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# REPLs in strange places: Lua, LaTeX, LPeg, LPegRex, TikZ
Eduardo Ochs - IRC: edrx, edrx <http://anggtwu.net/>, <mailto:eduardoochs@gmail.com>

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Many years ago, when I started programming, my dream was to write
games. I failed miserably in that, but I became fascinated by
languages, and I discovered Forth - that was perfect for writing
languages whose syntax was as simple as possible. Then I switched to
GNU/Linux and I had a series of favorite languages; at some point I
discovered Lua, that became not only my favorite language but also my
favorite tool for implementing other languages. One of the main
libraries of Lua is something called LPeg, that lets "people"
implement complex parsers in just a few lines of code using PEGs -
Parsing Expression Grammars.

I've put the "people" in the last paragraph between quotes because for
many years I wasn't included in the "people who can implement complex
parsers with LPeg"&#x2026; lots of things in LPeg didn't make sense to me,
and I couldn't visualize how it worked. Something was missing - some
diagrams, maybe?

The main tool for drawing diagrams in LaTeX is something called TikZ,
that is HUGE - its manual has over 1000 pages. TikZ includes lots of
libraries and extensions, and each one of these libraries and
extensions extends TikZ's core language with some extra constructs.

I don't know anyone - except for a handful of experts - who knows what
is the "core language" of Tikz, that lies, or that should lie, below
all these extensions&#x2026; all of my friends who use TikZ are just
"users" of TikZ - they've learned some parts of TikZ by starting with
exemples, and by then modifying these examples mostly by trial and
error. In particular, no one among my friends knows how styles in TikZ
really work; styles are implemented using "keys", that are hard to
inspect from a running TeX - see [1] - and I found the chapter on "key
management" in the manual very hard to understand. It feels as if
something is missing from it&#x2026; some diagrams, maybe?

In my day job I am a mathematician. I work in a federal university in
Brazil, and besides teaching I do some research - mostly in areas in
which the papers and theses have lots of diagrams, of many different
kinds, and in which people use zillions of different programs to draw
their diagrams. Every time that I see those diagrams I think "wow, I
<span class="underline">need</span> to learn how to draw diagrams like that!", but until a few
months ago this seemed to be impossible, or very hard, or very
painful&#x2026;

This presentation will be about a point in which all these ideas
intersect. I am the author of an Emacs package called eev, that
encourages using REPLs in a certain way; Lua can be used in several
different styles, and if we use it in a certain way that most people
hate - with lots of globals, and with an implementation of OO that
makes everything inspectable and modifiable - then it becomes very
REPL-friendly; there is an extension of LPeg called LPegRex ([2],
[3]), that I found promising but hard to use, so I rewrote some parts
of it to make them more REPL-friendly, and to make it print its ASTs
in 2D ASCII art. The core of my presentation will be about how I am
using REPLs written in Lua to write grammars, parsers, and tools to
generate many kinds of diagrams, and how I am using these diagrams to
document both my own code and other people's programs - the parts of
them in which some diagrams seem to be missing. My hope is that people
will find these ideas easy to port to other languages besides Lua, to
other tools for generating diagrams besides LaTeX - SVG, maybe? - and
to other ways to use REPLs in Emacs besides eev. Some ideas in this
presentation were inspired by the blog post [4].

[1]
<https://tex.stackexchange.com/questions/663740/alternative-to-edef-c-pgfkeys-a>
[2] <https://github.com/edubart/lpegrex>
[3] <https://github.com/edubart/lpegrex/blob/main/parsers/lua.lua>
[4] <https://ianthehenry.com/posts/my-kind-of-repl/>

About the speaker:

I am this person here: http://anggtwu.net/eepitch.html


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