Workaround/resolution for Swift compiler error related to Void callback defined in struct

Question

Due to the lack of generic support in typealias definitions in Swift, I wrote a generic ResultHandler<T> callback like so:

struct ResultHandler<T> {
    let f: (result: T) -> ()
}

This works great for handlers that use anything other than Void:

Dispatch<Bool>.dispatch({ () -> (Result<Bool>) in
    Result<Bool>(true)
}).onResult(ResultHandler() { (result: Bool) in
    if result {
        self.doSomething() // Method executes successfully
    }
})

But when attempting to use Void:

Dispatch<Void>.dispatch({ () -> (Result<Void>) in
    Result<Void>()
}).onResult(ResultHandler() { (result: Void) in // Compiler error here
    self.doSomething()
})

There is a compilation error:

Function signature '(Void) -> ()' is not compatible with expected type '(result: Void) -> ()'

Which seems misleading/a compiler bug, as if I change the signature to use a wrongly-typed value:

Dispatch<Void>.dispatch({ () -> (Result<Void>) in
    Result<Void>()
}).onResult(ResultHandler() { (result: Void?) in // Compiler error here
    self.doSomething()
})

The error message picks up the optional-indicating ?, but still does not properly detect my result: label:

Function signature '(Void?) -> ()' is not compatible with expected type '(result: Void) -> ()'

What am I doing wrong?

Nate Cook · Accepted Answer · 2015-05-31 19:18:38Z

1

You don't need to specify parameter names in closure type declarations—that's what's giving the compiler problems:

struct ResultHandler<T> {
    let f: T -> ()
}

Now you can create ResultHandler instances that use a Bool parameter, a Void parameter, or leave out the parameter altogether:

let r1 = ResultHandler() { (result: Bool) in
    if result {
        // do something
    }
}

let r2 = ResultHandler() { (_: Void) in
    // do something
}

let r3 = ResultHandler() {
    // do something
}

answered May 31, 2015 at 19:18

Nate Cook

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