feat: add toString visitor

Author: kelvin-olaiya

src/main/scala/io/github/kelvindev15/prolog/core/Term.scala

@@ -1,6 +1,6 @@
 package io.github.kelvindev15.prolog.core
 
-import io.github.kelvindev15.prolog.utils.TermVisitor
+import io.github.kelvindev15.prolog.utils.{TermVisitor, ToStringTermVisitor}
 
 trait Visitable:
   self: Term =>
@@ -10,3 +10,4 @@ trait Term extends Visitable:
   def isGround: Boolean
   def variables: Seq[Variable]
   def asTerm: Term
+  override def toString: String = accept(ToStringTermVisitor())

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

@@ -0,0 +1,25 @@
+package io.github.kelvindev15.prolog.utils
+import io.github.kelvindev15.prolog.core.Goals.{Conjunction, Disjunction}
+import io.github.kelvindev15.prolog.core.RecursiveStruct.BinaryRecursiveStruct
+import io.github.kelvindev15.prolog.core.Struct.Clause
+import io.github.kelvindev15.prolog.core.{Constant, PrologList, RecursiveStruct, Struct, Variable}
+
+class ToStringTermVisitor extends TermVisitor[String]:
+
+  override def visit(atom: Constant.Atom): String = atom.value
+  override def visit(numeric: Constant.Numeric): String = numeric.value.toString
+  override def visit(variable: Variable): String = variable.name
+  override def visit(clause: Struct.Clause): String = clause.head match
+    case Some(h) => s"${visit(h)} :- ${visit(clause.body)}."
+    case _ => visit(clause.body)
+  override def visit(struct: Struct): String = struct match
+    case atom: Constant.Atom => visit(atom)
+    case clause: Clause => visit(clause)
+    case list: PrologList => list.linearizedArguments.map(visit).mkString("[", ", ", "]")
+    case binaryRecursiveStruct: BinaryRecursiveStruct => visit(binaryRecursiveStruct)
+    case _ => s"${visit(struct.functor)}${struct.arguments.map(visit).mkString("(", ", ", ")")}"
+  override def visit(recursiveStruct: BinaryRecursiveStruct): String =
+    val linearizedArgs = recursiveStruct.linearizedArguments.map(visit)
+    recursiveStruct match
+      case _: Conjunction => linearizedArgs.mkString(", ")
+      case _: Disjunction => linearizedArgs.mkString("; ")

src/test/scala/io/github/kelvindev15/visitors/TestStringTermVisitor.scala

@@ -0,0 +1,29 @@
+package io.github.kelvindev15.visitors
+
+import io.github.kelvindev15.prolog.core.Struct.Fact
+import io.github.kelvindev15.prolog.dsl.{DeclarativeDSL, PrologDSL}
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class TestStringTermVisitor extends AnyFunSuite with Matchers with PrologDSL with DeclarativeDSL:
+
+  test("Test structs"):
+    "rainbow"("orange").toString shouldEqual "rainbow(orange)"
+    "father"("abraham", X).toString shouldEqual "father(abraham, X)"
+
+  test("Test facts"):
+    Fact("abraham").toString shouldEqual "abraham :- true."
+    Fact("father"(X)).toString shouldEqual "father(X) :- true."
+
+  test("Test rules"):
+    ("a" :- "b").toString shouldEqual "a :- b."
+    ("a" :- &&("b", "c", "d")).toString shouldEqual "a :- b, c, d."
+    ("a" :- ||("b", "c", "d")).toString shouldEqual "a :- b; c; d."
+
+  test("Test lists"):
+    list("a", "b", "d").toString shouldEqual "[a, b, d]"
+    list("a").toString shouldEqual "[a]"
+
+  test("Test numeric"):
+    "pred"(1.3).toString shouldEqual "pred(1.3)"
+    "pred"(25).toString shouldEqual "pred(25)"