Pattern matching

pattern.rb

# This module is a light implementation of pattern matching that allows us to
# match against hashes and arrays as we would with the new pattern matching
# syntax.
module Pattern
  class ArrayPattern
    attr_reader :values

    def initialize(values)
      @values = values
    end

    def ===(other)
      deconstructed = other.deconstruct

      values.each_with_index.all? do |value, index|
        value === deconstructed[index]
      end
    end
  end

  class HashPattern
    attr_reader :values

    def initialize(values)
      @values = values
    end

    def ===(other)
      deconstructed = other.deconstruct_keys(values.keys)

      values.all? do |key, value|
        value == :any ? deconstructed.key?(key) : value === deconstructed[key]
      end
    end
  end

  def self.[](value)
    case value
    when Array
      ArrayPattern.new(value.map { |child| self[child] })
    when Hash
      HashPattern.new(value.transform_values { |child| self[child] })
    else
      value
    end
  end
end

require "json"
source = '{ "a": 1, "b": [2, 3], "c": "Hello, world!" }'

case JSON.parse(source, symbolize_names: true)
when Pattern[a: 1] # <-- match against individual values
  puts "Matched value!"
when Pattern[b: [2, 3]] # <-- match against lists of values
  puts "Matched list!"
when Pattern[a: Integer] # <-- match against classes
  puts "Matched class!"
when Pattern[c: /Hello/] # <-- match against regular expressions
  puts "Matched regular expression!"
when Pattern[a: :any] # <-- match against any value
  puts "Matched wildcard!"
else
  puts "No match!"
end