✨ Add a `--watch` flag to `gleam build`.

[hayleigh-dot-dev]

After a bit of a discussion about how cool elm reactor is the idea of having the compiler automatically rebuild whenever files change came up. Mostly opening this here so we have a ticket to track any of that work and a place for further discussion.

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Below are some reasons why this feature would be useful for a project like lustre.

• We could implement live reloading on top of esbuild's dev server so folks could have a quick iterative dev experience. Equivalent projects in elm have had to implement this feature from scratch by rolling their own fs watchers: this was a route lustre was considering taking too.

• we could implement live rebuilds of projects that want to build individual web components

If this feature also included a way to communicate with other processes in a similar fashion to how the language server works, we could also...

• re-run lustre_ssg to generate whole static sites iteratively.

[lpil]

Sounds good!

What commands would we want it for? run, test, build and check seem like obvious ones.

Making it a flag means there's yet another flag that can't be passed easily down into the Gleam program, and --watch seems like a common one that would be wanted. Perhaps it could be a command itself: gleam watch test

If this feature also included a way to communicate with other processes in a similar fashion to how the language server works, ...

What do you mean by this?

[hayleigh-dot-dev]

Making it a flag means there's yet another flag that can't be passed easily down into the Gleam program

No flags can be easily passed down into a gleam program in my experience:

% gleam run -m lustre/try --include-styles
error: Found argument '--include-styles' which wasn't expected, or isn't valid in this context

        If you tried to supply `--include-styles` as a value rather than a flag, use `-- --include-styles`

USAGE:
    gleam run --module <MODULE>

For more information try --help

But a watch command sounds fine in any case.

What do you mean by this?

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I'd like to be able to spawn gleam watch as a (os) process and have some way of it telling my program when/which files have changed/been compiled. Maybe that's just output on stdout or maybe its events on a socket, but some way of knowing when this stuff happened would be immensely useful I think.

I'd like to, for example, re-run lustre_ssg any time some source files change. If the compiler already has a filesystem watcher it feels a bit silly if I have to re-implement all that functionality (and it is seemingly non-trivial to do this cross-platform) so if the binary could just expose that info it'd be super great.

[lpil]

No flags can be easily passed down into a gleam program in my experience:

That's a bug with Clap when only flags are passed. It doesn't in any way impact how we design the CLI API.

I'd like to be able to spawn gleam watch as a (os) process and have some way of it telling my program when/which files have changed/been compiled.

Gleam won't offer anything like this as part of this feature as it means we're accidentally shifting into being a generic build tool rather than a specific one, and I want to do a lot of R&D before we consider moving into that space as it's extremely easy to make a rubbish generic build tool / task runner.

For this specific example you don't actually need anything new from the compiler to run the site generator, you just run gleam run -m lustre_ssg --watch (or whatever the command will be).

[matobet]

A bit related to this issue: I'd find support for gleam test --watch specifically an immensely useful feature for TDD-style rapid iteration and testing :+1:

[inoas]

A bit related to this issue: I'd find support for gleam test --watch specifically an immensely useful feature for TDD-style rapid iteration and testing 👍

https://hexdocs.pm/glacier/ does that.

[lpil]

@matobet yup we would want it for all the main commands