Install require()ed executables

[yawn]

While very useful, I often find that I cannot use the require() function for executables if I also want to provide an installation recipe (which cannot run if the executable is not there because of require()).

Maybe we can introduce something like this:

foo := require("foo")

hello:
    {{ foo }} bar
    
[macos]
[install(foo)]
install-foo:
    brew install foo

When running just hello (without foo being installed) on Windows this would yield the current status quo:

error: Call to function `require` failed: could not find executable `foo`

The same on macOS would change the error message to this:

error: Call to function `require` failed: could not find executable `foo` - install it with `just install-foo`

Furthermore we could (should IMHO) add a configuration automatic-install flag (true by default) to just (including a matching environment variable) that automatically runs install recipes before they are needed. If that is too complex (not sure how this would pan out technically) this could be a great secondary feature on top of install().

This would give a great out of the box experience for getting started with complex projects.

Edit: changed "installs" keyword to "install" (singular)

[casey]

I think a simple alternative would be to allow require to take a second argument, which is a custom message, which is printed out if require fails:

foo := require("foo", "install it with `just install-foo")

$ just
error: Call to function `require` failed: could not find executable `foo`: install it with `just install-foo`

[yawn]

This is dramatically more simple and solves part of the problem, I like it!

It does not connect to the installation recipes (which I will almost always provide to make usage simpler) though. Without require pointing to a specific recipe we loose the chance to provide platform (e.g. macos) specific instructions (e.g. using brew).

Also we cannot do auto-installations of all missing executables.