Forth interpreter in 64 lines of Ruby

forth.rb

#!/usr/bin/env ruby

def pop; $stack.pop || raise(StackUnderflow); end
def push(expression); $stack << expression; end
def unary; -> { push(yield pop) }; end
def binary; -> { push(yield pop, pop) }; end
def unary_boolean; -> { push(if yield pop then 1 else 0 end) }; end
def binary_boolean; -> { push(if yield pop, pop then 1 else 0 end) }; end
def swap; $stack[-2,2] = $stack[-2,2].reverse; end
$stack = []

def new_word
  raise EmptyWord if $word.size < 1
  raise NestedDefinition if $word.include? ':'
  name, expression = $word.shift, $word.join(' ')
  $dictionary[name] = -> { parse(expression) }
  $word = nil
end

def parse(expression)
  puts "=> #{expression}"
  begin
    expression.split.each do |statement|
      case
        when !$skip.nil? && statement != 'fi'; next
        when !$word.nil? && statement != ';'; $word << statement
        when $dictionary.has_key?(statement); $dictionary[statement].call
        else push statement.to_i
      end
    end
  rescue
    puts "Error: #{$!}"
  end
end

$dictionary = {
  '+'     => binary { |a, b| a + b },
  '-'     => binary { |a, b| a - b },
  '*'     => binary { |a, b| a * b },
  '/'     => binary { |a, b| a * b },
  '%'     => binary { |a, b| a * b },
  '<'     => binary_boolean { |a, b| a < b },
  '>'     => binary_boolean { |a, b| a > b },
  '='     => binary_boolean { |a, b| a == b },
  '&'     => binary_boolean { |a, b| a && b },
  '|'     => binary_boolean { |a, b| a || b },
  'not'   => binary_boolean { |a, b| a == 0 },
  'neg'   => binary { |a| -a },
  '.'     => -> { puts(pop) },
  '..'    => -> { puts($stack) },
  ':'     => -> { $word = [] },
  ';'     => -> { new_word },
  'pop'   => -> { pop },
  'fi'    => -> { $skip = nil },
  'words' => -> { p $dictionary.keys.sort },
  'if'    => -> { $skip = true if pop == 0 },
  'dup'   => -> { push($stack.last || raise(StackUnderflow)) },
  'over'  => -> { push($stack.last(2).first || raise(StackUnderflow)) },
  'swap'  => -> { begin swap rescue raise(StackUnderflow) end }
}

puts "Ruby Forth interpreter: enter commands at the prompt"
while ARGV.size > 0; open(ARGV.shift).each { |line| parse(line) }; end
while true; print "> "; break unless gets; parse $_; end