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63 lines
1.4 KiB
Rust
63 lines
1.4 KiB
Rust
// errors3.rs
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// This is a program that is trying to use a completed version of the
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// `total_cost` function from the previous exercise. It's not working though--
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// we can't use the `?` operator in the `main()` function! Why not?
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// What should we do instead? Scroll for hints!
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use std::num::ParseIntError;
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fn main() {
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let mut tokens = 100;
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let pretend_user_input = "8";
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let cost = total_cost(pretend_user_input)?;
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if cost > tokens {
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println!("You can't afford that many!");
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} else {
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tokens -= cost;
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println!("You now have {} tokens.", tokens);
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}
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}
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pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
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let processing_fee = 1;
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let cost_per_item = 5;
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let qty = item_quantity.parse::<i32>()?;
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Ok(qty * cost_per_item + processing_fee)
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}
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// Since the `?` operator returns an `Err` early if the thing it's trying to
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// do fails, you can only use the `?` operator in functions that have a
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// `Result` as their return type.
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// Hence the error that you get if you run this code is:
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// ```
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// error[E0277]: the `?` operator can only be used in a function that returns `Result` (or another type that implements `std::ops::Try`)
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// ```
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// So we have to use another way of handling a `Result` within `main`.
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// Decide what we should do if `pretend_user_input` has a string value that does
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// not parse to an integer, and implement that instead of using the `?`
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// operator.
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