Reverse Polish Lisp
| Paradigm | stack, structured, object-oriented |
|---|---|
| Designed by | Hewlett-Packard |
| First appeared | 1984 (1986) |
| OS | HP calculators |
| Dialects | |
| System RPL, User RPL | |
| Influenced by | |
| RPN, Forth, Lisp | |
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.
The internal low- to medium-level variant of RPL, called System RPL (or SysRPL) is used on some earlier HP calculators as well as the aforementioned ones, as part of their operating system implementation language. In the HP 48 series this variant of RPL is not accessible to the calculator user without the use of external tools, but in the HP 49/50 series there is a compiler built in ROM to use SysRPL. It is possible to cause a serious crash while coding in SysRPL, so caution must be used while using it. The high-level User RPL (or UserRPL) version of the language is available on said graphing calculators for developing textual as well as graphical application programs. All UserRPL programs are internally represented as SysRPL programs, but use only a safe subset of the available SysRPL commands. The error checking that is a part of UserRPL commands, however, makes UserRPL programs noticeably slower than equivalent SysRPL programs. The UserRPL command SYSEVAL tells the calculator to process designated parts of a UserRPL program as SysRPL code.
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