Forth and Postscript are RPN computer languages that you can program in directly. (Likewise the JVM instruction set but most users don't program it directly or

Forth and Postscript are RPN computer languages that you can program in directly. (Likewise the JVM instruction set but most users don't program it directly or add new instructions.) Lisp is FPN ("Forward" Polish Notation). One reason Lisp needs parentheses while Forth doesn't is that Lisp allows a variable number of arguments to functions; Forth generally does not. It's possible to build variadic Forth functions but you have to push the number of arguments as a parameter, which is usually not very convenient. With Lisp, the closing parenthesis delimits the last argument so the Lisp compiler always knows how many arguments were passed automatically. Polish notation makes both Forth and Lisp very easy to parse compared to algebraic languages, and as the article points out it also makes expressing function composition very easy in both.