provided code

src/main/wacc/Main.scala

@@ -0,0 +1,15 @@
+package wacc
+
+import parsley.{Success, Failure}
+
+def main(args: Array[String]): Unit = {
+    println("hello WACC!")
+
+    args.headOption match {
+        case Some(expr) => parser.parse(expr) match {
+            case Success(x) => println(s"$expr = $x")
+            case Failure(msg) => println(msg)
+        }
+        case None => println("please enter an expression")
+    }
+}

src/main/wacc/lexer.scala

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+package wacc
+
+import parsley.Parsley
+import parsley.token.Lexer
+import parsley.token.descriptions.*
+
+object lexer {
+    private val desc = LexicalDesc.plain.copy(
+        // your configuration goes here
+    )
+    private val lexer = Lexer(desc)
+
+    val integer = lexer.lexeme.integer.decimal
+    val implicits = lexer.lexeme.symbol.implicits
+    def fully[A](p: Parsley[A]): Parsley[A] = lexer.fully(p)
+}

src/main/wacc/parser.scala

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+package wacc
+
+import parsley.{Parsley, Result}
+import parsley.expr.chain
+
+import lexer.implicits.implicitSymbol
+import lexer.{integer, fully}
+
+object parser {
+    def parse(input: String): Result[String, BigInt] = parser.parse(input)
+    private val parser = fully(expr)
+    
+    private val add = (x: BigInt, y: BigInt) => x + y
+    private val sub = (x: BigInt, y: BigInt) => x - y
+
+    private lazy val expr: Parsley[BigInt] =
+        chain.left1(integer | "(" ~> expr <~ ")")(
+            ("+" as add) | ("-" as sub)
+        )
+}

src/test/wacc/README.md

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+This is where you may wish to develop your tests. Remember that `scala-cli` picks up any files
+in a `test` sub-directory, or files ending in `.test.scala`.