Hi!

I would like to be able to detect within a justfile whether an optional import took place.

I found out a workaround to do that within recipe bodies, namely

# ./maybe.just
set export

IMPORTED := "true"

# ./justfile
import? 'maybe.just'

imported:
    ${IMPORTED:-false} && echo yes || echo no

however I did not find a way to have that information available in expressions.

If there's no way currently, here's a couple of things I tried that might be good features in their own right, and would solve this problem:

• when allow-duplicate-variables is set, one could have the position of the import be relevant for overriding. Reading the documentation of allow-duplicate-variables, I actually expected this to work:

  # ./maybe.just
  imported := "true"
  # ./justfile
  imported := ""
  import? "maybe.just"
  default:
      echo {{ if imported == "true" { "yes" } else { "no" } }}

  If imports are allowed anywhere in a just file, I would expect duplicates from imports to override previous definitions (if duplicates are allowed).
  If this held for recipes as well, I would probably even not need to use an if, and would just overwrite some recipes in maybe.just.

  # ./maybe.just
  default:
      echo yes
  # ./justfile
  default:
      echo no
  import? "maybe.just"

  Afterwards I could see this goes directly in contrast with the way imports are specified in the documentation... so maybe this route could be taken with a new directive like include/include??

• a ? postfix operator could allow using undefined variables: var? would evaluate to the value of var if var is defined, and to "" otherwise.

  # ./maybe.just
  imported := "true"
  # ./justfile
  import? "maybe.just"
  default:
      echo {{ if imported? != "" { "yes" } else { "no" } }}

• an imported function. It could be specified to return "true" for modules that were imported

  # ./maybe.just
  
  # ./justfile
  import? "maybe.just"
  default:
      echo {{ if imported('maybe.just') == "true" { "yes" } else { "no" } }}

• for completeness, though I don't particularly like such a solution, an optional variable name after an import? could be filled with "true" or "" depending on whether the import took place

  # ./maybe.just
  
  # ./justfile
  import? "maybe.just" imported
  default:
      echo {{ if imported == "true" { "yes" } else { "no" } }}