Practice Error Handling on rustlings.

error1.rs

// 5 tokens, and whenever you purchase items there is a processing fee of 1
// token. A player of the game will type in how many items they want to buy,
// and the `total_cost` function will calculate the total number of tokens.
// Since the player typed in the quantity, though, we get it as a string-- and
// they might have typed anything, not just numbers!

// Right now, this function isn't handling the error case at all (and isn't
// handling the success case properly either). What we want to do is:
// if we call the `parse` function on a string that is not a number, that
// function will return a `ParseIntError`, and in that case, we want to
// immediately return that error from our function and not try to multiply
// and add.

// There are at least two ways to implement this that are both correct-- but
// one is a lot shorter! Scroll down for hints to both ways.

use std::num::ParseIntError;

pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
    let processing_fee = 1;
    let cost_per_item = 5;
    let qty = item_quantity.parse::<i32>();
    match qty {
        Ok(x) => Ok(x * cost_per_item + processing_fee),
        Err(x) => Err(x)
    }

}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn item_quantity_is_a_valid_number() {
        assert_eq!(
            total_cost("34"),
            Ok(171)
        );
    }

    #[test]
    fn item_quantity_is_an_invalid_number() {
        assert_eq!(
            total_cost("beep boop").unwrap_err().to_string(),
            "invalid digit found in string"
        );
    }
}

















// One way to handle this is using a `match` statement on
// `item_quantity.parse::<i32>()` where the cases are `Ok(something)` and
// `Err(something)`. This pattern is very common in Rust, though, so there's
// a `?` operator that does pretty much what you would make that match statement
// do for you! Take a look at this section of the Error Handling chapter:
// https://doc.rust-lang.org/stable/book/second-edition/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
// and give it a try!

option1.rs

// option1.rs
// This example panics because the second time it calls `pop`, the `vec`
// is empty, so `pop` returns `None`, and `unwrap` panics if it's called
// on `None`. Handle this in a more graceful way than calling `unwrap`!
// Scroll down for hints :)

fn main() {
    let mut list = vec![3];

    let last = list.pop().unwrap();
    println!("The last item in the list is {:?}", last);

    let second_to_last = list.pop().unwrap_or(3);
    println!("The second-to-last item in the list is {:?}", second_to_last);
}

























// Try using a `match` statement where the arms are `Some(thing)` and `None`.
// Or set a default value to print out if you get `None` by using the
// function `unwrap_or`.
// Or use an `if let` statement on the result of `pop()` to both destructure
// a `Some` value and only print out something if we have a value!

result1.rs

// result1.rs
// Make this test pass! Scroll down for hints :)

#[derive(PartialEq,Debug)]
struct PositiveNonzeroInteger(u64);

#[derive(PartialEq,Debug)]
enum CreationError {
    Negative,
    Zero,
}

impl PositiveNonzeroInteger {
    fn new(value: i64) -> Result<PositiveNonzeroInteger, CreationError> {
        if value == 0 {
            Err(CreationError::Zero)
        }else if value < 0{
            Err(CreationError::Negative)
        }else{
            Ok(PositiveNonzeroInteger(value as u64))
        }

    }
}

#[test]
fn test_creation() {
    assert!(PositiveNonzeroInteger::new(10).is_ok());
    assert_eq!(Err(CreationError::Negative), PositiveNonzeroInteger::new(-10));
    assert_eq!(Err(CreationError::Zero), PositiveNonzeroInteger::new(0));
}
















// `PositiveNonzeroInteger::new` is always creating a new instance and returning an `Ok` result.
// It should be doing some checking, returning an `Err` result if those checks fail, and only
// returning an `Ok` result if those checks determine that everything is... okay :)