Why isn't is possible to append a String! via += operator?

Question

As the title says I'm trying to append text to an implicitly unwrapped optional String via += operator which gives me

'String!' is not identical to 'UInt8'

To let you see what I mean:

var myString: String = "Hello " myString += "world" // works great var myOptionalString: String! = "Foo " myOptionalString += " bar" // error: String! is not identical to 'UInt8'

however if I append it while the assignment it works

var myOptionalString: String! = "Foo " myOptionalString = myOptionalString + " bar" // works great

can anyone tell me the reason for that, or is there something I missed in optionals?

Update

mySecondOpString: String? = "Hello " mySecondOpString! += "world" // works great too

I don't know swift very much but be aware that x+=y is not semantically equivalent to x = x+y. In the first case you have something like append y to x (x pointing to a mutable string), and in the second you have catenate x and y, and associate the result to x. Optional may change things when append... — Jean-Baptiste Yunès
– Jean-Baptiste Yunès, Commented Mar 27, 2015 at 10:20
The x = x +y thing is just there so nobody can say "hey try this" but you're right it's not equivalent — Nitro.de
– Nitro.de, Commented Mar 27, 2015 at 10:22
See this for an answer... stackoverflow.com/questions/26583300/… — Jean-Baptiste Yunès
– Jean-Baptiste Yunès, Commented Mar 27, 2015 at 10:25

Abdullah · Accepted Answer · 2015-03-27 13:05:00Z

String! is implicitly unwrapped optional type which is special case of optional type.

As you may know String! is not identical to String. So when you write:

var myOptionalString: String! = "Foo " myOptionalString += " bar" // error: String! is not identical to 'UInt8'

it will try to find += operator with String! which it could not and the hence error.

if you explicitly unwrap it (you could say then it defies the purpose) works:

 myOptionalString! += " bar"

Community · Accepted Answer · 2017-05-23 11:43:25Z

There's nothing wrong in your code. It looks like in Swift's standard library there's no += operator overload made to work with String optionals.

Taken from the standard library, += is overloaded for String (not for String?)

func +=(inout lhs: String, rhs: String)

Just follow this nice SO answer to view the contents of the Swift standard library to check that

Note

Your code will be better written as:

var myOptionalString: String? = "Foo " myOptionalString myOptionalString! += " bar"

Ty but dont worry this isn't copied code. I was working with UITextView.text which is a String!, thats why the question came up

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