A further enhancement that would be nice is a way for the condition to be evaluated without invoking the shell e.g.:

foo:
  ?{{ assert(foo == 'yes') }}
  # the rest of the recipe

@liquidaty There would have to be a different syntax for this, since I believe the pattern of having an interpolation first is common.

For example, if someone wanted to use Python to perform a test, they might do:

python := '/usr/bin/python3'

foo:
  ?{{ python }} …

There would have to be a different syntax for this

Got it. Any syntax would be fine by me, but as I'm a new just user, that doesn't count for much.

That said, if it can be done, then, it seems logical (to me at least) to allow {{ assert(...) }} inside the recipe (and if it wasn't allowed before, it wouldn't seem to cause any compatibility issues to allow it). Currently, I can do this outside of a recipe:

a := if b == c { ... } else { ... }
d := assert(...)

and I can do this inside a recipe:

    {{ if b == c { ... } else { ... } }}

So it seemed logical to me that anything to the right of := outside a recipe, could also be put inside {{ }} inside a recipe, and that therefore this would also work inside a recipe:

    {{ assert(...) }}

Though I guess maybe the issue is that assert raises an error or returns a different data type than a shell command's exit code.

it seemed logical to me that anything to the right of := outside a recipe, could also be put inside {{ }} inside a recipe

Correct.

and that therefore this would also work inside a recipe:

    {{ assert(...) }}

Yes it does work. I have justfiles that use assert() like that.