Swift: how can String.join() work custom types?

Question

for example:

var a = [1, 2, 3]    // Ints
var s = \",\".join(a)  // EXC_BAD_ACCESS

Is it possible to make the join function return \"1,2,3\

Answer 1

And just to make your life more complete, starting from Xcode 8.0 beta 1 in Swift 3 you should NOW use [String].joined(separator: ",").

This is the new "ed/ing" naming rule for Swift APIs:

Name functions and methods according to their side-effects

• Those without side-effects should read as noun phrases, e.g. x.distance(to: y), i.successor().
• Those with side-effects should read as imperative verb phrases, e.g., print(x), x.sort(), x.append(y).
• Name Mutating/nonmutating method pairs consistently. A mutating method will often have a nonmutating variant with similar semantics, but that returns a new value rather than updating an instance in-place. Swift: API Design Guidelines