A pure Ruby interpreter for Forth

forth.rb

#!/usr/bin/env ruby

# the stack
$stack = []
def pop() $stack.pop || ufe end
def push(f) $stack<<f end
# poor man's Exception class
def ufe() raise("Stack underflow") end

# lambda constructor helpers
def mkUnary(&fn) lambda{push(fn.call(pop))} end
def mkBinary(&fn) lambda{push(fn.call(pop,pop))} end
def mkBool1(&b) lambda{push(b.call(pop)?1:0)} end
def mkBool2(&b) lambda{push(b.call(pop,pop)?1:0)} end

# the dictionary of Forth words (String => Proc)
$dict = {'+'   =>mkBinary{|a,b|a+b}, '-'   =>mkBinary{|a,b|a-b},
		 '*'   =>mkBinary{|a,b|a*b}, '/'   =>mkBinary{|a,b|a/b},
		 '%'   =>mkBinary{|a,b|a%b}, '='   =>mkBool2 {|a,b|a==b},
		 '<'   =>mkBool2 {|a,b|a<b}, '>'   =>mkBool2 {|a,b|a>b},
		 '&'   =>mkBool2 {|a,b|a&b}, '|'   =>mkBool2 {|a,b|a|b},
		 'not' =>mkBool1 {|a| a==0}, 'neg' =>mkUnary {|a|-a},
		 '.'   =>lambda  {puts pop}, '..'  =>lambda  {p $stack},
		 ':'   =>lambda  {$word=[]}, ';'   =>lambda  {makeword},
		 'pop' =>lambda  {pop},      'fi'  =>lambda  {$skip=nil},
		 'words'=>lambda {p $dict.keys.sort},
		 'if'   =>lambda {$skip=true if pop==0},
		 'dup'  =>lambda {push $stack[-1] || ufe},
		 'over' =>lambda {push $stack[-2] || ufe},
		 'swap' =>lambda {begin swap rescue ufe end},
	 	}
def swap
	$stack[-2,2] = $stack[-2,2].reverse
end

# adds user-defined word to the dictionary
def makeword
	($word=nil;raise("Empty word definition"))  unless $word && $word.size > 1
	($word=nil;raise("Nested word definition")) if $word.include? ':'
	name, code = $word.shift, $word.join(' ')
	$dict[name] = lambda{parse(code)}
	$word = nil
end

# meat and potatoes
def parse(str)
	begin
	str.split.each do |w|
		if $skip and w != 'fi'
			next 				# skipping over conditional
		elsif $word and w != ';'
			$word << w 			# reading word definition
		elsif $dict.has_key? w
			$dict[w].call 		# calling pre-defined word
		else
			push w.to_i 		# pushing int literal
		end
	end
	rescue
		puts "Error: #{$!}"
	end
end

puts "Ruby Forth interpreter: enter commands at the prompt"
# read definitions files
while ARGV.size > 0
	File.open(ARGV.shift).each{|line| parse(line)}
end
# REPL
loop do
	">> ".display
	break unless gets
	parse($_)
end