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You can extend RangeReplaceableCollection, constrain its elements to Equatable and declare your method as mutating. If you want to return Bool in case the appends succeeds you can also make the result discardable. Your extension should look like this:

extension RangeReplaceableCollection where Element: Equatable {
    @discardableResult
    mutating func appendIfNotContains(_ element: Element) -> (appended: Bool, memberAfterAppend: Element) {
        if !contains(element) {
            append(element)
            return (true, element)
        }
        return (false, element)
    }
}

You can extend RangeReplaceableCollection, constrain its elements to Equatable and declare your method as mutating. If you want to return Bool in case the appends succeeds you can also make the result discardable. Your extension should look like this:

extension RangeReplaceableCollection where Element: Equatable {
    @discardableResult
    mutating func appendIfNotContains(_ element: Element) -> (appended: Bool, memberAfterAppend: Element) {
        if let index = firstIndex(of: element) {
            return (false, self[index])
        } else {
            append(element)
            return (true, element)
        }
    }
}

You will need to constrain your array elements to equatable and make your method mutating. If you want to return Bool in case the appends succeeds you can also make the result discardable. Your extension should look like this:

extension Array where Element: Equatable {
    @discardableResult
    mutating func appendIfNotContains(_ element: Element) -> (appended: Bool, memberAfterAppend: Element) {
        if !contains(element) {
            append(element)
            return (true, element)
        }
        return (false, element)
    }
}

You can extend RangeReplaceableCollection, constrain its elements to Equatable and declare your method as mutating. If you want to return Bool in case the appends succeeds you can also make the result discardable. Your extension should look like this:

extension RangeReplaceableCollection where Element: Equatable {
    @discardableResult
    mutating func appendIfNotContains(_ element: Element) -> (appended: Bool, memberAfterAppend: Element) {
        if !contains(element) {
            append(element)
            return (true, element)
        }
        return (false, element)
    }
}

You will need to constrain your array elements to equatable and make your method mutating. If you want to return Bool in case the appends succeeds you can also make the result discardable. Your extension should look like this:

extension Array where Element: Equatable {
    @discardableResult
    mutating func appendIfNotContains(_ element: Element) -> Bool {
        if !contains(element) {
            append(element)
            return true
        }
        return false
    }
}

You will need to constrain your array elements to equatable and make your method mutating. If you want to return Bool in case the appends succeeds you can also make the result discardable. Your extension should look like this:

extension Array where Element: Equatable {
    @discardableResult
    mutating func appendIfNotContains(_ element: Element) -> (appended: Bool, memberAfterAppend: Element) {
        if !contains(element) {
            append(element)
            return (true, element)
        }
        return (false, element)
    }
}