diff --git a/src/interpreter/Preprocessor.kt b/src/interpreter/Preprocessor.kt
index 4b36614..5bd2ed7 100644
--- a/src/interpreter/Preprocessor.kt
+++ b/src/interpreter/Preprocessor.kt
@@ -67,51 +67,51 @@ open class Preprocessor {
                 when {
                     // TODO Remove hardcoding by storing the functors as constants in operators?
 
-                    term.functor == ":-/2" -> Rule( args[0] as Head, args[1] as Body )
+                    term.functor == FunctorInfo.of(":-/2") -> Rule( args[0] as Head, args[1] as Body )
 
                     // Logic
-                    term.functor == "=/2" -> Unify(args[0], args[1])
-                    term.functor == "\\=/2" -> NotUnify(args[0], args[1])
-                    term.functor == ",/2" -> Conjunction(args[0] as LogicOperand, args[1] as LogicOperand)
-                    term.functor == ";/2" -> Disjunction(args[0] as LogicOperand, args[1] as LogicOperand)
-                    term.functor == "\\+/1" -> Not(args[0] as Goal)
-                    term.functor == "\\==/2" -> NotEquivalent(args[0], args[1])
-                    term.functor == "==/2" -> Equivalent(args[0], args[1])
+                    term.functor == FunctorInfo.of("=/2") -> Unify(args[0], args[1])
+                    term.functor == FunctorInfo.of("\\=/2") -> NotUnify(args[0], args[1])
+                    term.functor == FunctorInfo.of(",/2") -> Conjunction(args[0] as LogicOperand, args[1] as LogicOperand)
+                    term.functor == FunctorInfo.of(";/2") -> Disjunction(args[0] as LogicOperand, args[1] as LogicOperand)
+                    term.functor == FunctorInfo.of("\\+/1") -> Not(args[0] as Goal)
+                    term.functor == FunctorInfo.of("\\==/2") -> NotEquivalent(args[0], args[1])
+                    term.functor == FunctorInfo.of("==/2") -> Equivalent(args[0], args[1])
 
-                    term.functor == "=\\=/2" && args.all { it is Expression } -> EvaluatesToDifferent(args[0] as Expression, args[1] as Expression)
-                    term.functor == "=:=/2" && args.all { it is Expression } -> EvaluatesTo(args[0] as Expression, args[1] as Expression)
-                    term.functor == "is/2" && args.all { it is Expression } -> Is(args[0] as Expression, args[1] as Expression)
+                    term.functor == FunctorInfo.of("=\\=/2") && args.all { it is Expression } -> EvaluatesToDifferent(args[0] as Expression, args[1] as Expression)
+                    term.functor == FunctorInfo.of("=:=/2") && args.all { it is Expression } -> EvaluatesTo(args[0] as Expression, args[1] as Expression)
+                    term.functor == FunctorInfo.of("is/2") && args.all { it is Expression } -> Is(args[0] as Expression, args[1] as Expression)
 
                     // Arithmetic
 
-                    term.functor == "-/1" && args.all { it is Expression } -> Negate(args[0] as Expression)
-                    term.functor == "-/2" && args.all { it is Expression } -> Subtract(args[0] as Expression, args[1] as Expression)
-                    term.functor == "+/1" && args.all { it is Expression } -> Positive(args[0] as Expression)
-                    term.functor == "+/2" && args.all { it is Expression } -> Add(args[0] as Expression, args[1] as Expression)
-                    term.functor == "*/2" && args.all { it is Expression } -> Multiply(args[0] as Expression, args[1] as Expression)
-                    term.functor == "//2" && args.all { it is Expression } -> Divide(args[0] as Expression, args[1] as Expression)
-                    term.functor == "between/3" && args.all { it is Expression } -> Between(args[0] as Expression, args[1] as Expression, args[2] as Expression)
-                    term.functor == "succ/2" && args.all { it is Expression } -> Successor(args[0] as Expression, args[1] as Expression)
+                    term.functor == FunctorInfo.of("-/1") && args.all { it is Expression } -> Negate(args[0] as Expression)
+                    term.functor == FunctorInfo.of("-/2") && args.all { it is Expression } -> Subtract(args[0] as Expression, args[1] as Expression)
+                    term.functor == FunctorInfo.of("+/1") && args.all { it is Expression } -> Positive(args[0] as Expression)
+                    term.functor == FunctorInfo.of("+/2") && args.all { it is Expression } -> Add(args[0] as Expression, args[1] as Expression)
+                    term.functor == FunctorInfo.of("*/2") && args.all { it is Expression } -> Multiply(args[0] as Expression, args[1] as Expression)
+                    term.functor == FunctorInfo.of("//2") && args.all { it is Expression } -> Divide(args[0] as Expression, args[1] as Expression)
+                    term.functor == FunctorInfo.of("between/3") && args.all { it is Expression } -> Between(args[0] as Expression, args[1] as Expression, args[2] as Expression)
+                    term.functor == FunctorInfo.of("succ/2") && args.all { it is Expression } -> Successor(args[0] as Expression, args[1] as Expression)
 
                     // Database
-                    term.functor == "dynamic/1" -> Dynamic((args[0] as Atom).name)
-                    term.functor == "retract/1" -> Retract(args[0])
-                    term.functor == "retractall/1" -> RetractAll(args[0])
-                    term.functor == "assert/1" -> {
+                    term.functor == FunctorInfo.of("dynamic/1") -> Dynamic(FunctorInfo.of((args[0] as Atom).name))
+                    term.functor == FunctorInfo.of("retract/1") -> Retract(args[0])
+                    term.functor == FunctorInfo.of("retractall/1") -> RetractAll(args[0])
+                    term.functor == FunctorInfo.of("assert/1") -> {
                         if (args[0] is Rule) {
                             Assert(args[0] as Rule)
                         } else {
                             Assert(Fact(args[0] as Head))
                         }
                     }
-                    term.functor == "asserta/1" -> {
+                    term.functor == FunctorInfo.of("asserta/1") -> {
                         if (args[0] is Rule) {
                             AssertA(args[0] as Rule)
                         } else {
                             AssertA(Fact(args[0] as Head))
                         }
                     }
-                    term.functor == "assertz/1" -> {
+                    term.functor == FunctorInfo.of("assertz/1") -> {
                         if (args[0] is Rule) {
                             AssertZ(args[0] as Rule)
                         } else {
@@ -120,10 +120,10 @@ open class Preprocessor {
                     }
 
                     // Other
-                    term.functor == "write/1" -> Write(args[0])
-                    term.functor == "read/1" -> Read(args[0])
-                    term.functor == "initialization/1" -> Initialization(args[0] as Goal)
-                    term.functor == "forall/2" -> ForAll(args[0] as LogicOperand, args[1] as Goal)
+                    term.functor == FunctorInfo.of("write/1") -> Write(args[0])
+                    term.functor == FunctorInfo.of("read/1") -> Read(args[0])
+                    term.functor == FunctorInfo.of("initialization/1") -> Initialization(args[0] as Goal)
+                    term.functor == FunctorInfo.of("forall/2") -> ForAll(args[0] as LogicOperand, args[1] as Goal)
 
                     else -> {
                         term.arguments = args
diff --git a/src/prolog/ast/Database.kt b/src/prolog/ast/Database.kt
index be2da7a..28589ef 100644
--- a/src/prolog/ast/Database.kt
+++ b/src/prolog/ast/Database.kt
@@ -6,14 +6,14 @@ import prolog.Substitutions
 import prolog.ast.logic.Clause
 import prolog.ast.logic.Predicate
 import prolog.ast.logic.Resolvent
-import prolog.ast.terms.Functor
+import prolog.ast.terms.FunctorInfo
 import prolog.ast.terms.Goal
 
 /**
  * Prolog Program or Database
  */
 open class Database(val sourceFile: String) {
-    var predicates: Map<Functor, Predicate> = emptyMap()
+    var predicates: Map<FunctorInfo, Predicate> = emptyMap()
 
     /**
      * Initializes the database by running the initialization clauses of that database.
@@ -23,14 +23,14 @@ open class Database(val sourceFile: String) {
 
         if (sourceFile !== "") {
             Logger.debug("Moving clauses from $sourceFile to main database")
-            predicates.filter { it.key != "/_" }
+            predicates.filter { it.key != FunctorInfo.of("/_") }
                 .forEach { (_, predicate) -> db.load(predicate, force = true) }
         }
 
         Logger.info("Initializing database from $sourceFile")
-        if (predicates.contains("/_")) {
+        if (predicates.contains(FunctorInfo.of("/_"))) {
             Logger.debug("Loading clauses from /_ predicate")
-            predicates["/_"]?.clauses?.forEach {
+            predicates[FunctorInfo.of("/_")]?.clauses?.forEach {
                 Logger.debug("Loading clause $it")
                 val goal = it.body as Goal
                 goal.satisfy(emptyMap()).toList()
diff --git a/src/prolog/ast/logic/Clause.kt b/src/prolog/ast/logic/Clause.kt
index 1080a9f..be3cc62 100644
--- a/src/prolog/ast/logic/Clause.kt
+++ b/src/prolog/ast/logic/Clause.kt
@@ -1,8 +1,8 @@
 package prolog.ast.logic
 
 import prolog.Answers
-import prolog.ast.Database.Program
 import prolog.Substitutions
+import prolog.ast.Database.Program
 import prolog.ast.terms.*
 import prolog.builtins.True
 import prolog.flags.AppliedCut
@@ -20,7 +20,7 @@ import prolog.logic.unifyLazy
  * @see [Predicate]
  */
 abstract class Clause(var head: Head, var body: Body) : Term, Resolvent {
-    val functor: Functor = head.functor
+    val functor: FunctorInfo = head.functor
 
     override fun solve(goal: Goal, subs: Substitutions): Answers = sequence {
         // If the clause is a rule, unify the goal with the head and then try to prove the body.
@@ -45,7 +45,14 @@ abstract class Clause(var head: Head, var body: Body) : Term, Resolvent {
                         onSuccess = { bodySubs ->
                             // If the body can be proven, yield the (combined) substitutions
                             val goalToHeadResult = goalToHeadSubs.mapValues { applySubstitution(it.value, bodySubs) }
-                            val headResult = headToGoalSubs.filterKeys { key -> goalToHeadSubs.any { occurs(key as Variable, it.value) } }
+                            val headResult = headToGoalSubs.filterKeys { key ->
+                                goalToHeadSubs.any {
+                                    occurs(
+                                        key as Variable,
+                                        it.value
+                                    )
+                                }
+                            }
                             yield(Result.success(goalToHeadResult + headResult))
                         },
                         onFailure = { error ->
diff --git a/src/prolog/ast/logic/Predicate.kt b/src/prolog/ast/logic/Predicate.kt
index 25fb737..e935468 100644
--- a/src/prolog/ast/logic/Predicate.kt
+++ b/src/prolog/ast/logic/Predicate.kt
@@ -2,25 +2,25 @@ package prolog.ast.logic
 
 import prolog.Answers
 import prolog.Substitutions
-import prolog.ast.terms.Functor
+import prolog.ast.terms.FunctorInfo
 import prolog.ast.terms.Goal
 import prolog.flags.AppliedCut
 
 /**
- * Collection of [Clause]s with the same [Functor].
+ * Collection of [Clause]s with the same [FunctorInfo].
  *
  * If a goal is proved, the system looks for a predicate with the same functor, then uses indexing
  * to select candidate clauses and then tries these clauses one-by-one.
  */
 class Predicate : Resolvent {
-    val functor: Functor
+    val functor: FunctorInfo
     val clauses: MutableList<Clause>
     var dynamic = false
 
     /**
      * Creates a predicate with the given functor and an empty list of clauses.
      */
-    constructor(functor: Functor, dynamic: Boolean = false) {
+    constructor(functor: FunctorInfo, dynamic: Boolean = false) {
         this.functor = functor
         this.clauses = mutableListOf()
         this.dynamic = dynamic
diff --git a/src/prolog/ast/terms/Atom.kt b/src/prolog/ast/terms/Atom.kt
index 5bc2004..2dae78d 100644
--- a/src/prolog/ast/terms/Atom.kt
+++ b/src/prolog/ast/terms/Atom.kt
@@ -4,9 +4,10 @@ import prolog.Answers
 import prolog.Substitutions
 import prolog.ast.logic.Resolvent
 import prolog.logic.unifyLazy
+import prolog.ast.arithmetic.Integer
 
 open class Atom(val name: String) : Goal(), Head, Body, Resolvent {
-    override val functor: Functor = "$name/_"
+    override val functor: FunctorInfo = FunctorInfo(this, Integer(0))
 
     override fun solve(goal: Goal, subs: Substitutions): Answers = unifyLazy(goal, this, subs)
 
diff --git a/src/prolog/ast/terms/FunctorInfo.kt b/src/prolog/ast/terms/FunctorInfo.kt
new file mode 100644
index 0000000..eb61ef7
--- /dev/null
+++ b/src/prolog/ast/terms/FunctorInfo.kt
@@ -0,0 +1,29 @@
+package prolog.ast.terms
+
+import prolog.Substitutions
+import prolog.ast.arithmetic.Integer
+
+data class FunctorInfo(val name: Atom, val arity: Integer) : Term {
+    companion object {
+        fun of(functor: String): FunctorInfo {
+            // Split the functor string into name and arity, by splitting the last "/"
+            val lastSlash = functor.lastIndexOf('/')
+            val name = Atom(functor.substring(0, lastSlash))
+            val arity = Integer(functor.substring(lastSlash + 1).toIntOrNull() ?: 0)
+            return FunctorInfo(name, arity)
+        }
+    }
+
+    override fun toString(): String = "${name.name}/$arity"
+    override fun applySubstitution(subs: Substitutions) : FunctorInfo = this
+
+    override fun equals(other: Any?): Boolean {
+        if (this === other) return true
+        if (other == null) return false
+        if (other !is FunctorInfo && other !is String) return false
+        if (other is FunctorInfo) {
+            return this.name.toString() == other.name.toString() && this.arity == other.arity
+        }
+        return this.toString() == other
+    }
+}
diff --git a/src/prolog/ast/terms/Goal.kt b/src/prolog/ast/terms/Goal.kt
index 95f9016..0ba62f0 100644
--- a/src/prolog/ast/terms/Goal.kt
+++ b/src/prolog/ast/terms/Goal.kt
@@ -1,8 +1,8 @@
 package prolog.ast.terms
 
 import prolog.Answers
-import prolog.ast.Database.Program
 import prolog.Substitutions
+import prolog.ast.Database.Program
 import prolog.ast.logic.LogicOperand
 
 /**
@@ -13,7 +13,7 @@ import prolog.ast.logic.LogicOperand
  * or it fails if Prolog fails to prove it.
  */
 abstract class Goal : LogicOperand(), Term {
-    abstract val functor: Functor
+    abstract val functor: FunctorInfo
 
     override fun satisfy(subs: Substitutions): Answers = Program.solve(this, subs)
 }
\ No newline at end of file
diff --git a/src/prolog/ast/terms/Head.kt b/src/prolog/ast/terms/Head.kt
index 71b4e8a..fa5bd8d 100644
--- a/src/prolog/ast/terms/Head.kt
+++ b/src/prolog/ast/terms/Head.kt
@@ -4,7 +4,5 @@ package prolog.ast.terms
  * Part of a [Clause][prolog.ast.logic.Clause] before the neck operator.
  */
 interface Head : Term {
-    val functor: Functor
+    val functor: FunctorInfo
 }
-
-typealias Functor = String
diff --git a/src/prolog/ast/terms/Structure.kt b/src/prolog/ast/terms/Structure.kt
index afed37c..4aadf01 100644
--- a/src/prolog/ast/terms/Structure.kt
+++ b/src/prolog/ast/terms/Structure.kt
@@ -5,13 +5,14 @@ import prolog.Substitutions
 import prolog.ast.logic.Resolvent
 import prolog.logic.applySubstitution
 import prolog.logic.unifyLazy
+import prolog.ast.arithmetic.Integer
 
 typealias Argument = Term
 
 typealias CompoundTerm = Structure
 
 open class Structure(val name: Atom, var arguments: List<Argument>) : Goal(), Head, Body, Resolvent {
-    override val functor: Functor = "${name.name}/${arguments.size}"
+    override val functor: FunctorInfo = FunctorInfo(name, Integer(arguments.size))
 
     override fun solve(goal: Goal, subs: Substitutions): Answers {
         return unifyLazy(goal, this, subs)
diff --git a/src/prolog/builtins/analysingAndConstructionOperators.kt b/src/prolog/builtins/analysingAndConstructionOperators.kt
new file mode 100644
index 0000000..ba589ef
--- /dev/null
+++ b/src/prolog/builtins/analysingAndConstructionOperators.kt
@@ -0,0 +1,65 @@
+package prolog.builtins
+
+import prolog.Answers
+import prolog.Substitutions
+import prolog.ast.arithmetic.Integer
+import prolog.ast.terms.AnonymousVariable
+import prolog.ast.terms.Atom
+import prolog.ast.terms.Head
+import prolog.ast.terms.Structure
+import prolog.ast.terms.Term
+import prolog.ast.terms.Variable
+import prolog.logic.applySubstitution
+import prolog.logic.atomic
+import prolog.logic.nonvariable
+import prolog.logic.unifyLazy
+import prolog.logic.variable
+
+class Functor(private val term: Term, private val functorName: Term, private val functorArity: Term) :
+    Structure(Atom("functor"), listOf(term, functorName, functorArity)) {
+    override fun satisfy(subs: Substitutions): Answers {
+        if (nonvariable(term, subs)) {
+            val t = applySubstitution(term, subs) as Head
+
+            return Conjunction(
+                Unify(t.functor.arity, functorArity),
+                Unify(t.functor.name, functorName)
+            ).satisfy(subs)
+        }
+
+        if (variable(term, subs)) {
+            require(atomic(functorName, subs) && atomic(functorArity, subs)) {
+                "Arguments are not sufficiently instantiated"
+            }
+
+            val name = applySubstitution(functorName, subs) as Atom
+            val arity = applySubstitution(functorArity, subs) as Integer
+
+            val result = if (arity.value == 0) {
+                functorName
+            } else {
+                val argList = mutableListOf<Term>()
+                for (i in 1..arity.value) {
+                    argList.add(AnonymousVariable.create())
+                }
+                Structure(name, argList)
+            }
+
+            if (nonvariable(term, subs)) {
+                return unifyLazy(term, result, subs)
+            }
+
+            return sequenceOf(Result.success(mapOf(term to result)))
+        }
+
+        throw IllegalStateException()
+    }
+
+    override fun applySubstitution(subs: Substitutions): Functor = Functor(
+        term.applySubstitution(subs),
+        functorName.applySubstitution(subs),
+        functorArity.applySubstitution(subs)
+    )
+
+    override fun toString(): String = "functor($term, $functorName, $functorArity)"
+}
diff --git a/src/prolog/builtins/databaseOperators.kt b/src/prolog/builtins/databaseOperators.kt
index ea36376..05f228c 100644
--- a/src/prolog/builtins/databaseOperators.kt
+++ b/src/prolog/builtins/databaseOperators.kt
@@ -3,26 +3,21 @@ package prolog.builtins
 import io.Logger
 import prolog.Answers
 import prolog.Substitutions
-import prolog.ast.logic.Clause
-import prolog.ast.terms.Atom
-import prolog.ast.terms.Structure
-import prolog.ast.logic.Predicate
-import prolog.ast.Database.Program
-import prolog.ast.terms.Functor
-import prolog.ast.terms.Term
-import prolog.ast.logic.Fact
 import prolog.ast.Database
-import prolog.ast.terms.Body
-import prolog.ast.terms.Goal
-import prolog.ast.terms.Operator
+import prolog.ast.Database.Program
+import prolog.ast.arithmetic.Integer
+import prolog.ast.logic.Clause
+import prolog.ast.logic.Fact
+import prolog.ast.logic.Predicate
+import prolog.ast.terms.*
 import prolog.logic.applySubstitution
 import prolog.logic.unifyLazy
 
 /**
- * (Make) the [Predicate] with the corresponding [Functor] dynamic.
+ * (Make) the [Predicate] with the corresponding [FunctorInfo] dynamic.
  */
-class Dynamic(private val dynamicFunctor: Functor): Goal(), Body {
-    override val functor: Functor = "dynamic/1"
+class Dynamic(private val dynamicFunctor: FunctorInfo) : Goal(), Body {
+    override val functor = FunctorInfo(Atom("dynamic"), Integer(1))
 
     override fun satisfy(subs: Substitutions): Answers {
         val predicate = Program.db.predicates[dynamicFunctor]
@@ -48,7 +43,7 @@ class Dynamic(private val dynamicFunctor: Functor): Goal(), Body {
 }
 
 class Assert(clause: Clause) : AssertZ(clause) {
-    override val functor: Functor = "assert/1"
+    override val functor = FunctorInfo(Atom("assert"), Integer(1))
 }
 
 /**
diff --git a/tests/interpreter/PreprocessorTests.kt b/tests/interpreter/PreprocessorTests.kt
index bf54869..4d9ed7f 100644
--- a/tests/interpreter/PreprocessorTests.kt
+++ b/tests/interpreter/PreprocessorTests.kt
@@ -612,7 +612,7 @@ class PreprocessorTests {
                     Atom("declaration/1")
                 )
             )
-            val expected = Dynamic("declaration/1")
+            val expected = Dynamic(FunctorInfo.of("declaration/1"))
 
             val result = preprocessor.preprocess(input)
 
diff --git a/tests/parser/grammars/LogicGrammarTests.kt b/tests/parser/grammars/LogicGrammarTests.kt
index 57a1f99..7ee3fcc 100644
--- a/tests/parser/grammars/LogicGrammarTests.kt
+++ b/tests/parser/grammars/LogicGrammarTests.kt
@@ -15,6 +15,7 @@ import prolog.ast.terms.CompoundTerm
 import prolog.ast.terms.Structure
 import prolog.ast.terms.Variable
 import prolog.builtins.Conjunction
+import prolog.ast.terms.FunctorInfo
 
 class LogicGrammarTests {
     private lateinit var parser: Grammar<List<Clause>>
@@ -94,13 +95,13 @@ class LogicGrammarTests {
 
         assertTrue(rule.head is Structure, "Expected head to be a structure")
         val head = rule.head as Structure
-        assertEquals("parent/2", head.functor, "Expected functor 'parent/2'")
+        assertEquals(FunctorInfo.of("parent/2"), head.functor, "Expected functor 'parent/2'")
         assertEquals(Variable("X"), head.arguments[0], "Expected first argument 'X'")
         assertEquals(Variable("Y"), head.arguments[1], "Expected second argument 'Y'")
 
         assertTrue(rule.body is Structure, "Expected body to be a structure")
         val body = rule.body as Structure
-        assertEquals("father/2", body.functor, "Expected functor 'father/2'")
+        assertEquals(FunctorInfo.of("father/2"), body.functor, "Expected functor 'father/2'")
         assertEquals(Variable("X"), body.arguments[0], "Expected first argument 'X'")
         assertEquals(Variable("Y"), body.arguments[1], "Expected second argument 'Y'")
     }
@@ -125,12 +126,12 @@ class LogicGrammarTests {
 
         assertEquals(1, result.size, "Expected 1 rule")
         val rule = result[0] as Rule
-        assertEquals("guest/2", rule.head.functor, "Expected functor 'guest/2'")
-        assertEquals(",/2", (rule.body as CompoundTerm).functor, "Expected functor ',/2'")
+        assertEquals(FunctorInfo.of("guest/2"), rule.head.functor, "Expected functor 'guest/2'")
+        assertEquals(FunctorInfo.of(",/2"), (rule.body as CompoundTerm).functor, "Expected functor ',/2'")
         val l1 = (rule.body as CompoundTerm).arguments[0] as CompoundTerm
-        assertEquals(",/2", l1.functor, "Expected functor ',/2'")
+        assertEquals(FunctorInfo.of(",/2"), l1.functor, "Expected functor ',/2'")
         val l2 = l1.arguments[0] as CompoundTerm
-        assertEquals("invited/2", l2.functor, "Expected functor 'invited/2'")
+        assertEquals(FunctorInfo.of("invited/2"), l2.functor, "Expected functor 'invited/2'")
     }
 
     @Test
@@ -157,6 +158,6 @@ class LogicGrammarTests {
         assertEquals(1, result.size, "Expected 1 rule")
         assertTrue(result[0] is Rule, "Expected a rule")
         val rule = result[0] as Rule
-        assertEquals("/_", rule.head.functor, "Expected a constraint")
+        assertEquals(FunctorInfo.of("/0"), rule.head.functor, "Expected a constraint")
     }
 }
\ No newline at end of file
diff --git a/tests/prolog/builtins/AnalysingAndConstructionOperatorsTests.kt b/tests/prolog/builtins/AnalysingAndConstructionOperatorsTests.kt
new file mode 100644
index 0000000..6e91bfc
--- /dev/null
+++ b/tests/prolog/builtins/AnalysingAndConstructionOperatorsTests.kt
@@ -0,0 +1,91 @@
+package prolog.builtins
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertInstanceOf
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+import prolog.ast.arithmetic.Integer
+import prolog.ast.terms.AnonymousVariable
+import prolog.ast.terms.Atom
+import prolog.ast.terms.Structure
+import prolog.ast.terms.Variable
+
+class AnalysingAndConstructionOperatorsTests {
+    @Test
+    fun `functor(foo, foo, 0)`() {
+        val functor = Functor(Atom("foo"), Atom("foo"), Integer(0))
+
+        val result = functor.satisfy(emptyMap()).toList()
+
+        assertEquals(1, result.size, "Expected 1 result")
+        assertTrue(result[0].isSuccess, "Expected success")
+        assertTrue(result[0].getOrNull()!!.isEmpty(), "Expected empty substitutions")
+    }
+
+    @Test
+    fun `functor(foo(X), foo, Y)`() {
+        val functor = Functor(
+            Structure(Atom("foo"), listOf(Variable("X"))),
+            Atom("foo"),
+            Variable("Y")
+        )
+
+        val result = functor.satisfy(emptyMap()).toList()
+
+        assertEquals(1, result.size, "Expected 1 result")
+        assertTrue(result[0].isSuccess, "Expected success")
+        val subs = result[0].getOrNull()!!
+        assertEquals(1, subs.size, "Expected 1 substitution")
+        assertEquals(Integer(1), subs[Variable("Y")])
+    }
+
+    @Test
+    fun `functor(foo, X, Y)`() {
+        val atom = Atom("foo")
+        val functor = Functor(atom, Variable("X"), Variable("Y"))
+
+        val result = functor.satisfy(emptyMap()).toList()
+
+        assertEquals(1, result.size, "Expected 1 result")
+        assertTrue(result[0].isSuccess, "Expected success")
+        val subs = result[0].getOrNull()!!
+        assertEquals(2, subs.size, "Expected 2 substitutions")
+        assertEquals(atom.functor.name, subs[Variable("X")])
+        assertEquals(atom.functor.arity, subs[Variable("Y")])
+    }
+
+    @Test
+    fun `functor(X, foo, 1)`() {
+        val functor = Functor(Variable("X"), Atom("foo"), Integer(1))
+
+        val result = functor.satisfy(emptyMap()).toList()
+
+        assertEquals(1, result.size, "Expected 1 result")
+        assertTrue(result[0].isSuccess, "Expected success")
+        val subs = result[0].getOrNull()!!
+        assertEquals(1, subs.size, "Expected 1 substitution")
+        assertInstanceOf(Structure::class.java, subs[Variable("X")])
+        val structure = subs[Variable("X")] as Structure
+        assertEquals(Atom("foo"), structure.name)
+        assertEquals(1, structure.arguments.size)
+        assertInstanceOf(AnonymousVariable::class.java, structure.arguments[0])
+    }
+
+    @Test
+    fun `functor(foo(a), foo, 0)`() {
+        val functor = Functor(Structure(Atom("foo"), listOf(Atom("a"))), Atom("foo"), Integer(0))
+
+        val result = functor.satisfy(emptyMap()).toList()
+
+        assertTrue(result.isEmpty(), "Expected no results")
+    }
+
+    @Test
+    fun `functor(foo(X), foo, 0)`() {
+        val functor = Functor(Structure(Atom("foo"), listOf(Variable("X"))), Atom("foo"), Integer(0))
+
+        val result = functor.satisfy(emptyMap()).toList()
+
+        assertTrue(result.isEmpty(), "Expected no results")
+    }
+}
\ No newline at end of file
diff --git a/tests/prolog/builtins/DatabaseOperatorsTests.kt b/tests/prolog/builtins/DatabaseOperatorsTests.kt
index 73cfef0..cbb5bb6 100644
--- a/tests/prolog/builtins/DatabaseOperatorsTests.kt
+++ b/tests/prolog/builtins/DatabaseOperatorsTests.kt
@@ -8,14 +8,13 @@ import org.junit.jupiter.api.Nested
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.params.ParameterizedTest
 import org.junit.jupiter.params.provider.ValueSource
-import prolog.ast.Database
 import prolog.ast.Database.Program
 import prolog.ast.logic.Clause
 import prolog.ast.logic.Fact
 import prolog.ast.logic.Predicate
 import prolog.ast.logic.Rule
 import prolog.ast.terms.Atom
-import prolog.ast.terms.Functor
+import prolog.ast.terms.FunctorInfo
 import prolog.ast.terms.Structure
 import prolog.ast.terms.Variable
 
@@ -40,7 +39,7 @@ class DatabaseOperatorsTests {
             createAssert(fact).satisfy(emptyMap())
 
             assertEquals(1, Program.db.predicates.size, "Expected 1 predicate")
-            assertEquals(fact, Program.db.predicates["a/_"]!!.clauses[0])
+            assertEquals(fact, Program.db.predicates[FunctorInfo.of("a/_")]!!.clauses[0])
         }
 
         @Test
@@ -49,7 +48,7 @@ class DatabaseOperatorsTests {
             createAssert(fact).satisfy(emptyMap())
 
             assertEquals(1, Program.db.predicates.size, "Expected 1 predicate")
-            assertEquals(fact, Program.db.predicates["a/1"]!!.clauses[0])
+            assertEquals(fact, Program.db.predicates[FunctorInfo.of("a/1")]!!.clauses[0])
         }
 
         @Test
@@ -61,7 +60,7 @@ class DatabaseOperatorsTests {
             createAssert(rule).satisfy(emptyMap())
 
             assertEquals(1, Program.db.predicates.size, "Expected 1 predicate")
-            assertEquals(rule, Program.db.predicates["a/1"]!!.clauses[0])
+            assertEquals(rule, Program.db.predicates[FunctorInfo.of("a/1")]!!.clauses[0])
         }
     }
 
@@ -92,8 +91,8 @@ class DatabaseOperatorsTests {
             AssertA(rule2).satisfy(emptyMap())
 
             assertEquals(1, Program.db.predicates.size, "Expected 1 predicate")
-            assertEquals(rule2, Program.db.predicates["a/1"]!!.clauses[0])
-            assertEquals(rule1, Program.db.predicates["a/1"]!!.clauses[1])
+            assertEquals(rule2, Program.db.predicates[FunctorInfo.of("a/1")]!!.clauses[0])
+            assertEquals(rule1, Program.db.predicates[FunctorInfo.of("a/1")]!!.clauses[1])
         }
     }
 
@@ -117,8 +116,8 @@ class DatabaseOperatorsTests {
             AssertZ(rule2).satisfy(emptyMap())
 
             assertEquals(1, Program.db.predicates.size, "Expected 1 predicate")
-            assertEquals(rule1, Program.db.predicates["a/1"]!!.clauses[0])
-            assertEquals(rule2, Program.db.predicates["a/1"]!!.clauses[1])
+            assertEquals(rule1, Program.db.predicates[FunctorInfo.of("a/1")]!!.clauses[0])
+            assertEquals(rule2, Program.db.predicates[FunctorInfo.of("a/1")]!!.clauses[1])
         }
     }
 
@@ -309,20 +308,20 @@ class DatabaseOperatorsTests {
 
         val control = Program.query(Structure(Atom("a"), listOf(Variable("X")))).toList()
         assertEquals(3, control.size, "Expected 3 results")
-        assertEquals(3, Program.db.predicates["a/1"]!!.clauses.size, "Expected 3 clauses")
+        assertEquals(3, Program.db.predicates[FunctorInfo.of("a/1")]!!.clauses.size, "Expected 3 clauses")
 
         val retract = RetractAll(Structure(Atom("a"), listOf(Variable("X"))))
         val result = retract.satisfy(emptyMap()).toList()
 
         assertEquals(1, result.size, "Expected 1 result")
         assertTrue(result[0].isSuccess, "Expected success")
-        assertEquals(0, Program.db.predicates["a/1"]!!.clauses.size, "Expected 0 clauses")
+        assertEquals(0, Program.db.predicates[FunctorInfo.of("a/1")]!!.clauses.size, "Expected 0 clauses")
     }
 
     @Test
     fun `If Head refers to a predicate that is not defined, it is implicitly created as a dynamic predicate`() {
         val predicateName = "idonotyetexist"
-        val predicateFunctor = "$predicateName/1"
+        val predicateFunctor = FunctorInfo.of("$predicateName/1")
 
         assertFalse(predicateFunctor in Program.db.predicates, "Expected predicate to not exist before")