Arg

src/prolog/builtins/analysingAndConstructionOperators.kt

@@ -3,56 +3,43 @@ 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
+import prolog.ast.terms.*
+import prolog.logic.*
+import java.util.Locale
+import java.util.Locale.getDefault
 
+/**
+ * [True] when [Term] is a term with [FunctorInfo] Name/Arity.
+ *
+ * If Term is a [Variable] it is unified with a new term whose arguments are all different variables.
+ * If Term is [atomic], Arity will be unified with the integer 0, and Name will be unified with Term.
+ */
 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 when {
+            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"
+                Conjunction(
+                    Unify(t.functor.arity, functorArity),
+                    Unify(t.functor.name, functorName)
+                ).satisfy(subs)
             }
 
-            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())
+            variable(term, subs) -> {
+                require(atomic(functorName, subs) && atomic(functorArity, subs)) {
+                    "Arguments are not sufficiently instantiated"
                 }
-                Structure(name, argList)
+
+                val name = applySubstitution(functorName, subs) as Atom
+                val arity = applySubstitution(functorArity, subs) as Integer
+                val result = Structure(name, List(arity.value) { AnonymousVariable.create() })
+                sequenceOf(Result.success(mapOf(term to result)))
             }
 
-            if (nonvariable(term, subs)) {
-                return unifyLazy(term, result, subs)
-            }
-
-            return sequenceOf(Result.success(mapOf(term to result)))
+            else -> throw IllegalStateException()
         }
-
-        throw IllegalStateException()
     }
 
     override fun applySubstitution(subs: Substitutions): Functor = Functor(
@@ -60,6 +47,51 @@ class Functor(private val term: Term, private val functorName: Term, private val
         functorName.applySubstitution(subs),
         functorArity.applySubstitution(subs)
     )
-
-    override fun toString(): String = "functor($term, $functorName, $functorArity)"
+}
+
+class Arg(private val arg: Term, private val term: Term, private val value: Term) :
+    Structure(Atom("arg"), listOf(arg, term, value)) {
+    override fun satisfy(subs: Substitutions): Answers = sequence {
+        require(nonvariable(term, subs)) { "Arguments are not sufficiently instantiated" }
+        require(compound(term, subs)) {
+            val expected = CompoundTerm::class.simpleName?.lowercase()
+            val actual = term::class.simpleName?.lowercase()
+            "Type error: `$expected' expected, found `$term' ($actual)"
+        }
+
+        val t = applySubstitution(term, subs) as Structure
+
+        when {
+            variable(arg, subs) -> {
+                // Value will be unified with the successive arguments of term.
+                // On successful unification, arg is unified with the argument number.
+                // Backtracking yields alternative solutions.
+                var count = 0
+                for (argument in t.arguments) {
+                    unifyLazy(value, argument, subs).forEach { result ->
+                        result.map {
+                            val sub = arg to Integer(count + 1)
+                            yield(Result.success(it + sub))
+                        }
+                    }
+                    count++
+                }
+
+            }
+
+            else -> {
+                val a = applySubstitution(arg, subs) as Integer
+
+                require(0 <= a.value) { "Domain error: not_less_than_zero" }
+
+                // Fail silently if the argument is out of bounds
+                if (0 == a.value || t.arguments.size < a.value) {
+                    return@sequence
+                }
+
+                val argument = t.arguments[a.value - 1]
+                yieldAll(unifyLazy(argument, value, subs))
+            }
+        }
+    }
 }