a RPN calculator in 19 lines of Ruby

RPM calculator.rb

def RPN(*data)
  stack = []
  data.each do |el|
    case el
    when :+, :-, :*, :/, :&, :|, :<<        # 1,2,3,:*,:+    == 1 + 2 $ 3
      a, b = stack.pop(2)
      stack.push a.send(el.to_sym, b)              
    when Class                              # 3,Float        ==  Float(3)
      stack.push send(el.name, stack.pop)      
    when Symbol                             # '1+2',:eval    ==  eval('1+2')
      stack.push send(el, stack.pop)           
    when Proc                               # 3,->(n){2*n}   ==  ->(n){2*n}.call(3)
      stack.push el.(*stack.pop(el.arity))
    else
      stack << el                           
    end
  end
  stack.pop
end

#  Tests : 

def assert_eq(l,r)
  l==r or fail "#{l} != #{r}"
end

assert_eq "AZ",     RPN("A", "Z", ->(a, b){a+b} )
assert_eq 3,        RPN('1+2', :eval)
assert_eq 15,       RPN(5, ->(v){3*v})
assert_eq (Float(3) / 2),    RPN(3, Float, 2, :/)
assert_eq (1+2*3),  RPN(1, 2, 3, :*, :+)
assert_eq (1+2),    RPN(1, 2, :+)
assert_eq (16<<2),    RPN(16, 2, :<<)
assert_eq 'ab',     RPN('a', 'b', :+)
assert_eq (10/2),   RPN(10, 2, :/)
assert_eq (3-2),    RPN(3, 2, :-)
assert_eq (3*2),    RPN(3, 2, :*)
assert_eq false,    RPN(true, false, :&)
assert_eq true,     RPN(true, false, :|)
# assert_eq false,    RPN(true, false, :'&&')     !!does not work
# assert_eq true,     RPN(true, false, :'||')     !!does not work