refactor(dsl)!: visitors are now sigletons

Author: kelvin-olaiya

src/main/scala/io/github/kelvindev15/prolog/visitors/BinaryToFlatVisitor.scala

@@ -7,7 +7,7 @@ import io.github.kelvindev15.prolog.core.Term
 /** Linearizes a [[BinaryRecursiveStruct]], e.g a ','(a, ','(b, ','(c, [])))
   * becomes [a, b, c].
   */
-class BinaryToFlatVisitor extends TermVisitor[Seq[Term]]:
+object BinaryToFlatVisitor extends TermVisitor[Seq[Term]]:
   override def visit(tuple: BinaryRecursiveStruct): Seq[Term] = tuple match
     case Tuple(l, r @ Tuple(_, _)) => Seq(l) ++ visit(r)
     case Tuple(l, r)               => Seq(l, r)

src/main/scala/io/github/kelvindev15/prolog/visitors/ToInstanceTermVisitor.scala

@@ -0,0 +1,12 @@
+package io.github.kelvindev15.prolog.visitors
+
+import io.github.kelvindev15.prolog.core.{Constant, PrologList, Struct, Term, Variable}
+import io.github.kelvindev15.prolog.solver.Solver.Substitution
+
+class ToInstanceTermVisitor(substitution: Substitution) extends TermVisitor[Term]:
+  override def visit(struct: Struct): Term = struct match
+    case l: PrologList => visit(l)
+    case _ => Struct(struct.functor, struct.arguments.map(visit)*)
+  override def visit(list: PrologList): Term = PrologList(list.linearizedArguments.map(visit)*)
+  override def visit(variable: Variable): Term = substitution.getOrElse(variable, variable)
+  override def visit(atom: Constant.Atom): Term = atom

src/main/scala/io/github/kelvindev15/prolog/visitors/ToStringTermVisitor.scala

@@ -5,7 +5,7 @@ import io.github.kelvindev15.prolog.core.RecursiveStruct.BinaryRecursiveStruct
 import io.github.kelvindev15.prolog.core.Struct.Clause
 import io.github.kelvindev15.prolog.core.*
 
-class ToStringTermVisitor extends TermVisitor[String]:
+object ToStringTermVisitor extends TermVisitor[String]:
 
   override def visit(atom: Constant.Atom): String = atom.value
   override def visit(numeric: Constant.Numeric): String = numeric.value.toString