How to highlight HSL colors with `mini.hipatterns`

editor.lua

-- plugins/editor.lua
return {
	{
		"echasnovski/mini.hipatterns",
		event = "BufReadPre",
		opts = {
			highlighters = {
				hsl_color = {
					pattern = "hsl%(%d+,? %d+,? %d+%)",
					group = function(_, match)
						local utils = require("craftzdog.utils")
						local h, s, l = match:match("hsl%((%d+),? (%d+),? (%d+)%)")
						h, s, l = tonumber(h), tonumber(s), tonumber(l)
						local hex_color = utils.hslToHex(h, s, l)
						return MiniHipatterns.compute_hex_color_group(hex_color, "bg")
					end,
				},
			},
		},
	},
}

utils.lua

-- craftzdog/utils.lua
-- https://github.com/EmmanuelOga/columns/blob/master/utils/color.lua

local M = {}

local hexChars = "0123456789abcdef"

function M.hex_to_rgb(hex)
	hex = string.lower(hex)
	local ret = {}
	for i = 0, 2 do
		local char1 = string.sub(hex, i * 2 + 2, i * 2 + 2)
		local char2 = string.sub(hex, i * 2 + 3, i * 2 + 3)
		local digit1 = string.find(hexChars, char1) - 1
		local digit2 = string.find(hexChars, char2) - 1
		ret[i + 1] = (digit1 * 16 + digit2) / 255.0
	end
	return ret
end

--[[
 * Converts an RGB color value to HSL. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSL_color_space.
 * Assumes r, g, and b are contained in the set [0, 255] and
 * returns h, s, and l in the set [0, 1].
 *
 * @param   Number  r       The red color value
 * @param   Number  g       The green color value
 * @param   Number  b       The blue color value
 * @return  Array           The HSL representation
]]
function M.rgbToHsl(r, g, b)
	local max, min = math.max(r, g, b), math.min(r, g, b)
	local h = 0
	local s = 0
	local l = 0

	l = (max + min) / 2

	if max == min then
		h, s = 0, 0 -- achromatic
	else
		local d = max - min
		if l > 0.5 then
			s = d / (2 - max - min)
		else
			s = d / (max + min)
		end
		if max == r then
			h = (g - b) / d
			if g < b then
				h = h + 6
			end
		elseif max == g then
			h = (b - r) / d + 2
		elseif max == b then
			h = (r - g) / d + 4
		end
		h = h / 6
	end

	return h * 360, s * 100, l * 100
end

--[[
 * Converts an HSL color value to RGB. Conversion formula
 * adapted from http://en.wikipedia.org/wiki/HSL_color_space.
 * Assumes h, s, and l are contained in the set [0, 1] and
 * returns r, g, and b in the set [0, 255].
 *
 * @param   Number  h       The hue
 * @param   Number  s       The saturation
 * @param   Number  l       The lightness
 * @return  Array           The RGB representation
]]
function M.hslToRgb(h, s, l)
	local r, g, b

	if s == 0 then
		r, g, b = l, l, l -- achromatic
	else
		function hue2rgb(p, q, t)
			if t < 0 then
				t = t + 1
			end
			if t > 1 then
				t = t - 1
			end
			if t < 1 / 6 then
				return p + (q - p) * 6 * t
			end
			if t < 1 / 2 then
				return q
			end
			if t < 2 / 3 then
				return p + (q - p) * (2 / 3 - t) * 6
			end
			return p
		end

		local q
		if l < 0.5 then
			q = l * (1 + s)
		else
			q = l + s - l * s
		end
		local p = 2 * l - q

		r = hue2rgb(p, q, h + 1 / 3)
		g = hue2rgb(p, q, h)
		b = hue2rgb(p, q, h - 1 / 3)
	end

	return r * 255, g * 255, b * 255
end

function M.hexToHSL(hex)
	local hsluv = require("solarized-osaka.hsluv")
	local rgb = M.hex_to_rgb(hex)
	local h, s, l = M.rgbToHsl(rgb[1], rgb[2], rgb[3])

	return string.format("hsl(%d, %d, %d)", math.floor(h + 0.5), math.floor(s + 0.5), math.floor(l + 0.5))
end

--[[
 * Converts an HSL color value to RGB in Hex representation.
 * @param   Number  h       The hue
 * @param   Number  s       The saturation
 * @param   Number  l       The lightness
 * @return  String           The hex representation
]]
function M.hslToHex(h, s, l)
	local r, g, b = M.hslToRgb(h / 360, s / 100, l / 100)

	return string.format("#%02x%02x%02x", r, g, b)
end

function M.replaceHexWithHSL()
	-- Get the current line number
	local line_number = vim.api.nvim_win_get_cursor(0)[1]

	-- Get the line content
	local line_content = vim.api.nvim_buf_get_lines(0, line_number - 1, line_number, false)[1]

	-- Find hex code patterns and replace them
	for hex in line_content:gmatch("#[0-9a-fA-F]+") do
		local hsl = M.hexToHSL(hex)
		line_content = line_content:gsub(hex, hsl)
	end

	-- Set the line content back
	vim.api.nvim_buf_set_lines(0, line_number - 1, line_number, false, { line_content })
end

return M

[...] On the return MiniHipatterns.compute_hex_color_group(hex_color, "bg") I have a warning Undefined global 'MiniHiPatterns'.. I can't understand how is this line working, maybe that could be the source of my problem ?

In the original snippet, MiniHiPatterns is the plugin mini.patterns assigned to a variable.
Its definition isn't in the gist, which is why you're seeing an error.

You should define it yourself in the group function (or wherever else you need it):

group = function(_, match)
  local MiniHiPatterns = require('mini.hipatterns')`
  -- ...rest of function
end