RPL (programming language)

RPL (derived from Reverse Polish Lisp according to its original developers, whilst for a short while in 1987 HP marketing attempted to coin the backronym ROM-based Procedural Language for it.) is a handheld calculator operating system and application programming language used on Hewlett-Packard's scientific graphing RPN (Reverse Polish Notation) calculators of the HP 28, 48, 49 and 50 series, but it is also usable on non-RPN calculators, such as the 38, 39 and 40 series.

RPL is a structured programming language based on 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 originated from HP's Corvallis, Oregon development facility in 1984 as a replacement for the previous practice of implementing the operating systems of calculators in assembly language. The last calculator supporting RPL, the HP 50g, was discontinued in 2015.

Programming language

A programming language is a formal constructed language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs to control the behavior of a machine or to express algorithms.

The earliest known programmable machine preceded the invention of the digital computer and is the automatic flute player described in the 9th century by the brothers Musa in Baghdad, at the time a major centre of knowledge. From the early 1800s, "programs" were used to direct the behavior of machines such as Jacquard looms and player pianos. Thousands of different programming languages have been created, mainly in the computer field, and many more still are being created every year. Many programming languages require computation to be specified in an imperative form (i.e., as a sequence of operations to perform), while other languages use other forms of program specification such as the declarative form (i.e. the desired result is specified, not how to achieve it).