Awesome tool, I'm mostly very happy with it, and this is a relatively minor QOL issue issue with two workarounds I mention below.

My use-case is something like:

build_dir := "build"
file_name := "out"

default arg="": (html arg) (pdf arg)

html arg="":
    pandoc input.md -o "{{build_dir}}/{{arg}}/{{file_name}}.html"

pdf arg="":
    pandoc input.md -o "{{build_dir}}/{{arg}}/{{file_name}}.pdf"

I would like a way to deduplicate the specific string formatting {{build_dir}}/{{arg}}/{{file_name}} so that my uses of it don't become out of sync. My current solution is to use arg as a just variable instead of an argument, i.e. arg := "" and file_path := build_dir + "/" + arg + "/" + file_name at the top of the Justfile. This is imperfect because it requires passing the argument to the CLI as just pdf arg=foo instead of just pdf foo, and also it puts into global scope something that shouldn't necessarily be there.

Another workaround is something like this:

[...]

default arg="": (_default (build_dir + "/" + arg + "/" + file_name))
_default arg: (html arg) (pdf arg)

html arg="":
    pandoc input.md -o "{{arg}}.html"

pdf arg="":
    pandoc input.md -o "{{arg}}.pdf"

But this is inelegant and doesn't allow the html and pdf targets to be run directly.

Is there a better way to accomplish this currently? If not, is it possible to add some syntax for declaring functions on strings? I'm imagining something like (this is obviously just a first draft at a syntax):

buildpath(s) := build_dir + "/" + s + "/" + file_name

pdf arg="":
    pandoc input.md -o "{{buildpath(arg)}}.pdf"

I haven't looked super closely into the code base so I'm not sure how easy this would be. I'd be happy to look into writing a PR for this if it's desirable and feasible. Regardless, thanks for a great tool.