Mathn Compatibility

RubyconceptualbeginnerParslet-compatible

Ruby mathn library compatibility

How the Code Works

This section explains how the parsing rules map to the format semantics.

The Mathn Problem

# Without mathn:
3 / 2  # => 1 (integer division)

# With mathn:
3 / 2  # => (3/2) (Rational)

Ruby's deprecated `mathn` library changed integer division behavior: This broke Parslet's internal calculations.

The Grammar

cephalopod =
  str('octopus') |
  str('squid')

parenthesized_cephalopod =
  str('(') >>
  possible_whitespace >>
  cephalopod >>
  possible_whitespace >>
  str(')')

Simple cephalopod matching:

The Compatibility Fix

def attempt_parse
  parser = possible_whitespace >>
           parenthesized_cephalopod >>
           possible_whitespace

  # This would hang without the fix
  parser.parse %{(\nsqeed)\n}
rescue Parsanol::ParseFailed
end

Parsanol includes a fix that works regardless of mathn:

Version Check

if RUBY_VERSION.gsub(/[^\d]/, '').to_i < 250
  require 'mathn'
end

The example checks Ruby version: Mathn was deprecated in Ruby 2.5, so it's only loaded on older Rubies.

Version Check

if RUBY_VERSION.gsub(/[^\d]/, '').to_i < 250
  require 'mathn'
end

The example checks Ruby version: Mathn was deprecated in Ruby 2.5, so it's only loaded on older Rubies.

Source Code

Ruby

# frozen_string_literal: true

# Demonstrates that we have a compatibility fix to mathn's weird idea of
# integer mathematics.
# Originally contributed to Parslet, ported to Parsanol as an example.

$LOAD_PATH.unshift "#{File.dirname(__FILE__)}/../lib"

require "parsanol/parslet"
require "parsanol/convenience"
include Parsanol::Parslet

def attempt_parse
  possible_whitespace = match['\s'].repeat

  cephalopod =
    str("octopus") |
    str("squid")

  parenthesized_cephalopod =
    str("(") >>
    possible_whitespace >>
    cephalopod >>
    possible_whitespace >>
    str(")")

  parser =
    possible_whitespace >>
    parenthesized_cephalopod >>
    possible_whitespace

  # This parse fails, but that is not the point. When mathn is in the current
  # ruby environment, it modifies integer division in a way that makes
  # parslet loop indefinitely.
  parser.parse %{(\nsqeed)\n}
rescue Parsanol::ParseFailed
end

attempt_parse
puts "it terminates before we require mathn"

puts "requiring mathn now"
# mathn was deprecated as of Ruby 2.5
require "mathn" if RUBY_VERSION.gsub(/[^\d]/, "").to_i < 250
puts "and trying again (will hang without the fix)"
attempt_parse # but it doesn't terminate after requiring mathn
puts "okay!"