Quus

A Ruby implementation of Saul Kripke's "quus" function from Wittgenstein on Rules and Private Language.

In Kripke's famous thought experiment, he introduces the "quus" function (⊕) to illustrate the problem of rule-following:

x ⊕ y = x + y, if x, y ≤ 57
        otherwise 5

This gem brings this philosophical concept to Ruby, allowing you to explore the distinction between "plus" and "quus" in your code.

Add this line to your application's Gemfile:

gem 'quus'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install quus

require 'quus'

# Using the ⊕ operator (Kripke's notation)
68 ⊕ 57  # => 5
2 ⊕ 4    # => 6

# Using the method form
68.quus(57)  # => 5
2.quus(4)    # => 6

# Using the module method
Quus.quus(68, 57)  # => 5
Quus.quus(2, 4)    # => 6

# For arguments ≤ 57, quus behaves like plus
57 ⊕ 57  # => 114

# But for larger arguments, it returns 5
68 ⊕ 57  # => 5

You can customize the limit and constant values:

Quus.configure do |config|
  config.quus_limit = 100      # Change the limit from 57 to 100
  config.quus_constant = 10    # Change the constant from 5 to 10
end

68 ⊕ 57   # => 125 (now adds because both ≤ 100)
101 ⊕ 50  # => 10  (returns the new constant)

# Reset to defaults
Quus.reset_configuration!

For a dramatic demonstration of the indistinguishability between "plus" and "quus", you can temporarily replace the + operator:

# WARNING: This modifies Integer#+ globally within the block!
Quus.change_game do
  68 + 57  # => 5 (plus becomes quus!)
  2 + 4    # => 6
end

68 + 57  # => 125 (back to normal)

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests.

To install this gem onto your local machine, run bundle exec rake install.

Bug reports and pull requests are welcome on GitHub at https://github.com/a5-stable/quus.

The gem is available as open source under the terms of the MIT License.