Naming Convention for Optional Binding

Naming convention for optional binding

Just use the same name:

if let bananasInBarrel = bananasInBarrel {
  print("We have \(bananasInBarrel) bananas in the barrel.")
}

Don't use hungarian notation - the compiler will complain if you are using an unwrapped optional.

Is redeclaring an optional parameter using optional binding supported?

Yes, that is generally a recommended pattern in swift.

There's generally no need to cast a passed optional to an internal variable unless you're wanting to mutate it internally, and it looks like you're using guard properly, when you're wanting to exit a function before any other code runs if the parameter is nil.

Are there Rust variable naming conventions for things like Option T ?

I'm unsure if there is a convention per say, but I often see (and use) maybe for Options. i.e.

let maybe_thing: Option<Thing> = ...
let thing: Thing = ...

Also, in regards to your use of u and user in this situation, it is fine to use user in both places. i.e.

let user = match maybe_user {
    Some(user) => user,
    ...

This is because the match expression will be evaluated prior to the let assignment.

However (slightly off topic) @Manishearth is correct, in this case it would be nicer to use or_else. i.e.

let user = maybe_user.or_else(|| new("guest", "guest")).unwrap();

I'd recommend becoming familiar with the rest of Option's methods too as they are excellent for reducing match boilerplate.

How to assign an optional Binding parameter in SwiftUI?

@Binding var searchTxt: String?

init(searchTxt: Binding<String?>?) {
    self._searchTxt = searchTxt ?? Binding.constant(nil)
}

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Update: I prefer this one. TextField("", text: $text ?? "default value")

https://stackoverflow.com/a/61002589/4728060

func ??<T>(lhs: Binding<Optional<T>>, rhs: T) -> Binding<T> {
    Binding(
        get: { lhs.wrappedValue ?? rhs },
        set: { lhs.wrappedValue = $0 }
    )
}

Optional Binding, Capturing References and Closures [possible bug]

The error you're receiving is a little misleading, but the underlying problem here is that you're trying to do something that Swift doesn't support. In Swift, you can't use the result of assignment in a condition.

That being said, both of your proposed alternative methods will work, although I tend to think that the first of the two is a little more Swifty.

What is the swift underscore's official name?

It's called a wildcard pattern:

Wildcard Pattern

A wildcard pattern matches and ignores any value and consists of an underscore (_). Use a wildcard pattern when you don’t care about the values being matched against. For example, the following code iterates through the closed range 1...3, ignoring the current value of the range on each iteration of the loop:

for _ in 1...3 {
   // Do something three times.
}

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Patterns.html#//apple_ref/doc/uid/TP40014097-CH36-ID420

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