Make the `else` branch of a conditional optional about just

[casey]

All just expressions return a value, so we could only make this work if the if branch diverges, e.g., does not return. So it could work for the error case, but couldn't work in general. One possibility is to add an error_if(CONDITION, MESSAGE) function.

[xavdid]

Awesome, thank you!

[Julian]

One reason I'm not totally sold is that it seems like this could be done in shell.

Personally I want to write literally the minimum amount of shell ever possible. I realize part of that is counter to using just at all, but certainly I try to minimize any shell I write regardless.

Here are two places (the other a few lines down, where the condition is inverted) where I basically use the workaround discussed here -- I do have to say I personally find it surprising to be missing but I'm simply someone who's used / evaluated just once (to write that justfile) -- and certainly ensure(...) would be better than nothing so as a user I'd certainly prefer that if that's what you'd be comfortable with!

[Julian]

Why is that? I too was tempted into simply using else { "" } to workaround this (before finding this issue). Surely even if the if doesn't diverge it could simply default to an else branch with some default value like "" as its "return value", which is highly likely to be ignored?

[mike-lloyd03]

I'm all for an error_if function. I'm currently using else {""} as a workaround but it's not very clean. I'm migrating most of my stuff over from make and enjoy the cleaner and simpler syntax of just so this solution seems like a good fit.

[casey]

I'm still not totally sold, but if we did implement this, how about a function called ensure(CONDITION, MESSAGE), which takes a boolean CONDITION, and errors out with MESSAGE if it's false?

One reason I'm not totally sold is that it seems like this could be done in shell.

For example, for the use-case in the issue, you could do:

foo:
  test -e {{("Formula" / package + ".rb"}} || exit "no formula for package {{package}}"

[xavdid]

+1 - I'm using just's functions because I want to avoid finicky shell scripts.

I think ensure would work here! sort of like an assert in other languages

[casey]

assert is probably a better name, since it's more familiar. Yah, agree about shell being tricky. And even if the just syntax is slightly worse, being statically typed is a big advantage.

Okay, I think I'm sold. If anyone wants to take a crack at this, it would make a good first PR!

[xavdid]

Nice, appreciate it!

I won't have time to look at this for a few months (and I have to brush up on my Rust). But if it's still available then, I'll take a crack at it!