RPL

RPL is a structured programming language based on reverse Polish notation(RPN), but equally capable of processing algebraic expressions and formulae, implemented as a threaded interpreter. RPL has many similarities to Forth, both languages being stack-based, as well as the list-based Lisp. Contrary to previous HP RPN calculators, which had a fixed four-level stack, the stack used by RPL is only limited by available calculator RAM.

RPL (an acronym for '''Reverse Polish Lisp''') is the onboard calculator language for the HP-28 and HP 48 lines of graphing calculators. It is a stack language (not a Lisp-like language, as its name implies), tightly integrated with the calculator system, with many instructions similar to Forth.

It can natively handle unsigned integers (named ''binary integers''), floating-point numbers (named ''real numbers''), complex numbers, vectors and matrices (named ''arrays''), lists and algebraic expressions.

There are actually two languages with the name RPL: '''User RPL''' and '''System RPL'''. System RPL is used in implementing the operating system, and User RPL is the language users use to write programs on the calculator.

The language 2 is a recreation of this language for POSIX machines.

https://en.wikipedia.org/wiki/RPL_(programming_language)