Checkpoint

src/prolog/builtins/verification.kt

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+package prolog.builtins
+
+import prolog.components.terms.CompoundTerm
+import prolog.components.terms.Term
+import prolog.components.terms.Variable
+
+/**
+ * True if [Term] is bound (i.e., not a variable) and is not compound.
+ * Thus, atomic acts as if defined by:
+ *
+ * atomic(Term) :-
+ *         nonvar(Term),
+ *         \+ compound(Term).
+ */
+fun atomic(term: Term): Boolean = nonvariable(term) && !compound(term)
+
+/**
+ * True if [Term] is bound to a compound term.
+ * See also functor/3 =../2, compound_name_arity/3 and compound_name_arguments/3.
+ */
+fun compound(term: Term): Boolean {
+    val isCompound = term is CompoundTerm
+    val isVariableCompound = term is Variable && term.alias().isPresent && compound(term.alias().get())
+    return isCompound || isVariableCompound
+}
+
+/**
+ * True if [Term] currently is not a free variable.
+ */
+fun nonvariable(term: Term): Boolean = !variable(term)
+
+/**
+ * True if [Term] currently is a free variable.
+ */
+fun variable(term: Term): Boolean {
+    if (term is Variable) {
+        return term.alias().isEmpty || term.alias().get() === term || variable(term.alias().get())
+    }
+
+    return false;
+}