fix: do not allow negation of a positive int literal

src/main/wacc/parser.scala

@@ -2,7 +2,7 @@ package wacc
 
 import parsley.Result
 import parsley.Parsley
-import parsley.Parsley.{atomic, many, pure}
+import parsley.Parsley.{atomic, many, notFollowedBy, pure}
 import parsley.combinator.{countSome, sepBy}
 import parsley.expr.{precedence, SOps, InfixL, InfixN, InfixR, Prefix, Atoms}
 import parsley.errors.combinator._
@@ -11,7 +11,7 @@ import cats.data.NonEmptyList
 
 object parser {
   import lexer.implicits.implicitSymbol
-  import lexer.{ident, integer, charLit, stringLit}
+  import lexer.{ident, integer, charLit, stringLit, negateCheck}
   import ast._
 
   def parse(input: String): Result[String, Program] = parser.parse(input)
@@ -32,7 +32,7 @@ object parser {
       SOps(InfixL)(Mul from "*", Div from "/", Mod from "%") +:
       SOps(Prefix)(
         Not from "!",
-        Negate from "-",
+        Negate from (notFollowedBy(negateCheck) ~> "-"),
         Len from "len",
         Ord from "ord",
         Chr from "chr"