Rust's Block Pattern

I have one better: the try block pattern.

https://doc.rust-lang.org/beta/unstable-book/language-featur...

More significantly the new variables x and y in the block are Drop'd at the end of the block rather than at the end of the function. This can be significant if:

- Drop does something, like close a file or release a lock, or

- x and y don't have Send and/or Sync, and you have an await point in the function or are doing multi-threaded stuff

This is why you should almost always use std::sync::Mutex rather than tokio::sync::Mutex. std's Mutex isn't Sync/Send, so the compiler will complain if you hold it across an await. Usually you don't want mutex's held across an await.

Reminds of Brian Wills OOP rant video from 2016. He advocates exactly for this pattern: https://www.youtube.com/watch?v=QM1iUe6IofM&t=2235s

There are some situations with tricky lifetime issues that are almost impossible to write without this pattern. Trying to break code out into functions would force you to name all the types (not even possible for closures) or use generics (which can lead to difficulties specifying all required trait bounds), and `drop()` on its own is of no use since it doesn't effect the lexical lifetimes.

This is one of those natural consequences of "everything is an expression" languages that I really like! I like more explicit syntax like Zig's labelled blocks, but any of these are cool.

Try this out, you can actually (technically) assign a variable to `continue` like:

let x = continue;

Funnily enough, one of the few things that are definitely always a statement are `let` statements! Except, you also have `let` expressions, which are technically different, so I guess that's not really a difference at all.

This seems like a great way to group semantically-related statements, reduce variable leakage, and reduce the potential to silently introduce additional dependencies on variables. Seems lighter weight (especially from a cognitive load perspective) than lambdas. Appropriate for when there is a single user of the block -- avoids polluting the namespace with additional functions. Can be easily turned into a separate function once there are multiple users.

Block expression https://doc.rust-lang.org/reference/expressions/block-expr.h...

Also in Kotlin, Scala, and nim.

You can also de-mut-ify a variable by simply shadowing it with an immutable version of itself:

let mut data = foo(); data.mutate(); let data = data;

May be preferable for short snippets where adding braces, the yielded expression, and indentation is more noise than it's worth.

It's idiomatic in Kotlin as well!

https://kotlinlang.org/docs/scope-functions.html

I love that this is part of the syntax.

I typically use closures to do this in other languages, but the syntax is always so cumbersome. You get the "dog balls" that Douglas Crockford always called them:

``` const config = (() => { const raw_data = ...

  ...

  return compiled;

const result = config.whatever;

// carry on

return result; ```

Really wish block were expressions in more languages.

GCC adds similar syntax as an extension to C: https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html

It's used all throughout the Linux kernel and useful for macros.

The first example given is not at all convincing. Its is clear as the sky that loading the config file should be be a separate function of its own. Coupling sending HTTP requests with it makes no sense.

The second example "erasure of mutability" makes more sense. But this effectively makes it a Rust-specific pattern.