Every turtle function should have an optional "done" callback last argument.

[davidbau]

Every turtle function should have an optional "done" callback last argument.

For example, "lt 90, -> write 'finished'" should wait until the left turn is finished before writing the message.

Here is why: when learning "await done defer()" with iced coffeescript, the question sometimes comes up, "why can't I wrap await and defer around any function?" By implementing callbacks uniformly around turtle methods that do queuing, the answer can be, "you can do that for any function that can queue an async process."

await lt 90, defer()

This would affect the signatures of a lot of methods: fd, bk, lt, rt, slide, movexy, moveto, jump, jumpto, turnto, home, pen, fill, dot, pause, st, ht, pu, pd, pe, pf, play, speed, wear, reload, twist, scale... However "read" methods that are synchronous such as getxy would not get the optional argument.

The change would look something like this:
https://github.com/PencilCode/jquery-turtle/compare/continuationstyle

Opinions?