How does ArrayLiteralElement get defaulted in SetAlgebra?

CTMacUser · August 21, 2019, 5:46am

The SetAlgebra protocol refines Equatable and ExpressibleByArrayLiteral. There is a default implementation when the set's and array-literal's element types are the same. I just made up a set type and had "public typealias ArrayLiteralElement = Element" in it to get the provided implementation. I then commented out the line, and everything still worked, without a request to define ArrayLiteralElement. I did a search over the SetAlgebra file and the entire repository, and I don't see how an unspecified ALE gets a default assignment to Element.

DevAndArtist · August 21, 2019, 7:12am

That's exactly how it gets defaulted to that type:

github.com

apple/swift/blob/main/stdlib/public/core/SetAlgebra.swift#L563


      
            ///     print(attendees.isStrictSubset(of: attendees))
            ///     // Prints "false"
            ///
            /// - Parameter other: A set of the same type as the current set.
            /// - Returns: `true` if the set is a strict subset of `other`; otherwise,
            ///   `false`.
            @inlinable // protocol-only
            public func isStrictSubset(of other: Self) -> Bool {
              return other.isStrictSuperset(of: self)
            }
          }
          
          extension SetAlgebra where Element == ArrayLiteralElement {
            /// Creates a set containing the elements of the given array literal.
            ///
            /// Do not call this initializer directly. It is used by the compiler when
            /// you use an array literal. Instead, create a new set using an array
            /// literal as its value by enclosing a comma-separated list of values in
            /// square brackets. You can use an array literal anywhere a set is expected
            /// by the type context.
            ///

The where clause is simply ignored.

For more information on that read this thread:

Are there any further stdlib development related questions, otherwise I'd move this thread into Using Swift?!

CTMacUser · August 22, 2019, 4:58am

I agree with you in that thread; it looks like a mis-feature where the compiler is being too smart. The line looks descriptive, and almost always is, but here (and only when there's exactly one such extension) it's being proscriptive.