数组扩展通用Equatable-“无法使用…调用'=='

Question

I am trying to write an extension of the Swift Array, but I am getting some wierd errors as I try to compile.

My code:

extension Array
{
    func itemExists<T: Equatable>(item: T) -> Bool
    {
        for elt in self
        {
            if elt == item
            {
                return true
            }
        }
        return false
    }
}

Error:

Cannot invoke '==' with an argument list of type '(T, T)'

Why am I getting this? I am using the Equatable protocol?

What I also tried was:

extension Array
{
    func itemExists<T: Equatable>(item: T) -> Bool
    {
        var array:[T] = self
        for elt in array
        {
            if elt == item
            {
                return true
            }
        }
        return false
    }
}

Where I got a funny error:

'T' is not identical to 'T'

What am I missing? I read the Apple Documentation about it but I am already using the Equatable protocol to be able to use the == operator on 'T' .

Answer 1

With Swift 3.0 (and I think some of the later 2.x versions), you can solve this by referencing the associated type Element with a where clause:

extension Array where Element: Equatable
{
    func itemExists(item: Element) -> Bool
    {
        for elt in self
        {
            if elt == item
            {
                return true
            }
        }
        return false
    }
}

Answer 2

I found my solution.

As MartinR alreary said, my method would only be applicable to arrays of Equatable elements, and you cannot write a method on a generic type that is more restrictive on the template.

There are two ways - writing my code as a function and not as an extension, or use this trick:

extension Array
{
    func itemExists<T: Equatable>(item: T) -> Bool
    {
        return self.filter({$0 as? T == item}).count > 0
    }
}