Feature #19013: Error Tolerant Parser

• Status: Closed
• Assignee: -
• Target version: -

[yui-knk (Kaneko Yuichiro)]

Background

Implementation for Language Server Protocol (LSP) sometimes needs to parse incomplete ruby script for example users want to complement expressions in the middle of statement like below:

class A
  def m
    a = 10
    if # here users want to run completion
  end
end

In such case, LSP implementation wants to get partial AST instead of syntax error.

Proposal

At the moment I want to propose 3 types of tolerance

1. Complement end when lexer hits to end-of-input but end is not enough

This is a case. Lexer will generate 1 end before generates end-of-input.

describe "1" do
  describe "2" do
    describe "3" do
      it "here" do
    end
  end
end

2. Extract "end" as keyword not identifier based on an indent

This is a case. Normal parser recognizes "end" on line 4 as "local variable or method".
This causes not only syntax error but also bar method definition is assumed as Z::Foo#bar.
Other approach is suppress !IS_lex_state(EXPR_DOT) checks for "end".

module Z
  class Foo
    foo.
  end

  def bar
  end
end

3. Change locations of error

Currently error is put into top_stmts and stmts like top_stmts: error top_stmt and stmts: error stmt.
However these are too strict to catch syntax error then want to move it to stmt: error and expr_value: error.

Interface

• Adding error_tolerant option to RubyVM::AbstractSyntaxTree.parse
• Adding --error-tolerant-parser option to ruby command for debugging
  • This option is valid only when –dump=yydebug, --dump=parsetree or --dump=parsetree_with_comment is passed

Compatibility

Changing the location of error can lead incompatibility. At least I observed 2 test cases in ruby/ruby are broken by this change.
I think both of them depend on how ripper behaves after ripper raises syntax error.

• RDoc: https://github.com/yui-knk/ruby/commit/1dabbe508f0cc3dd4f83aa72502bbf347029dd8c
  • However ruby script in heredoc is invalid...
• irb: https://github.com/yui-knk/ruby/commit/e18be19ecd044eb26a56f6f9ba4f19d40c01a9c7
  • Range of error coloring is changed

All other changes are related to not parser but lexer and they are controlled by error_tolerant option. Therefore no behavior change is expected for ruby parser and ripper.

Implementation

https://github.com/yui-knk/ruby/tree/error_recovery_indent_aware

[duerst (Martin Dürst)]

The topic of parsing incomplete syntax also came up in Kevin Newton's talk (see https://rubykaigi.org/2022/presentations/kddnewton.html) at RubyKaigi 2022. In the talk, he said he is working on a new parser. Maybe these efforts could be combined?

[matz (Yukihiro Matsumoto)]

Kevin's work has broader goals, e.g. being faster, consuming less memory, which should be free from yacc/bison limitation.
I consider this work as an experiment to explore error-tolerant-ness.

Matz.

[yui-knk (Kaneko Yuichiro)]

• Status changed from Open to Closed

Applied in changeset git|fbbdbdd8911ffb24d98bb71c7c33d24609ce7dfe.

---

Add error_tolerant option to RubyVM::AST

If this option is enabled, SyntaxError is not raised and Node is
returned even if passed script is broken.

[Feature #19013]