Chapter 2 Closure Example won't compile

[andradei]

Thank you for this project. Has helped me a ton!

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File

2-structs-enums-lifetimes.md

Example

let mut answer = 42;
let set = |v| answer = v;

//let get = || answer;

set(58);
assert_eq!(answer, 58);

Won't compile. Compiler yells set must be mutable, then it complains about the assertion because answer is currently mutably borrowed by the closure.

One Solution

let mut answer = 42;

{
    let mut set = |v| answer = v;
    //let get = || answer;
    set(58);
}

assert_eq!(answer, 58);

Using the extra scope might throw beginners off a little... But at least they'll know there is more to closures (lifetimes and scopes).

[stevedonovan]

I agree, and actually decided that was the best way even before I read all the way. (I get embarrassed about compile failures in published work and fix them quickly). Mutable borrowing is a strict business! Thanks for the keen eye. The general solution I have is that I must always use rustc as my first proof-reader ;)

[andradei]

rustc is the best proof reader for Rust code there is :)

And let mut set = // closure... could be a great point to bring back later when you explain how Fn, FnMut, and FnOnce work (maybe a Closures in Depth section).

Thanks again for this great material.

[andradei]

Well, you were fast. I'm closing this unless you object.