A Dictionary of Single-Letter Variable Names

Haskell’s expressive type system means that type signatures can carry a lot of information. Haskell’s polymorphism means that you sometimes write functions that work across an enormous range of types, and are left wondering “what do I actually call my variables?”. It is often the case that there’s nothing to say beyond “this variable is a Functor”, or “this variable is a monadic action”, and so a single-letter variable name is appropriate. An unofficial and largely undocumented convention has emerged around these variable names, and so I wanted to write them all down in one place.

It should go without saying that single-letter variable names are not always the answer. Like point-free style, it sometimes obscures more than it helps and people get carried away with it. But when you have a highly polymorphic function and no good words to use, choosing the right letter can convey a surprising amount of meaning.

This dictionary is not and cannot be exhaustive. Variable naming often relies on context to convey information, and shorter variable names should only be used when they make sense in context. That context could be:

• The function’s (...) => context to reference “nearby” concepts, as in Monoid and Monad;
• Some nearby variable context, like when using (k, v) to reference the key and value of a Map entry; or
• Domain context. position a u t = u * t + (a * t * t) / 2 is reasonable if the reader knows you’re talking classical mechanics.

With the warnings out of the way, the dictionary is after the jump. The bulk of the dictionary documents type variables, where overly long variable names can blow out complicated type signatures. Important value-level variable names are also documented, and are explicitly labelled as such.

Travel Tip: USB-C Desktop Chargers

I have travelled a lot this year, and after yet another trip where I lugged too many things around, I’ve been thinking about ways to cut back. The classic guide for this is onebag.com, which covers a very interesting mix of techniques and some carefully-chosen lightweight gear that will take you to the farthest corners of the map. Perma-nomad Vitalik Buterin has his own take on living out of a 40L backpack; one of his key points is to run everything you can off of USB-C. The benefits should be obvious: you cut down the number of charging cables you need to carry and your power bank can recharge any of your devices. While I’m a bit of a luddite, I can see a lot of people travelling with at least a laptop and phone, and possibly also a tablet, earbuds, and/or a smartwatch, all of which need power.

Vitalik’s guide mentions a USB-C “wall wart” charger that he uses to power his stuff. I disagree with this choice, at least for international travel: I think you want a desktop USB-C charger that takes an IEC C7 (“figure 8”) cable.

I’m not aware of any good articles that spell out the “desktop charger + replaceable cable” trick and how to actually find a suitable one, so a full explanation, research procedure, and some tentative recommendations are spelled out in laborious detail after the jump.