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Comment by pdubroy | Hacker News Reader

I think most people associate bytecode VMs / interpreters with general-purpose programming languages, but it's a surprisingly useful concept in other contexts.

Sometimes bytecode VMs appear in unexpected places! A few that I'm aware of:

- eBPF, an extension mechanism in the Linux kernel - DWARF expression language, with interpreters in debuggers like GDB and LLDB - The RAR file format includes a bytecode encoding for custom data transformation

More here: https://dubroy.com/blog/bytecode-vms-in-surprising-places/

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Bytecode is great for tiny domain languages and I use them in many projects.

Ultima IV used one to animate sprites on it's title screen map. For the next version of XU4 I implemented three bytecode interpreters to script the entire title sequence. There's a high level presentation interpreter, a GPU rendering interpreter, and one for the Ultima IV bytecode.

The original TeX Fonts stored their metrics in TFM (short for TeX font metrics) files, which contains a bytecode interpreter for calculating ligatures and kerning between characters. I learned about that when I tried reading the files myself.

From what I can tell, modern fonts using OpenType just have tables to accomplish something similar now, in the form of the GSUB and GPOS tables?

Documentation for the TFM format here: https://tug.org/TUGboat/Articles/tb02-1/tb02fuchstfm.pdf (search for lig/kern)

TrueType (and OpenType, which is an evolution of TT) absolutely includes a bytecode instruction set: https://developer.apple.com/fonts/TrueType-Reference-Manual/...

Oh yes, thank you for the link to that! Looks like that is an instruction set for representing the font glyphs themselves? I was talking about the instruction set in TFM which is for representing meta information, like ligatures and kerning between glyphs, and not for the actual glyphs. The glpyhs for the original TeX fonts are described using Metafont which is an interpreted language.

Glyphs are generally defined as pure outlines (the "glyf" table [1]), and the instruction set is an optional system for things like grid fitting. Ligatures, kerning etc. are normal tables.

[1] https://developer.apple.com/fonts/TrueType-Reference-Manual/...

I believe these fall under "interpreter pattern":

https://en.wikipedia.org/wiki/Interpreter_pattern

https://gameprogrammingpatterns.com/bytecode.html

https://wiki.c2.com/?InterpreterPattern

I first read about it from the "Design Patterns" book, which I would recommend to everyone.

They are frequently used in games to run scripting languages.

Yep, Greenspun's tenth rule:

Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp.

Also regular expression engines.

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