Abstract Nonsense

A place for musings, observations, design notes, code snippets - my thought gists.

Irregular Expressions

computer-science

Someone at work asked if it’s possible to validate credit card numbers with the Luhn algorithm in regex.

Technically, a regular language could recognise valid fixed-length credit card numbers by brute-force enumerating all possible sequences. But as a more general solution, I don’t think DFAs can support the modular arithmetic required for arbitrary sequence lengths…

Possible or not, I feel incredibly nerd sniped.

Open uBanking

In Australia, we have the Consumer Data Right, a government mandated interoperability standard for the Banking (and now Energy) sector.

Also known as Open Banking, the idea is to ensure that banks expose APIs that allow safe access to your transactions history and account data without having to rely on hacky methods like screen scraping1.

Unfortunately, due to how the standard is enforced, it’s pretty tough to get access to your own transactions without going through an “approved data broker” that charges a premium for it (like Basiq), or a free app with limited export support like Frollo.

Well, if you’re a Ubank customer, it appears some genius has reverse-engineered the internal Ubank API from examining the API requests the frontend makes, and has exposed it as a Python-consumable API with Passkey authentication support. Genius.

As an aside, the API mentions Python “descriptors”, which I’d not heard of, but seems like an intriguing functionality worth exploring…

  1. A big player in this space is Yodlee: you provide your bank login password, and they scrape your transactions data on some interval. For obvious reasons, this breaches most banks’ T&Cs. ↩︎

Surely you're `jq`ing

software-engineering

Today I read through the jq manual cover-to-cover. For those unaware, jq is a popular CLI tool to query and manipulate JSON. It’s also a Turing-complete mini-language with nice functional semantics that fits well into the ethos of composable CLI tools.

It was an exemplar of well-written technical documentation. Concise, well-written, littered with examples, and linking to an interactive playground to test-and-learn.

Some learnings:

  1. It’s surprisingly functional! You can implement recursive functions and use higher-order functions! For example, here’s factorial in jq:
$ jq '[.,1]|until(.[0] < 1; [.[0] - 1, .[1] * .[0]])|.[1]'
  1. It supports string interpolation - this is really nice if you’re piping stuff from JSON into a string. Coupled with format strings this becomes frictionless:
$ echo '{"search":"hello; world"}' | jq -r '@uri "https://www.google.com/search?q=\(.search)"'
# https://www.google.com/search?q=hello%3B%20world
  1. You can define functions that accept functions1, and control structures that allow labelling.
$ echo '[[1,2],[10,20]]' | jq -r 'def addvalue(f): . + [f]; map(addvalue(.[0]))'
#[[1,2,1], [10,20,10]]
  1. You can traverse complex data structures with first-class pathing support. And you can easily modify nested structures to extend objects.
  2. For the category theorists/polyglots, there’s a denotational semantics paper written about jq.
  3. Bonus: You can build a Brainfuck interpreter in jq; and you can build a jq interpreter in jq - how’s that for bootstrapping!

Side note: This is one of my goals for 2025 - read through documentation end-to-end to develop mastery over tools. I’m trying to prioritise selectively depth over breadth.

  1. The line between “functions” and “filters” is a little blurry to me. ↩︎

Shellshocked? Brace yourselves!

software-engineering

I just discovered that to capture multiple lines of stdout from a shell script and redirect them to a file, you can simply wrap them in braces!

For example, my “Create a blog post via a GitHub Action triggered on an Issue creation” workflow uses this snippet:

{
  echo "---"
  jq 'del(.content)' "parsed_issue.json" | yq -P
  echo "---"
  echo ''
  
  # Inline "content" key for the body
  jq -r '.content' "parsed_issue.json"
} > content/micro-blog/"$FILENAME"

Developer Ergonomics

design software-engineering

“I wonder how much it is insightful to watch someone doing a workflow and to note when discomfort kicks in. That’s a really insightful thing to realize what matters from bitter experience, right? … Experience tells you when to worry about something and when not to worry about it” - Ben Sparks

That is - the rising discomfort of a programmer when employing a new tool, framework, or library is a good window into the ergonomics of how one uses your tool, framework, or library. Source: How I animate 3Blue1Brown | A Manim demo with Ben Sparks. The whole video is worth checking out! It’s a masterclass on how to construct a programatic-animation library and demonstrate how to work within it.

When I get a chance, I’d love to unpack the source code for how sprite-to-sprite transforms (part of 3B1B’s signature look) and the vectorised rendering engine are implemented with OpenGL.