usure

#!/usr/bin/env ruby
require 'pp'

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 functions; -> { push(yield pop, pop)}; end