Cann't assign variables to ExpressibleByStringLiteral?

Question

Created Jan ’19

Replies 2

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Views 1.6k

Participants 3

I used xcode 10.1

enum PriceType {
    case cny
    case usd
}
extension PriceType: ExpressibleByStringLiteral {
    init(stringLiteral value: String) {
        self.init(string: value)
    }
    init(unicodeScalarLiteral value: String) {
        self.init(string: value)
    }
    init(extendedGraphemeClusterLiteral value: String) {
        self.init(string: value)
    }

    init(string: String) {
        switch string {
        case "0":
            self = .cny
        case "1":
            self = .usd
        default:
            switch Language.use {
            case .cn(.hans):
                self = "0"
            case .en, .cn(.hant):
                self = "1"
            }
        }
    }
    
    static var Symbol: String {
        let priceStr: String = "0"
        let priceType: PriceType = "priceType"
        let priceType1: PriceType = priceStr // erorr: Cannot convert value of type 'String' to specified type 'PriceType'
        return priceType.symbol
    }
    var symbol: String {
        switch self {
        case .cny:
            return ""
        case .usd:
            return "$"
        }
    }
}

when biuld, line 29 is erorr : Cannot convert value of type 'String' to specified type 'PriceType'

I hope can use :

     let priceStr: String = "0"
     let priceType1: PriceType = priceStr

How can I do? or it's cann't?

Answered by DTS Engineer in 347034022

I hope can use:

That’s not going to work. Note the Literal part of the name

ExpressibleByStringLiteral

. That protocol is only involved when dealing with literals. In your second code snippet the string literal is on line 1. Once you get to line 2 you’re dealing with a string value, and thus

ExpressibleByStringLiteral

never comes into play.

To see

ExpressibleByStringLiteral

in action, you need to use a literal. For example:

let p1: PriceType = "xxx"

Or:

let p2 = "xxx" as PriceType

Those are not particularly useful examples. The

ExpressibleByStringLiteral

facility works best when you already have established the type context in play. For example:

func discount(price: PriceType) -> PriceType {
    fatalError()
}
let p3 = discount(price: "xxx")

The

discount(price:)

function takes a parameter of

PriceType

, and that establishes the type context that allows you to pass in a string literal.

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Answer 1

Claude31 OP

Jan ’19

That is a bit convoluted, but I tested the following and it compiles:

       let priceType1: PriceType = PriceType(stringLiteral: priceStr) // erorr: Cannot convert value of type 'String' to specified type 'PriceType'

0

Answer 2

DTS Engineer OP

Apple