Add timer:send_interval/3

[erikareads]

Can we add timer:send_interval/3 to gleam/erlang/process?

It behaves similarly to send_after, except it spins up a process that repeatedly sends the message every time interval. It's useful for a low-fidelity Tick.

Since Subject(a) is opaque, I can't implement it myself in projects without resorting to FFI workarounds unpacking the Subject tuple by hand.

[erikareads]

This would be the implementation, similar to send_after:

@external(erlang, "timer", "send_interval")
fn timer_send_interval(a: Int, b: Pid, c: msg) -> Timer

pub fn send_interval(subject: Subject(msg), delay: Int, message: msg) -> Timer {
  timer_send_interval(delay, subject.owner, #(subject.tag, message))
}

[lpil]

Could be cool! Is there a reason not to use send_after? It can help avoid overloading a process, but I guess most the time that's not a risk.

[erikareads]

1. The timer:send_interval function does exactly what I need, without me needing to do additional work. (Outside of needing to sidestep the opaque type currently)
2. It's not clear to me how you use send_after to achieve the same thing, and after OTP 25, timer is pretty efficient to use: https://www.erlang.org/doc/efficiency_guide/commoncaveats#timer-module

[lpil]

You'd use send_after again when you receive the previous tick message.

It's not that the timer is expensive, it's that if you ever take too long to handle a tick message you end up with multiple tick messages in the inbox, potentially never managing to catch up resulting in the inbox growing. This then has performance implications and any other messages sent may have delay before they are handled. With send_after this failure mode is impossible.

We could add this function, but I think send_after is what you want for TUIs given the short interval and want to remain responsive even when under high load.