structopt | Parse command line arguments by defining a struct | Parser library

A working version is

Cargo.toml

Your problem lies with how you're handling messages after clients have connected:

I don't believe this is currently possible. I opened up an RFC for a parser that gets a vec::Drain, or a parser that get the rest of the command line. I also asked for alternative ways to accomplish this. The response was,

The clap infrastructure (which structopt only expose on top of struct attribute) doesn't support this as far as I know. Validators in clap (that is named parser in structopt, as they validate and parse) are implemented on each argument, not on a set of arguments.

You can convert a String into a OsString with an infallible conversion, but you can not convert a OsString into a String.

This is because an OsStr accepts things that are not valid Strings so there is no way to ensure an OsStr that structopt parses into can be converted to a String.

You likely don't need to parse() at all,

I don't think it is possible to do this with structopt. The idiomatic way to solve this problem is to instead use an Option instead of usize (as documented here):

Clap has a global() method for arguments that does what you want. You can access this method vis Structopt using raw attributes:

From the example, it seems you're getting the arguments from the command line. If you instead wanted the program to wait for a user to enter some text, interpret that text as a command and run it, and then wait for input again, until the user exits the program, then you'll probably want https://doc.rust-lang.org/std/io/struct.Stdin.html or possibly a higher level crate such as https://docs.rs/rustyline/8.0.0/rustyline/

If you were using stdin directly, you can call io::stdin().read_line() which will wait until the user has entered a line of text and pressed enter before the function will return. You can then parse that string to get the action to perform. The input/parsing/action code can be surrounded in a loop {} and one of the actions can be an exit command that will exit the loop.

After much messing around I think I have something that works, although I'm happy to accept other answers that are better as I feel this is a bit messy:

structopt fields can take default values, but there is also an ENV fallback which will go and get a default value from the envvar if the option is not provided explicitely.

If you can assume running on a POSIX system, then HOME will be set to the current user's home.

If you can not assume POSIX, then I think there are two ways:

• you can define the option as, well, an Option. If unspecified it will be set to None, and at use-site you can just swap out the None for the user's homedir
• define a custom type with all the right bits such that it defaults to the homedir if unspecified (see the structopt doc for how to do that, it involves FromStr and optionally Default and ToString)

Either way, you can use dirs::home_dir to get the home directory.