Filtering a Swift [AnyObject] array by type

Question

Say I have an array of AnyObject.

let grabBag: [AnyObject] = [ "Tom", 4, "Dick", NSObject(), "Harry" ] 

and I want to cast it to an array of Strings, extracting only those elements that are in fact Strings. I would expect this to work:

let strings = grabBag.filter{ $0 is String } as! [String]      // 1 

but it gives the error 'Bool' is not convertible to 'String'. Yet this works:

let definitelyStrings = grabBag.filter{ $0 is String }         // 2 let strings = definitelyStrings as! [String]                   // 

Why does 2 work when 1 does not? And is there a simpler (than 2) way of extracting and casting the elements of an [AnyObject] to whatever [T]?

Answer 1

It's better to use compactMap for a nice one-liner:

let strings = grabBag.compactMap { $0 as? String } 

Now strings is of type [String].

Answer 2

This is what flatMap is for:

let strings = grabBag.flatMap{ $0 as? String } 

This takes a closure that returns an optional; if the optional is non-nil, then it is added to the result.

(Note that this doesn't match the meaning of flatMap from other languages, and doesn't even match the other meaning of flatMap in Swift. A better name would have been mapOptional or mapSome. But it's still kind of intuitive, even if inconsistent. It "maps to optionals, then flattens out all the nils." Rob Mayoff notes that if Optionals were SequenceTypes, which they probably should be, this would be a sensible name.)