Swift: seemingly not allowed to compare a String? with a String? - swift

Question

I am asking for user input (this works) and trying to output different results depending on if the input was nil, an empty string, or a non-empty string using a switch clause (doesn't work).
The first attempt got me an error because I'm trying to compare an optional string with a non-optional string:
import Foundation
print("Hi, please enter a text:")
let userInput = readLine(stripNewline: true)
switch userInput {
case nil, "": // Error: (!) Expression pattern of type ‘String’ cannot match values of type ‘String?’
print("You didn’t enter anything.")
default:
print("You entered: \(userInput)")
}
Fair enough, so I create a optional empty string to compare to:
import Foundation
print("Hi, please enter a text:")
let userInput = readLine(stripNewline: true)
let emptyString: String? = "" // The new optional String
switch userInput {
case nil, emptyString: // Error: (!) Expression pattern of type ‘String?’ cannot match values of type ‘String?’
print("You didn’t enter anything.")
default:
print("You entered: \(userInput)")
}
So that gives me an error saying I cannot compare a ‘String?’ to a ‘String?’.
Why is that? Are they somehow still not the same type?
PS. I have the feeling that I'm missing something fundamental here, such as what Troy pointed out about optionals not being "the same type as the corresponding non-otpional type but with the additional possibility of not having a value" (not an exact quote, see his UPDATE at the end of the question: What does an exclamation mark mean in the Swift language?). But I'm stuck connecting the final dots why this isn't ok.

Answer 1

Replace
case `nil`, "":
with
case nil, .Some(""):
Note that optional is an enum with two possible values: .None and .Some(value).
As for your second example with String?, note that the matching in case is performed using ~= operator which is not defined for optionals.
If you define:
#warn_unused_result
public func ~=<T: Equatable>(lhs: T?, rhs: T?) -> Bool {
return lhs == rhs
}
both your examples will start to work (not recommended).

Answer 2

Did you try to get your userInput text as a String:
let response = userInput ?? ""
switch response {
case "":
print("You didn’t enter anything.")
default:
print("You entered: \(response)")
}
Then you would be sure it is the same type String.
I hope it works

Answer 3

Just as a supplement to the existing good answers: Instead of testing
against nil and the empty string you can go the other way around
and test against a non-nil string (with the input? pattern) and
add a where constraint:
switch userInput {
case let input? where !input.isEmpty:
print("You entered", input)
default:
print("You did not enter anything")
}
Equivalently:
if case let input? = userInput where !input.isEmpty {
print("You entered", input)
} else {
print("You did not enter anything")
}
However: readLine() returns nil only on an end-of-file
condition, which means that no data can be read anymore from standard
input. Therefore it might make more sense to terminate the
program in that case:
guard let userInput = readLine(stripNewline: true) else {
fatalError("Unexpected end-of-file")
}
// `userInput` is a (non-optional) String now
if userInput.isEmpty {
print("You did not enter anything")
} else {
print("You entered", userInput)
}

Answer 4

see the self-explanatory example ...
let txtArr: [String?] = [nil, "", "some text"]
for txt in txtArr {
switch txt {
case nil:
print("txt is nil")
case let .Some(t) where t.isEmpty:
print("txt is empty string")
default:
print("txt is \(txt)")
}
}
the code prints
txt is nil
txt is empty string
txt is Optional("some text")