Pattern match in ruby

match.rb

# Copyright (C) 2012 Kazuki Tsujimoto, All rights reserved.
#
# You can redistribute it and/or modify it under the terms of the 2-clause BSDL.

class Pattern
  attr_accessor :parent
  attr_reader :env

  def initialize(*children)
    @parent = nil
    @children = children.map {|i| i.is_a?(Pattern) ? i : PatternValue.new(i) }
    @children.each do |i|
      i.parent = self
    end
    @env = {}
  end

  def method_missing(name, *)
    @as = PatternVariable.new(name)
    @as.parent = self
    self
  end

  def bind(name, val)
    if @parent
      @parent.bind(name, val)
    else
      @env[name] =  val
    end
  end
end

class PatternExtractor < Pattern
  def initialize(extractor, *patterns)
    super(*patterns)
    @extractor = extractor
  end

  def match(val)
    unapplied_val = @extractor.unapply(val)
    return false unless unapplied_val.length == @children.length
    @children.zip(unapplied_val).all? do |pat, v|
      pat.match(v)
    end.tap {|ret| @as.match(val) if ret and @as }
  end
end

class PatternVariable < Pattern
  def initialize(name)
    super()
    @name = name
  end

  def match(val)
    bind(@name, val)
    true
  end
end

class PatternValue < Pattern
  def initialize(val)
    super()
    @val = val
  end

  def match(v)
    @val === v
  end
end

class PatternMatchingEnv < Object
  def initialize(ctx, val)
    @ctx = ctx
    @val = val
    @matched = false
  end

  def pattern(pat, &block)
    if (! @matched) and pat.match(@val)
      @matched = true
      with_tmpenv(@ctx, pat.env, &block)
    end
  rescue PatternNotMatchError
  end

  def method_missing(name, *)
    PatternVariable.new(name)
  end

  private

  def with_tmpenv(obj, env, &block)
    tmpenv_module(obj).instance_eval do
      begin
        env.each do|name, val|
          define_method(name) { val }
        end
        obj.instance_eval(&block)
      ensure
        env.each do |name, _|
          remove_method(name)
        end
      end
    end
  end

  def tmpenv_module(obj)
    m = class << obj; self.ancestors.find{|i| i.respond_to? :tmpenv? }; end
    unless m
      m = Module.new do |m|
        def tmpenv?; true; end
        def m.method_added(name, *); end
      end
      obj.extend(m)
    end
    m
  end
end

module Kernel
  def match(expr, &block)
    PatternMatchingEnv.new(self, expr).instance_eval(&block)
  end
end

class PatternNotMatchError < Exception
end

module Extractor
  def call(*patterns)
    PatternExtractor.new(self, *patterns)
  end
end

class Class
  include Extractor
end

def Array.unapply(value)
  case value
  when Array
    value
  else
    raise PatternNotMatchError.new
  end
end

class Regexp
  include Extractor

  def unapply(value)
    m = self.match(value.to_s)
    raise PatternNotMatchError.new unless m
    m.captures.empty? ? [m[0]] : m.captures
  end
end

if $0 == __FILE__
  class EMail
    def self.unapply(value)
      value.to_s.split(/@/).tap {|parts| raise PatternNotMatchError.new unless parts.length == 2}
    end
  end

  match([1, "[email protected]"]) {
    pattern(Array.(i, EMail.(/(\w+)-(\w+)/.(firstname, 'bar') :: name, domain) :: mail)) {
      p [i, firstname, name, domain, mail] # => [1, "foo", "foo-bar", "example.com", "[email protected]"]
    }
  }
end