CODEWITHSUNDEEP
swiftswift3iteratorsequence
horseshoe7
horseshoe7

Reputation: 2817

Why can't I get type-specific information in my Sequence extension?

Why does this work, where PageBreak is a NSManagedObject

extension Sequence where Iterator.Element : PageBreak {

    var landscape: [PageBreak] {
        return self.filter({ (pageBreak) -> Bool in
            return !pageBreak.isPortraitOrientation
        })
    }

    var portrait: [PageBreak] {
        return self.filter({ (pageBreak) -> Bool in
            return pageBreak.isPortraitOrientation
        })
    }
}

But not this:

extension Sequence where Iterator.Element : String {
    var onlyDumbOnes: [String] {
        return self.filter({ (string) -> Bool in
            if string.hasPrefix("Dumb") {
                return true
            }
            return false
        })
    }
}

The compiler fails. Value of type 'Self.Iterator.Element' has no member 'hasPrefix'

So it doesn't seem to know that we're dealing with String objects.

Upvotes: 4

Views: 45

Answers (1)

vacawama
vacawama

Reputation: 154603

String is a struct and not a class or a protocol, so the syntax Iterator.Element : String doesn't make sense since Iterator.Element can't be a subclass of String or implement the protocol String. Instead, use Iterator.Element == String:

extension Sequence where Iterator.Element == String {
    var onlyDumbOnes: [String] {
        return self.filter { (string) -> Bool in
            if string.hasPrefix("Dumb") {
                return true
            }
            return false
        }
    }
}

Upvotes: 4

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