This is mostly copy-pasta'ed from my comment here.

Just is now 1.0, and has a strong commitment to backwards-compatibility. In practice, backwards-compatibility was important even before 1.0, since there have been enough justfiles out there for a while now that breaking things would have been really annoying for users.

However, time marches forward, and there will always be backwards-incompatible changes that seem desirable to introduce, and old mistakes that would be nice to fix.

To be able to make changes while preserving backwards compatibility, I suggest we steal Rust's concept of an edition. An edition is an opt-in set of backwards-incompatible changes. For just, it would work like this:

1. We add a new feature that would be a breaking change, but we make it an opt-in setting, i.e. set good-thing := true, so that it doesn't break existing justfiles.
2. We fix some old behavior that was bad, but which people might have been relying on, but we make it an opt-in setting, i.e. set dont-do-bad-thing := true, so that it doesn't break existing justfiles.
3. Some time passes, and we get confident that good-thing and dont-do-bad-thing are good and useful enough that they should be true by default
4. We create a new setting called edition, that takes a number. If you don't set it, it defaults to set edition := 1, which corresponds to the way just works now.
5. We announce edition 2, which, if you do set edition := 2, turns on both good-thing and dont-do-bad-thing.

Pros:

• We can make changes that are backwards compatible
• We don't have to try to justify breaking edge-case behavior by telling ourselves "well this is really a bug so it's okay if we break existing justfiles"
• Users don't need to micromanage their settings, they can just do set edition := LATEST_EDITION and get good defaults
• No existing justfiles will be broken

Cons:

• Users have to put set edition := 2 at the top of their justfile or else they get edition 1.