68 lines
1.6 KiB
Rust
68 lines
1.6 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 call the `try!` macro 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 = try!(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 = try!(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 `try!` macro returns an `Err` early if the thing it's trying to
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// do fails, you can only use the `try!` macro in functions that have a
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// `Result` as their return type.
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// The error that you get if you run this code is:
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// ```
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// error: mismatched types:
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// expected `()`,
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// found `std::result::Result<_, _>`
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// ```
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// which is saying that the expected return type of the `main` function is
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// the empty tuple, but we tried to return a `Result`-- and that's happening
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// in the implementation of `try!`. The `main` function never has a return type,
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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 calling the `try!`
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// macro.
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