Auto-rebuild on file change

[snoyberg]

Moving from an internal issue tracker:

Just an idea I had: what about having a command (maybe autobuild) that scans a directory for any changes to files (via fsnotify) and, on a change, automatically rebuilds? I know this is something that I would use regularly.

Not high priority, just an idea.

[chrisdone]

Yeah, it would be neat if Shake coud do this as it already knows what files to be tracking. @ndmitchell But I guess with a simple notification of "one file in this directory and subdirs changed" we can call build and it's fine.

There's definitely general demand for this; there are like 5 proper projects for recompiling like this out there.

[ndmitchell]

Via shakeLiveFiles you can get notified of which files Shake is interested in. There's no direct support for what you describe, but it is planned/vaguely thought about. Now I use file tracker in ghcid, it's much more likely.

[snoyberg]

If we want to do this properly, we'd have to go quite a bit deeper, like looking at all of the .hi files to track the addDependentFiles in them (something I'm dealing with right now on the yesod devel front).

[snoyberg]

Relevant links: yesodweb/yesod#955 (comment) and #105

[ndmitchell]

I would have thought a far simpler "anything under this directory changes, i rerun build" would be sufficient - as long as you get build fast enough. Generally people only put dependent files in the same folder as the project.

[snoyberg]

That could work. We'd probably get hit with some annoying corner cases, such as .git and dist, where there are lots of files and adding watchers for all of them could actually slow us down significantly. Special casing those may solve that problem though.

[chrisdone]

I suppose if your project generates a lot of files, watching the whole dir could come with a performance penalty? In any case watching the whole dir vs just the shakeLiveFiles vs more detail could be considered levels of implementation of the feature.

[snoyberg]

Working on this now, I want it too much to wait :)

[snoyberg]

Closing, this is implemented. If there are bugs, we can follow up in separate issues.