Feature request: Add support for --before/--then/--else flags

[felixSchl]

Pulp has a handful of very useful options to run a command when compilation starts, fails, and succeeds: --before <string>, --else <string>, and --then <string>, respectively. This is useful in combination with --watch in order to, for example, run some post-processing script on the output. Currently the only way to achieve the same is using an external file watcher and run spago build && foobar on every file change. Aside from the additional overhead of getting set up properly for every project, there's also the overhead of cold-starting a build every time (see also: #409, #110) which slows down rebuilds considerably.

[f-f]

Yeah this sounds reasonable! Though I'd be curious about your specific usecase, to see if we can provide some specific integration before opening up to the full before/then/else workflow

[felixSchl]

It's a very project specific workflow where we need to run a script over the generated JS to fix up some internal require paths based on the build output. To be honest I have not used --before at all and only discovered it when doing my research for opening this issue, but it seems to make sense to have if we do end up with some sort of --then and --else.

It's not a super urgent feature request by any means because it's really just been a workflow enhancement. That said, if you do think it's a worth while feature to bring in (in terms of maintenance cost going forward,) I'd be happy to take a stab at it.

[StephenWakely]

I used the --then flag to post a notification once pulp had finished building.

pulp --watch --then 'notify-send built' build -O -t ../dist/ps.js

It was quite handy as I didn't have to swap to the terminal to check to see when everything was ready to refresh the browser.

Would love to see this go into Spago.

[f-f]

Yeah this sounds good, let's go with the way pulp does it since it seems to be a nice design!

[felixSchl]

I wonder if we can achieve the same thing in a more composable manner. Imagine spago only reporting to stderr during --watch and only reporting build status updates line by line on stdout. Then one could compose quite easily downstream using ordinary shell tooling.

Imagine some structured output like this:

* Build starting
0 Build succeeded
1 Build failed

it would be trivial to do sth like:

spago build --watch | while read -r status; do ...; done

Then again, that precludes spago from cancelling long-running post-build commands. --then to pulp would be canceled when the user triggered a rebuild, which I thought was really useful, too.

[f-f]

@felixSchl yeah this is also a very interesting approach. Note that starting with #475 logging is starting to move to stderr, so this option is also totally viable (and good by me). On the other hand, the --before/--then/--else approach might be more flexible because of the cancelability aspect