diff --git a/src/libstd/primitive_docs.rs b/src/libstd/primitive_docs.rs index ce4bbfffc2e4..8905f7c9e160 100644 --- a/src/libstd/primitive_docs.rs +++ b/src/libstd/primitive_docs.rs @@ -112,6 +112,8 @@ mod prim_bool { } /// /// # `!` and generics /// +/// ## Infalliable errors +/// /// The main place you'll see `!` used explicitly is in generic code. Consider the [`FromStr`] /// trait: /// @@ -140,9 +142,59 @@ mod prim_bool { } /// [`Ok`] variant. This illustrates another behaviour of `!` - it can be used to "delete" certain /// enum variants from generic types like `Result`. /// +/// ## Infinite loops +/// +/// While [`Result`] is very useful for removing errors, `!` can also be used to removed +/// successes as well. If we think of [`Result`] as "if this function returns, it has not +/// errored," we get a very intuitive idea of [`Result`] as well: if the function returns, it +/// *has* errored. +/// +/// For example, consider the case of a simple web server, which can be simplified to: +/// +/// ```ignore (hypothetical-example) +/// loop { +/// let (client, request) = get_request().expect("disconnected"); +/// let response = request.process(); +/// response.send(client); +/// } +/// ``` +/// +/// Currently, this isn't ideal, because we simply panic whenever we fail to get a new connection. +/// Instead, we'd like to keep track of this error, like this: +/// +/// ```ignore (hypothetical-example) +/// loop { +/// match get_request() { +/// Err(err) => break err, +/// Ok((client, request)) => { +/// let response = request.process(); +/// response.send(client); +/// }, +/// } +/// } +/// ``` +/// +/// Now, when the server disconnects, we exit the loop with an error instead of panicking. While it +/// might be intuitive to simply return the error, we might want to wrap it in a [`Result`] +/// instead: +/// +/// ```ignore (hypothetical-example) +/// fn server_loop() -> Result { +/// Ok(loop { +/// let (client, request) = get_request()?; +/// let response = request.process(); +/// response.send(client); +/// }) +/// } +/// ``` +/// +/// Now, we can use `?` instead of `match`, and the return type makes a lot more sense: if the loop +/// ever stops, it means that an error occurred. +/// /// [`String::from_str`]: str/trait.FromStr.html#tymethod.from_str /// [`Result`]: result/enum.Result.html /// [`Result`]: result/enum.Result.html +/// [`Result`]: result/enum.Result.html /// [`Ok`]: result/enum.Result.html#variant.Ok /// [`String`]: string/struct.String.html /// [`Err`]: result/enum.Result.html#variant.Err