HSL to hex codes in Nix

Over the past month, I’ve been configuring my NixOS system.¹ In preparation for theming (and doing it well), I wanted to be able to specify colors in the HSB² format and have them be converted to RGB hex codes. So, I present to you, after a day or two worth of work, said function, in the hopes that it may be useful to you in some way (also, it’s neat to share random things like this):

# Converts HSB to RGB in hexadecimal form
# Note that B and V (brightness and value respectively) are the same value
# We implement the following algorithm:
# https://en.wikipedia.org/wiki/HSL_and_HSV#HSV_to_RGB_alternative
# h - A value from 0 to 360
# s - A value from 0 to 1
# v - A value from 0 to 1
hsbToHex = h: s: b:
	let
		# Nix lacks a built-in modulo operator, so we introduce our own.
		# We used trunctuated division, the variant used by JavaScript.
		# See: https://en.wikipedia.org/wiki/Modulo#Variants_of_the_definition
		mod = a: b: a - b * builtins.floor (a / b);

		# Get the higher or lower of two values
		max = x: y:
			if x < y then y
			else x;
		min = x: y:
			if x < y then x
			else y;

		# Round a number to the nearest integer
		round = num:
			let
				floored = builtins.floor num;
			in
				# Round up for 0.5 and above
				if (num - floored < 0.5)
				then floored
				else builtins.ceil num
		;

		f = n:
			let
				# Explicitly use floating points, otherwise things get rounded
				k = mod (n + h / 60.0) 6.0;
			in
				format (
					b - b*s*(max 0 (min k (min (4.0-k) 1)))
				)
		;

		# Convert an integer from 0 to 255 to a two-digit hex value
		format = i:
			let
				ten = round (i * 255);
				hex = lib.trivial.toHexString ten;
			in
				if builtins.stringLength hex == 1
				then "0" + hex
				else hex;
		
	in
		"#" + builtins.concatStringsSep "" [(f 5) (f 3) (f 1)];

I’m not sure if it’s the best way to do this in the context of using Nix (in other words, whether the code structure would be considered a “best practice”), but it works for me.

The comments come off to me as a displaying a bit more confidence about certain subjects than the confidence I have in myself as to how much I actually know/understand said subjects. For example, floating point numbers. While I’m pretty sure what I wrote is correct, it’s not an issue I run into often (I primarily do stuff³ in TypeScript) and I had to debug for a little bit in order to figure out why my colors were off.

Another would be the “we implement the following algorithm” bit. I was originally on a StackOverflow page about implementing the above function in JavaScript, but went to the Wikipedia page and implemented it myself, partly out of a desire to prove that I could do this myself. This is the first time I’ve read it.

I’d like to make the Git repository holding my NixOS configuration public, but I’ve run into issues regarding macOS-specific firmware—currently included within the repository—that I don’t want to publish.

Oh yeah, one last thing, the comments you (hopefully) see at the bottem of this page are rendered via Sukopon, a small project I had abandoned several months ago due to packaging-related issues.