traverse => traverse_ to avoid building list of units!

Author: j-mie6

parsley/shared/src/main/scala/parsley/combinator.scala

@@ -185,6 +185,7 @@ object combinator {
     def traverse[A, B](f: A => Parsley[B], xs: A*): Parsley[List[B]] = sequence(xs.map(f): _*)
     // this will be used in future!
     private [parsley] def traverse5[A, B](xs: A*)(f: A => Parsley[B]): Parsley[List[B]] = traverse(f, xs: _*)
+    private [parsley] def traverse_[A](xs: A*)(f: A => Parsley[_]): Parsley[Unit] = skip(unit, xs.map(f): _*)
 
     /** This combinator will parse each of `ps` in order, discarding the results.
       *

parsley/shared/src/main/scala/parsley/errors/tokenextractors/LexToken.scala

@@ -10,7 +10,7 @@ import parsley.Success
 import parsley.XAssert.assert
 import parsley.XCompat.unused
 import parsley.character.{item, stringOfSome}
-import parsley.combinator.{option, traverse5}
+import parsley.combinator.{option, traverse5, traverse_}
 import parsley.errors.{ErrorBuilder, Token, TokenSpan}
 import parsley.position
 
@@ -58,7 +58,7 @@ trait LexToken { this: ErrorBuilder[_] =>
     private lazy val makeParser: Parsley[Either[::[(String, (Int, Int))], String]] = {
         val toks = traverse5(tokens: _*)(p => option(lookAhead(attempt(p) <~> position.pos))).map(_.flatten).collect { case toks@(_::_) => toks }
         // this can only fail if either there is no input (which there must be), or there is a token at the front, in which case `rawTok` is not parsed anyway
-        val rawTok = stringOfSome(traverse5(tokens: _*)(notFollowedBy) *> item)
+        val rawTok = stringOfSome(traverse_(tokens: _*)(notFollowedBy) *> item)
         toks <+> rawTok
     }