def evaluate( expr ) =
  stack = []

  for token <- expr.split( '''\s+''' )
    case number( token )
      Some( n ) ->
        stack = n : stack
        println( "push $token: ${stack.reversed()}" )
      None ->
        case {'+': (+), '-': (-), '*': (*), '/': (/), '^': (^)}.>get( token )
          Some( op ) ->
            stack = op( stack.tail().head(), stack.head() ) : stack.tail().tail()
            println( "perform $token: ${stack.reversed()}" )
          None -> error( "unrecognized operator '$token'" )

  stack.head()

res = evaluate( '3 4 2 * 1 5 - 2 3 ^ ^ / +' )
println( res + (if res is Integer then '' else " or ${float(res)}") )