New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

No parse when using filter for associativity disambiguation + explicit parenthesis #34

danleh opened this Issue Aug 10, 2017 · 1 comment


None yet
2 participants
Copy link

danleh commented Aug 10, 2017

Given the following test case

import cats.Eval
import parseback._
import parseback.ast._
import parseback.compat.cats._

object Testcase extends App {

  sealed trait Expr extends Node
  case class Identifier(name: String) extends Expr with LeafNode
  case class Application(left: Expr, right: Expr) extends Expr with BinaryNode {
    override def assocLeft: Boolean = true
    override def sym: Symbol = 'app

  lazy val expr: Parser[Expr] = (
    """[a-zA-Z0-9]+""".r ^^ { (_, name) => Identifier(name) }
      | expr ~ " " ~ expr ^^ { (_, fun, _, arg) => Application(fun, arg) }
      | "(" ~> expr <~ ")"
    ) filter prec(Application)

  // Test 1, correct: gives the left assoc. AST
  println(expr(LineStream[Eval]("function1 function2 argument"))

  // Test 2, bug? gives no error but an empty list of ASTs
  println(expr(LineStream[Eval]("function1 (function2 argument)"))

where we define a simple grammar with just Identifiers and function Application. I intend application to be left-associative and thus write filter prec(Application) with BinaryNode and def assocLeft: Boolean = true.

The first example works fine, only the left-associative AST is returned. However, in the second example where we use parentheses, an empty list is returned (where we would expect the right-associative AST).

Am I using filter and/or BinaryNode wrong? Or is this a bug?


This comment has been minimized.

Copy link

djspiewak commented Mar 23, 2018

Hey! I'm back. :-) Sorry, longest not-seeing-of-an-issue ever. This definitely looks like a bug to me. You're using things correctly. Honestly, an empty list of results should never happen. The disambiguation filters are designed in such a way that they should never eliminate all results unless the grammar itself encodes precedence in an opposite way from the filters (that isn't happening here). I'll look into it.

@djspiewak djspiewak added the bug label Mar 23, 2018

osiire pushed a commit to osiire/parseback that referenced this issue Dec 30, 2018

osiire pushed a commit to osiire/parseback that referenced this issue Jan 12, 2019

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment