Pre and post execute hooks for REPL

[malmaud]

A cool feature for the REPL would be an API to specify functions that run before or after each invocation of a command. 3rd party plugins could then do things like reload a module before each command is executed, or implement some kind of custom logging after execution. IJulia and IPython support this.

Thoughts?

[mbauman]

I've been mulling over an API for the REPL myself — for allowing users to interactively change their keybindings. One of the surprising difficult things here is simply grabbing a reference to the REPL. It's designed in an amazingly asynchronous and re-entrant manner such that there can be multiple invocations within the same process — both nested and (I think) alongside eachother. It's really cool. But it makes this kind of API very tricky.

[chrisvwx]

I'd love to have hooks to call ClobberingReload.jl before each command as described above by malmaud. This is one thing that Matlab is ahead of Julia in; I don't ever need to 'reload' or 'creload' files; it's done automatically for me through the magic of ... computers.

[cstjean]

We could work around the lack of built-in hooks via a custom REPL mode, that behaves like normal Julia, except with preexecution hooks. Unfortunately, Base.REPL is completely undocumented AFAICT, and the R mode is a 1000 lines of code. Can anyone guess how much work that would be? Obviously, we wouldn't want to lose other REPL features.

[StefanKarpinski]

REPL code seriously needs a refactor + documentation pass. Having a real API would be great.

[stevengj]

Is this closed by #34626? You can now push a custom AST transformation to any REPL backend instance. This could add pre-execution or post-execution code as well as transforming the user's code in any other way.

[KristofferC]

Seems powerful enough to warrant this to be closed.