What's the difference between Optional and

问题:

Update - there is no difference between Optional and optional types in Swift - they are the same thing.

So in Swift they introduced the Type? construct, that creates an optional type which "forces" the programmer to check if the value actually exists.

Also, in Swift there is no exception handling. But there is this built-in optionality mechanism.

This optional feature ? is just an Optional<T> enum behind the scenes inspired from Haskell's Maybe.

I was wondering if there was any way of passing error information through the optional type. Can "abc123".toInt() return error information through the optional type Int? ? Could this replace Exceptions or NSError? Of course Cocoa still uses (and will use?) the NSError pattern.

Playing in the playground, I found this piece of code in the Swift "header":

protocol Printable {
    var description: String { get }
}

extension T? : Printable {
    var description: String { get }
}

Basically this adds a readonly property description to ANY optional type. This would be super powerful (and could lead to many problems, I guess).

But after trying to extend myself the T? I get an error:

Non-nominal type 'T?' cannot be extended

So why didn't they add a property to all optional types to carry with them error information? Wouldn't it be useful? I mean, you could return a tuple if you want multiple return types...

回答1:

this is already part of the language

extension Optional : DebugPrintable {

    /// A textual representation of `self`, suitable for debugging.
    var debugDescription: String { get }
}

回答2:

Option in swift is like Option in Scala (Maybe in Haskell): It represents a value that may be absent.

What you're looking for is something like the Try or the Either monads from Scala, which represent two possible states of a computation, typically a Success or a Failure.

This would be a different algebraic data type, beyond the purpose of an optional value.

Ultimately, since swift doesn't have full-fledged monads in the standard library, I think the default error handling pattern will stay the classic Cocoa NSError one.

回答3:

Optionals basically allow you to check whether or not a variable is nil. It's not meant to have an error message. For example take this Objective-C snippet:

if(self.someProperty == nil){
    self.someProperty = [[PropertyClass alloc] init]
}else{
    //property is not nil.
}

In swift you would do something like this:

var someProperty : PropertyClass?
if(someProperty){
    //property is not nil 
}else{
    //property is nil.
}

The apple docs say:

Optional chaining in Swift is similar to messaging nil in Objective-C, but in a way that works for any type, and that can be checked for success or failure.

回答4:

Per this answer, the correct syntax is:

extension Optional : Printable {
    //...
}

回答5:

Optional<T> is the definition of Optionals (see below). Type? is just the syntactic sugar for creating the type.

enum Optional<T> {
  case None
  case Some(T)
}

From the Swift Programming Language iBook, the two declarations below are equivalent

var optionalInteger: Int?
var opTionalInteger: Optional<Int>