In a previous post, I shortly discussed the concept of “effects” and the parallels between them. In an unrelated post since then, Yosh Wuyts writes about the problem of trying to write fallible code inside of an iterator adapter that doesn’t support it. In a previous discussion, the users of the Rust Internals forum hotly discuss the notion of closures which would maintain the so-called “Tennant’s Correspondence Principle” - that is, closures which support breaking to scopes outside of the closure, inside of the function they are in (you can think of this is closures capturing their control flow environment in addition to capturing variables).

I think it may not be obvious, but these discussions are all deeply related. They all arise from what is, in my opinion, one of the biggest problems with the design of the Rust language: its failure at 1.0 to give good support for handling common effects related to program control flow.

I’ve long been a proponent of having some sort of syntax in Rust for writing functions which return results which “ok-wrap” the happy path. This is has also always been a feature with very vocal, immediate, and even emotional opposition from many of our most enthusiastic users. I want to write, in one place, why I think this feature would be awesome and make Rust much better.

I don’t want to get into the details too much of the specific proposal, but here’s a sketch of one way this could work (there are a number of variables). We would add a syntactic modifier to the signature of a function, like this:

fn foo() -> usize throws io::Error {
    //..
}

This function returns Result<usize, io::Error>, but internally the return expressions return a value of type usize, not the Result type. They are “Ok-wrapped” into being Ok(usize) automatically by the language. If users wish to throw an error, a new throw expression is added which takes the error side (the type after throws in the signature). The ? operator would behave in this context the same way it behaves in a function that returns Result.