Swift enum - switch statement matching associated values warning-CodePudding

I have an enum declaring associated values in Swift, along with a method that is trying to get a value at a specific location in that entry:

enum myEnum {

    case entry0(Float=0.0, Float=0.0)

    ...

}

var entry0XValue: Float {
    switch self {
    case .entry0(let x, let _):
        return x
    default:
        return 0
    }
}

The code works, but the problem I'm having here is when I compile this, the compiler is giving this warning:

'let' pattern has no effect; sub-pattern didn't bind any variables

This is referring to the underscore in the case switches. Is there any way to rewrite this to remove this warning?

CodePudding user response：

Spell it like

case .entry0(let x, _):

Or like

case let .entry0(x, _):

which works more generally like:

case let .entry0(x, y):

CodePudding user response：

The effect of a let <subpattern> pattern is that it binds values to identifier patterns that appear in <subpattern>, when the pattern is matched against. For example, the x in .entry0(let x, let _) is bound to 0.0, when you match it against MyEnum.entry0(0.0, 0.0) in a switch statement. Another example: when you do let (x, y) = (1, 2), the x and y are bound to 1 and 2 respectively. Note that in this case, <subpattern> is a tuple pattern, with identifier patterns inside it.

The let pattern let _ "has no effect" because there are no identifier patterns in <subpattern> (the wildcard pattern, _, in this case). As a result, the let pattern will match if and only if <subpattern> matches, which means that you don't need let at all. You can just write <subpattern> directly.

case .entry0(let x, _):

This removes the warning.

Since the .entry(...) part is also a pattern (an enumeration case pattern), you can write that part as the <subpattern>:

case let .entry0(x, _):

For more info on the syntax of patterns, see the Patterns section of the language reference.