NSURL extension to adopt StringLiteralConvertible

Question

According to this post from NSHipster, we have extended the NSURL class to initialize NSURL objects using literals like this:

// the following is a full fledged NSURL object
let url: NSURL = "http://nshipster.com/"

Unfortunately, the post was written when Swift was first announced and it no longer compiles.

I was able to get my own custom object to conform to StringLiteralConvertible, and it looked like this:

final class Dog {
  let name: String

  init(name: String) {
    self.name = name
  }
}

// MARK: - StringLiteralConvertible
extension Dog: StringLiteralConvertible {
  typealias UnicodeScalarLiteralType = StringLiteralType
  typealias ExtendedGraphemeClusterLiteralType = StringLiteralType

  convenience init(unicodeScalarLiteral value: UnicodeScalarLiteralType) {
    self.init(stringLiteral: value)
  }

  convenience init(extendedGraphemeClusterLiteral value: ExtendedGraphemeClusterLiteralType) {
    self.init(stringLiteral: value)
  }

  convenience init(stringLiteral value: StringLiteralType) {
    self.init(name: value)
  }
}

This works great. For example, the following 2 lines of code will create a Dog object:

let dog = Dog(name: "Bob")
let dog: Dog = "Bob"

Unfortunately, using this strategy via extending NSURL was met with errors:

extension NSURL: StringLiteralConvertible {
  public typealias UnicodeScalarLiteralType = StringLiteralType
  public typealias ExtendedGraphemeClusterLiteralType = StringLiteralType

  convenience public init?(unicodeScalarLiteral value: UnicodeScalarLiteralType) {
    self.init(stringLiteral: value)
  }

  convenience public init?(extendedGraphemeClusterLiteral value: ExtendedGraphemeClusterLiteralType) {
    self.init(stringLiteral: value)
  }

  convenience public init?(stringLiteral value: StringLiteralType) {
    self.init(string: value)
  }
}

I've been trekking through the compiler errors, solving them 1 guess at a time. However, I can't get past the following error that occurs for each of the initializers:

Initializer requirement 'init(...)' can only be satisfied by a 'required' initializer in the definition of non-final class 'NSURL'

Adding the required keyword will give the error that you may not declared required initializers within extensions.

Looking for some directions :|

Answer 1

StringLiteralConvertible support

Unfortunately, StringLiteralConvertible support for NSURL seems to be not possible in the current Swift version (2.2). The closest I can get is the following:

extension NSURL: StringLiteralConvertible {

    public convenience init(stringLiteral value: String) {
        self.init(string: value)!
    }

    public convenience init(extendedGraphemeClusterLiteral value: String) {
        self.init(string: value)!
    }

    public convenience init(unicodeScalarLiteral value: String) {
        self.init(string: value)!
    }

}

But the compiler complains:

Playground execution failed: OS X Playground.playground:5:24: error: initializer requirement 'init(stringLiteral:)' can only be satisfied by a `required` initializer in the definition of non-final class 'NSURL'
    public convenience init(stringLiteral value: String) {
                       ^
OS X Playground.playground:3:24: error: initializer requirement 'init(extendedGraphemeClusterLiteral:)' can only be satisfied by a `required` initializer in the definition of non-final class 'NSURL'
    public convenience init(extendedGraphemeClusterLiteral value: String) {
                       ^
OS X Playground.playground:7:24: error: initializer requirement 'init(unicodeScalarLiteral:)' can only be satisfied by a `required` initializer in the definition of non-final class 'NSURL'
    public convenience init(unicodeScalarLiteral value: String) {

And required initializers cannot be implemented in an extension.

Alternative solution

We can simplify string-to-URL conversion from the other side!

extension String {

    var url: NSURL? {
        return NSURL(string: self)
    }

}

var url = "http://google.coom/".url
print(url?.scheme) // Optional("http")