How to Create a Clap Struct without StuctOpts

help
#1

Hello Rust Folks,

I have been attempting to create a command-line application with Clap and I am having trouble implementing the creation of the Matches() struct with the usage of a function.

Context: I am fully able to create and utilise the clap application within main(). However, for the sake of test-ability, and the desire for a clean code base, I would like to make some sort of abstraction layer so i can create the clap application within tests without repeating the same instantiation commands over and over again.

Specifically, I can make it work when I do the specified example format:

fn main(){
    let matches = App::new("example app").get_matches();
}

However, I really want to create something where I can simply call a function and return either the full app, or the matches.

fn make_app()->Matches{
    let matches = App::new("example app").get_matches();
}

fn main(){
    app = make_app()
}

Whenever I try the second variation, the compiler complains that the matches or app struct have temporary values that are dropped once the function is completed.

I know this has to do with lifetimes and the borrowing system. I've read the rust book chapter on this time and time again and I feel i understand what it's trying to say here. I understand that this is failing because matches is really a reference which dies at the end of the function.

But this leaves me with the question of how do I actually instantiate this outside of main? Is this where macros start coming in? Or is clap simply not designed to be instantiated outside of main and this means I need to start considering structopts to get the functionality i'm hoping for?

I can do structopts, but I just have the format built out nicely in YAML for the app and it feels like I should be able to abstract this part out without needing to rely on a completely different crate.

Thanks for your time and expertise.

Answer Found:

The problem I was running into was the implementation of the YAML API which could only use borrowed references, thus, making clean code implementations nearly impossible witout the usage of leak.

As such, i just need to rebuild the app using something else.

Thank you to all of the great responses I recieved and for following up even after the original question evolved.

#2

Maybe you could try structopt::StructOpt - Rust from_clap ?

#3

I think this would work if you restructure it like this. get_matches is just a call to get_matches_from(std::env::args_os) in the library.

use clap::{App,ArgMatches};

fn make_app<'a>(args: &'a mut std::env::ArgsOs)->ArgMatches<'a>{
    App::new("example app").get_matches_from(args)
}

fn main(){
    let app = make_app(&mut std::env::args_os());
}
1 Like
#4

I would recommend returning an App<'static, 'static> instead, this will allow you to use App::get_matches_from to provide the args from a vec in your tests: clap::App - Rust

#5

i tried doing this but it's still throwing the same reference issue:

fn create_app_from_yaml()->App<'static,'static>{
    let yaml = load_yaml!("file/path/string");
    App::from(yaml)
}

it throws a "contains reference that is owned by this functiuon" error.

#6

Everything that you create and borrow inside function has a lifetime limited to this function, and can't leave it. Lifetime annotations don't affect what happens, they only describe what the code is doing anyway. And your description as App<'static, 'static> is incorrect, because it says that your temporary yaml variable inside the function lives forever (the lifetime in the return type is the same as lifetime of the App::from arg, which makes it apply to the &yaml borrow).

If you want to leak some memory ('static is more or less leaked memory), then you can do it like this:

fn create_app_from_yaml()->App<'static,'static>{
    let yaml = load_yaml!("file/path/string");
    App::from(Box::leak(Box::new(yaml)))
}

but the interface of App is meant to be used like this:

fn create_app_from_yaml(yaml: &'a Yaml)->App<'a,'a>{  
    App::from(yaml)
}

fn main() {
   let yaml = load_yaml!("file/path/string");
   let app = create_app_from_yaml(&yaml);
}
1 Like
#7

Thank you!

After several hours, I was getting something like this. But this leaves the question of can I abstract the from_yaml call into its' own function?

My ultimate goal is something that makes testing these two functions easy in multiple cases (as well as creating a sense of clean code) so i'm wondering if I would be better off just defining the app from the creation function itself.

#8

Unfortunately clap's yaml API only works with borrowed Yaml structs, meaning separating the Yaml loading and App creation into its own function requires some workaround, such as Box::leak. I didn't realise this when posting as I hadn't used it for myself...

1 Like
#9

Okay,

This gave me the answer I was looking for, i would rather not mess around with leak logic, so it may be better to just build the app within my function.