require 'ruby-debug'
require 'dsl' # get it at http://raganwald.com/source/dsl_and_let.html

module Matchable

  def lit(obj)
    Literal.new(obj)
  end

  def match(args,&block)
    pmatch = PatternMatch.new(args,block.binding)
    pmatch.instance_eval(&block)
    pmatch.value
  end
  
  class PatternMatch < Let::Scope
    
    def initialize(args,binding)
      @args = args
      super(binding)
    end
    
    def with(pattern, &block)
      args = @args
      unless @matched
        mapping = {}
        if(pattern.patmatch(args,mapping))
          @matched = true
          if(mapping.length > 0)
            @value = Kernel.with(Let){let(mapping,&block)}
          else
            @value = block.call
          end
        end
      end
    end
    
    def otherwise(&by_default)
      @by_default = by_default
    end
    
    def value
      if (@matched)
        @value
      else
        if(@by_default)
          @by_default.call
        else
          raise NoMatchFoundError, "The arguments did not match any of the supplied patterns and no otherwise clause was provided"
        end
      end
    end
    
  end

  class NoMatchFoundError < StandardError; end

end

class Object
  def patmatch(arg,mapping)
    self == arg
  end
end

class Class
  def patmatch(arg,mapping)
    arg.is_a?(self)
  end
end

class Symbol
  
  def patmatch(arg,mapping)
    if(empty?)
      true
    else
      mapping[self] = arg unless mapping.nil?
      true
    end
  end
  
  def empty?
    self.to_s=="_"
  end

  def %(other)
    raise ArgumentError unless other.is_a?(Symbol)
    Destructurer.new(self,other)
  end

  def &(other)
    Namer.new(self,other)
  end

end

class Namer
  
  def initialize(sym, obj)
    @sym = sym
    @obj = obj
  end
  
  def patmatch(args,mapping)
    if(@obj.patmatch(args,mapping))
      mapping[@sym] = args
      true
    else
      false
    end
  end
  
end

class Destructurer
  
  def initialize(*names)
    @names = names
  end

  def patmatch(args,mapping)
    return false unless args.is_a?(Array)
    return false unless args.length>0
    @names[0...-1].each do |name|
      mapping[name]=args.shift
    end
    mapping[@names.last] = args
    true
  end

  def %(symbol)
    raise ArgumentError unless symbol.is_a?(Symbol)
    @names << symbol
    self
  end
  
end

module Enumerable

  def patmatch(args,mapping)
    return false if self.length != args.length || !args.is_a?(Enumerable)
    return self.zip(args).all? {|x_y| x_y[0].patmatch(x_y[1],mapping)}
  end

end

class String
  
  # Have to special case this, because String is enumerable but the 
  # zip method on string doesn't behave as expected
  def patmatch(args,mapping)
    return self==args
  end
  
end

class Literal
  
  def initialize(obj)
    @obj = obj
  end

  def patmatch(args,mapping)
    @obj == args
  end
  
end