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Instructions

"Result is a better version of the Option type that describes possible error instead of possible absence".

Create a structure called Flag which has the following elements:

• short_hand: String
• long_hand: String
• desc: String

This structure must have a function called opt_flag which initializes the structure. This function receives two strings references and returns a structure Flag. Here is an example of its usage:

    let d = Flag::opt_flag("diff", "gives the difference between two numbers");

    println!("short hand: {}, long hand: {}, description: {}", d.short_hand, d.long_hand, d.desc);
    // output: "short hand: -d, long hand: --diff, description: gives the difference between two numbers"

A second structure called FlagsHandler will be given which just has one element: flags: HashMap<(String, String), Callback> The following functions (methods) associated with FlagsHandler are for you to complete :

• add_flag, which adds to the HashMap the flag and the Callback function.
• exec_func, which executes the function using the flag provided and returns the result, which can be either a string with the value from the callback or an error.

A type called Callback will also be provided. It is a function which is going to be used in the structure and functions above. This function will be the callback for the flag associated to it.

You will have to create the following callback functions :

• div which converts the reference strings to floats and returns the Result, being the division of the floats or the standard (std) error: ParseFloatError.
• rem which converts the reference strings to floats and returns the Result, being the remainder of the division of the floats or the standard (std) error ParseFloatError.

Notions

• Result
• Method optflag

Expected Function

use std::collections::HashMap;

pub struct Flag {
    // expected public fields
}

impl Flag {
    pub fn opt_flag(l_h: &str, d: &str) -> Flag {

    }
}

pub type Callback = fn(&str, &str) -> Result<String, ParseFloatError>;

pub struct FlagsHandler {
    pub flags: HashMap<(String, String), Callback>,
}

impl FlagsHandler {
    pub fn add_flag(&mut self, flag: (String, String), func: Callback) {

    }
    pub fn exec_func(&mut self, flag: (String, String), argv: &[&str]) -> String {

    }
}

pub fn div(a: &str, b: &str) -> Result<String, ParseFloatError> {

}
pub fn rem(a: &str, b: &str) -> Result<String, ParseFloatError> {

}

Usage

Here is a program to test your function:

use banner::*;
use std::collections::HashMap;

fn main() {
    let mut handler = FlagsHandler { flags: HashMap::new() };

    let d = Flag::opt_flag("division", "divides the values, formula (a / b)");
    let r = Flag::opt_flag(
        "remainder",
        "remainder of the division between two values, formula (a % b)",
    );

    handler.add_flag((d.short_hand, d.long_hand), div);
    handler.add_flag((r.short_hand, r.long_hand), rem);

    println!("{:?}", handler.exec_func(("-d".to_string(), "--division".to_string()), &["1.0", "2.0"]));

    println!("{:?}",handler.exec_func(("-r".to_string(), "--remainder".to_string()), &["2.0", "2.0"]));

    println!("{:?}",handler.exec_func(("-d".to_string(), "--division".to_string()), &["a", "2.0"]));

    println!("{:?}",handler.exec_func(("-r".to_string(), "--remainder".to_string()), &["2.0", "fd"]));
}

And its output:

student@ubuntu:~/banner/test$ cargo run
"0.5"
"0"
"invalid float literal"
"invalid float literal"
student@ubuntu:~/banner/test$