Literal String vs. `String` in swift

Question

Playing with swift I found this surprising:

"123".integerValue // <= returns 123

var x = "123"
x.integerValue     // <= Error: String does not have a member named integerValue

Can someone explain?

Answer 1

My guess is that in the first example the compiler uses the call to integerValue as additional information to infer the type (choosing between NSString and a Swift String).

In the second example it probably defaults to a Swift String because it doesn't evaluate multiple lines.

Answer 2

I believe this is an example of type inference in action. When you do: "123".integerValue the compiler detects that in this case you want to use an NSString (which it also does when you use string literals as arguments to objective-c functions.

A similar example is:

// x is an Int
let x = 12

// here we tell the compiler that y is a Double explicitly
let y: Double  = 12

// z is a Double because 2.5 is a literal Double and so "12" is also
// parsed as a literal Double
let z = 12 * 2.5