Give Ruby 3 an Elixir flavor: provide pattern matching for recursion

fibonacci.rb

# frozen_string_literal: true

module PatternMatching
  def self.extended(base)
    base.class_eval do
      @__pn_methods = {}
    end
  end

  def __pn_methods
    @__pn_methods
  end

  def deff(method_name, pattern, &blk)
    __pn_methods[method_name] ||= {} # FIXME: make me thread-safe
    __pn_methods[method_name][pattern] = blk

    define_singleton_method(method_name) do |*args, **kwargs|
      __pn_methods[method_name].each do |pn, b|
        match = begin
          pn.call(*args, **kwargs)
          true
        rescue NoMatchingPatternError
          false
        end

        return b.call(*args, **kwargs) if match
      end

      raise NoMatchingPatternError
    end
  end

  def pn(&blk)
    blk
  end
end

class Fib
  extend PatternMatching

  deff :call, (pn { |n| n => 0 }) do
    0
  end

  deff :call, (pn { |n| n => 1 }) do
    1
  end

  deff :call, (pn { |n| n => Integer }) do |n|
    call(n - 1) + call(n - 2)
  end
end

puts Fib.call(10)