This is correct behavior. In Swift, literals don't have an intrinsic type, just a default one. So when you say 1.0 as Float or even 10 as Float, the compiler knows that the value should be a Float. It doesn't start with a Double or Int and then convert. 'as?', however, is only used for converting between runtime values, and so the compiler falls back to the default type for a particular literal. And indeed, different basic number types cannot be converted to one another in Swift.

NSNumber does come along and confuse things. It's mostly just there for compatibility with Objective-C, but it can hold any of the basic number types. Because of that, it's possible to convert to and from NSNumber.

Comment by Janusz Bossy (JIRA)

My point here is that there's a lot of inconsistency between the behavior. The simplest example I can create is:

1> let x = NSNumber(value: 0.1)
x: NSNumber = Double(0.1)
2> x.floatValue
$R0: Float = 0.100000001
3> x as? Float
$R1: Float? = nil

NSNumber does return a floatValue but cannot be casted to a Float.

The problem happens mostly when working with JSON based APIs – the data format does not give information about types so parsers use NSNumber to make sure everything works properly. Then it turns out I run into a lot of problems because values can or cannot be casted to Floats or any different numeric type and all of this depends on what data was actually sent from the server.

@phausler can probably explain this better than me, but the basic idea is that a cast to Float will fail whenever doing so would lose precision. Since "0.1" (a value written in JSON) can't be precisely represented by a binary floating-point value, both the Float and Double representations are approximations. The JSON parser in Foundation uses Double for all floating-point values because it's a better approximation.