问题
The question is very simple, but I just could not find the answer!
Why doesn't
return x == 0? "" : "Hello"
compile but
return x == 0 ? "" : "Hello"
does?
This is really weird because all the other operators don't need an extra white space. e.g.
let x = 1+1
let y = 1 + 1
are the same.
I think it has something to do with optionals. But when you use a ? operator on a variable, it must be used like this:
let s: String? = nil
let x = s?.startIndex
I mean it must follow another operator, right?
回答1:
I think it has something to do with optionals.
It does. The documentation on operators says:
There is one caveat to the rules [regarding whitespace around operators]. If the
!or?predefined operator has no whitespace on the left, it is treated as a postfix operator, regardless of whether it has whitespace on the right. To use the?as the optional-chaining operator, it must not have whitespace on the left. To use it in the ternary conditional (?:) operator, it must have whitespace around both sides.
回答2:
Yes, I'm pretty sure (as you presumed) the problem is with optionals.
I prefer to write my ternary operators like this...
let num = (isTrue) ? (1) : (0)
Of course, you can choose what's in the parentheses, whether it's simply a literal (as shown) or not.
来源：https://stackoverflow.com/questions/34553318/why-ternary-operator-in-swift-is-so-picky