I’m working with Swift and encountered an issue when using the contains method on an array. The following code works fine:
let result = ["hello", "world"].contains(Optional("hello")) // ✅ Works fine
However, when I try to use the same contains method with the array declared in a separate constant(or variable), I get a compile-time error:
let stringArray = ["hello", "world"]
let result = stringArray.contains(Optional("hello")) // ❌ Compile-time error
The compiler produces the following error message:
Cannot convert value of type 'Optional<String>' to expected argument type 'String'
Both examples seem conceptually similar, but the second one causes a compile-time error, while the first one works fine.
This confuses me because I know that Swift automatically promotes a non-optional value to an optional when comparing it with an optional value. This means "hello" should be implicitly converted to Optional("hello") for the comparison.
What I understand so far:
The contains(_:) method is defined as:
func contains(_ element: Element) -> Bool
Internally, it calls contains(where:), as seen in the Swift source code:
🔗 Reference
contains(where:) takes a closure that applies the == operator for comparison.
Since Swift allows comparing String and String? directly (String is implicitly promoted to String? when compared with an optional), I expected contains(where:) to work the same way.
My Questions:
Why does the first example work, but the second one fails with a compile-time error?
What exactly causes this error in the second case, even though both cases involve comparing an optional value with a non-optional value?
Does contains(_:) behave differently when used with an explicit array variable rather than a direct array literal? If so, why?
I know that there are different ways to resolve this, like using nil coalescing or optional binding, but what I’m really looking for is a detailed explanation of why this issue occurs at the compile-time level.
Can anyone explain the underlying reason for this behavior?
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I’m working with Swift and ran into an issue when using the contains(_:) method on an array. The following code works fine:
let result = ["hello", "world"].contains(Optional("hello")) // ✅ Works fine
But when I try to use the same contains method with the array declared in a separate variable, I get a compile-time error:
let stringArray = ["hello", "world"]
let result = stringArray.contains(Optional("hello")) // ❌ Compile-time error
Both examples seem conceptually similar, but the second one causes a compile-time error, while the first one works fine.
I understand that when comparing an optional value (Optional("hello")) with a non-optional value ("hello"), Swift automatically promotes the non-optional value to an optional (i.e., "hello" becomes Optional("hello")).
🔗 reference
What I don’t understand is why the first code works but the second one doesn’t, even though both cases involve comparing an optional value with a non-optional value. I know that there are different ways to resolve this, like using nil coalescing or optional binding, but what I’m really looking for is a detailed explanation of why this issue occurs at the compile-time level.
Can anyone explain the underlying reason for this behavior?