While I can't speak for MicroLua, this can indeed be accomplished with vanilla Lua.
There are two things in play here:
First, the _ENV variable here is special.
It is implicitly used to lookup any identifier that has no visible binding, locally or in the surrounding scopes.
Thus a script just containing print("Hello world") is really _ENV.print("Hello world").
Usually _ENV is just implicitly defined to be the global environment (available as _G), but it can be overridden within a lexical scope to a custom value.
Second, the jump labels just make use of the alternate function call syntax of object:method(args), which is equivalent to object.method(object, args).
The whitespace is non-significant, which allows it to be written like that.
In combination with metatables you can use _ENV to track variable reads, calls etc. within a function, which you can (abuse) to create DSLs.
You can get an idea of whats possible by just tracking what we can intercept with _ENV.
Adding the following code after the pio_timer function, and running the script with Lua 5.4, already gives us quite a bit for just the first three lines of the function.
local loggerMetatable = {}
function loggerMetatable:__bnot()
print("Called bnot on " .. rawget(self, "name"))
end
function loggerMetatable:__call(...)
local name = rawget(self, "name")
print("Invoked " .. name .. " with " .. #table.pack(...) .. " args")
-- Return another logger table to visualize variable interactions
return setmetatable({ name = name .. "(...)" }, loggerMetatable)
end
function loggerMetatable:__index(key)
local name = rawget(self, "name")
print("Accessed key " .. key .. " on " .. name)
-- Return another logger table to visualize variable interactions
return setmetatable({ name = name .. "." .. key }, loggerMetatable)
end
local pio_env = setmetatable({ name = "_ENV" }, loggerMetatable)
pio_timer(pio_env)
-- Output:
Accessed key public on _ENV
Accessed key start on _ENV
Invoked _ENV.public with 1 args
Accessed key set on _ENV.public(...)
Accessed key y on _ENV
Invoked _ENV.public(...).set with 3 args
Accessed key mov on _ENV
Accessed key y on _ENV
Accessed key y on _ENV
Called bnot on _ENV.y
Invoked _ENV.mov with 1 args
[...snip...]