Swift 2: Invalid Conversion from Throwing Function of Type to Non-Throwing Function

How to fix Invalid conversion from throwing function of type X to non-throwing function type Y with Do-Try-Catch

The do-catch is currently outside of the closure that has the try statement in it.

You'll need to move the do-catch into the closure. The error is because weather is expecting a closure that doesn't throw but it's getting on that does.

Invalid conversion from throwing function of type '(_, _) throws - ()' to non-throwing function type '(Bool, Error?) - Void

You cannot throw from within the non throwing completion handler. It is asynchronous! Just as you cannot return a value from within an asynchronous function, so too you cannot throw there. After all, to catch the throw, the throw would need to travel backwards in time to when the original function was called. And that is metaphysically impossible.

Instead you must provide another completion handler such that you can call it with an Error parameter from within the non throwing completion handler. That is exactly why the Result type was invented, to allow you to propagate an error through an asynchronous situation.

Invalid conversion from throwing function of type '(_) throws - ()' to non-throwing function type '(Response) - Void'

You cannot

throw ValidationError.athenticationFailure

because the request is asynchronous. What you can do is to change the completion type to Result<ResponseModel<SignUpModel>, ValidationError> to return

completion(.success(responseModel))

on success and

completion(.failure(athenticationFailure)

on failure. By the way I buy an u

Invalid conversion from throwing function to non-throwing function , trying to store a closure and it can't throw

Your throws is in the wrong place:

static func markdownReader() throws -> Self {

markdownReader() cannot throw. It just returns a Reader. The throwing, if any is going to happen, will happen in try file.readAsString, which is inside the closure. The throw does not magically somehow filter up out of that closure definition to the place where it is defined!

So basically you have two choices. One is to declare that the closure type can in fact throw:

var convert: (File) throws -> Page

The other is: don't let the throw percolate up out of the closure. Catch it:

do {
    let contents = try file.readAsString(encodedAs: .utf8)
    return Page(slug: "", title: "", content: contents, html: "")
} catch {
    // ???
}

The problem then is that in the catch block you are still obligated to return a Page (unless you'd like to fatalError at this point, on the grounds that the file can't be read so we may as well die). So you'd have to make up a page with fake content, since you failed to obtain the contents from the File.

Invalid conversion from throwing function of type '(_, _) throws - ()' to non-throwing function type '(JSON?, Error?) - Void'

Since this block is not expected to throw an error, you need to wrap your throwing call in a do catch block:

EduappRestClient.request(with: URLString, method: .post, parameters: parameters) { (json, error) in
    guard error == nil, let json = json else {
        completion(nil, error)
        return
    }
    do {
        let result = try JSONDecoder().decode(QuestionModel.self, from: json)
        completion(result, nil)
    } catch let error {
        completion(nil, error)
    }
}

Invalid conversion from throwing function of type '(_) throws - ()' to non-throwing function type '(DataSnapshot) - Void'

You are throwing an error in the completion block. This is not possible and causes the error.

The return value of the closure is not related to the return value of the enclosing function – strictly spoken throws is not a return value but is affected, too.

To be able to return something from the closure you have to implement a completion block rather than throws

func getUserList(completion : (Error?) -> ())

and use it

completion(value.isEmpty ? UserError.Empty : nil)

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Side-note: You are using too many question and exclamation marks. Use optional binding to unwrap optionals for example (and use Swift native collection types)

if let value = snapshot.value as? [String:Any] {
   for key in value.keys { ...

Alamofire 4 Invalid conversion from throwing function of type '(_) throws - ()' to non-throwing function type '(DataResponse Any ) - Void'

The error occurs because you don't handle the errors of the throwing functions.

Add a do - catch block

do {
     let clear = try ClearMessage(string: keyAndiv, using: .utf8)
     let encrypted = try clear.encrypted(with: publicKey, padding: .PKCS1)

     let data = encrypted.data
     let base64String = encrypted.base64Encoded
     print ("data", data);
     print ("base64String", base64String)
} catch { print(error) }

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And this is Swift, you don't need parentheses around if conditions and trailing semicolons

if response.result.value != nil { ...

or better

guard let result = response.result.value else { return }
let swiftyJSON = JSON(result)

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