3")
-    }
-}