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πŸ“Ÿ HP-RPL β€” The Stack-Based Language Hidden Inside Classic HP Calculators

#architecture #computerscience #programming

What is HP-RPL?

HP-RPL (Reverse Polish Lisp) is a hybrid stack-based programming language created by Hewlett-Packard for their advanced graphing calculators. The language combines two unusual design philosophies: Forth-style postfix stack execution and Lisp-style symbolic processing.

Because of this hybrid model, HP-RPL is capable of symbolic algebra, numeric computing, matrix manipulation, graphing, recursion, and even creating custom calculator apps β€” all inside a handheld device.

Specs

Language Type: Stack-based symbolic language

Released: Late 1980s

Used In: HP48, HP49, HP50 calculator series

Execution Model: Postfix evaluation with symbolic expression support

Typing: Dynamic, polymorphic

Primary Purpose: Scientific computing, calculus automation, engineering logic

Example Code (Hello World) 

"Hello, HP-RPL!" 1 DISP
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Math example:

3 4 + 2 * 1 +
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(Output: 15)

How It Works

HP-RPL uses a data stack for execution. Values are pushed first, then operators act on them. Instead of writing:

add(3,4)
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you write:

3 4 +
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The stack now contains: 7

The language supports:

Feature Available
Lists, matrices, symbolic algebra βœ”
Recursive functions βœ”
User-defined menus βœ”
Anonymous functions βœ”
Hardware buttons + UI customization βœ”

Conditional example:

IF DUP 10 > THEN "BIG" ELSE "SMALL" END

Strengths

  • Extremely powerful for symbolic and numeric computation
  • Built into millions of engineering calculators
  • Supports stack logic, functional style, and symbolic reasoning
  • Instant full execution on hardware β€” no compilation cycle

Weaknesses

  • Steep learning curve and unusual syntax
  • Debugging requires stack awareness
  • Mostly useful only in calculator ecosystems
  • Limited modern toolchain support

Where to Run

HP-RPL can run on:

  • Physical HP48/49/50 calculators
  • Emulator apps like Emu48, x48, HP-GX
  • TIO.run (partial sandbox support)

Should You Learn It?

βœ” Yes β€” if you enjoy retro computing, symbolic math, Forth/Lisp hybrids, or calculator hacking.

βœ– No β€” if you need a modern programming language for general software development.

Summary

HP-RPL is one of the most unusual yet capable embedded programming languages ever created. Blending postfix execution with symbolic reasoning, it turned calculators into programmable mathematical workstations β€” long before smartphones or microcontrollers became common.

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