Using a default value provided by a function call

[hansl]

Is there currently a way to call a function to provide a default value (instead of a static string)? This is in the context of Clap's proc_macro.

We have some resolution logic (config file, plus global configuration, plus some environment stuff) for default values, and that would be very useful.

For a concrete example, we have a --file argument for a server, which takes a filename and validates it's a valid path (user can write to it). If it's not passed in, it should be read from an environment variable. If that's not specified, it should be in a configuration file. If it's not there, it should be at the root of the project (if the user is running inside a project). All else, it should be a temporary file (using temp_file()). We could use an Option<PathBuf> and set it after arg matching, but it complicates the code using it (unwrapping everywhere). Simplest is to panic during arg parsing (e.g. if user has no right to the fs), else just be a PathBuf, from the clap code directly.

I know there is a parse argument to the proc_macro, but AFAIK it only applies if there is a value passed by the user, not for default values.

[CreepySkeleton]

If you're OK with precomputing the default only once per arg, you can use Arg::default_value.

#[structopt(long, default_value = default_function())]
file: PathBuf

This will be translated into

arg
    .long("file")
    .default_value(default_function())

In other words, the function will be executed even before the parsing starts. If this function creates a temporary file of sorts, this may be undesirable.

clap doesn't have an ability to compute a default value "on demand", i.e only when needed. Not yet.

[hansl]

Okay, good to know this isn't supported. Do you want me to file an issue? I couldn't find one.

[pksunkara]

There is one issue for this discussion already. But there is another issue for the execution during parsing thing also.

[CreepySkeleton]

Simplest is to panic during arg parsing

I would really like to discourage you from using panic! for error reporting. It's supposed to be used only for bug detection, like "out of bounds access" or "unreachable code entered".

Better to use Result + ? (presumably along with one of the tons of third party helpers out there), then print the error to stderr and exit(ERR_CODE).

[hansl]

I use panic as a general term, but yes this would actually be checked by a clap validation function.

[pksunkara]

Here is the link to the hooks issue which should solve this. #1634