Swift: Cannot convert Type to Type

Question

I am looking to create a function to handle potential errors in GraphQL mutation results, using the Apollo SDK. Apollo can have errors as part of its success data.

To keep it as generic as possible, I define my function as:

    func alertResult(result: Result<GraphQLResult<Any>, Error>) {
        print(result)
        switch result {
        case let .failure(error):
            self.presentAlert(title: "Operation Error", message: "API says: \(error)")
        case let .success(data):
            if let errors = data.errors {
                self.presentAlert(title: "Operation Error", message: "API says: \(errors)")
            }
        }
    }

When used with a specific Apollo mutation, InviteToCommunityMutation, the compiler rejects it with the following error:

Cannot convert value of type 'Result<GraphQLResult<InviteToCommunityMutation.Data>, any Error>' to expected argument type 'Result<GraphQLResult<Any>, any Error>'

I find this bizarre, as surely InviteToCommunityMutation.Data should be convertible to Any.

If I try to define my function more broadly with:

func alertResult(result: Result<Any, Error>)

then I get Type of expression is ambiguous without more context when trying to extract data.errors from .success(data).

Would love help! Also, I'm new to Stackoverflow, so tried to include only relevant data but happy to provide more.

Thanks - it works! Can you tell me why the Any conversion didn't work and how this solves it, or tell me where in the docs I can understand what you're doing? — Tom Petit, Sep 27 '22 at 03:43

vacawama · Accepted Answer · 2022-09-27T04:11:39.177

Make your function generic:

func alertResult<T>(result: Result<GraphQLResult<T>, Error>) {

By using T as a generic placeholder type, you allow Swift to synthesize an appropriate function for any subtype of GraphQLResult.

You can read about Swift Generics here.

Note: The error message tells you why Any doesn’t work. Result<GraphQLResult<InviteToCommunityMutation.Data>, any Error> and Result<GraphQLResult<Any>, any Error> are two different types. Generics allow Swift to generate a function with the properly matching type.

Swift: Cannot convert Type to Type

1 Answers1