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 RPL2 is a recreation of this language for POSIX machines.

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