Swift compiler error related to FutureKit callbacks

build_error.txt

PKLoginModelPromise.swift:62:22: error: enum element 'main' cannot be referenced as an instance member
            .onFail(.main, block:
                     ^

Cartfile

github "FutureKit/FutureKit" "v3"
github "ReSwift/ReSwift" == 3.0.0
github "ReSwift/ReSwiftRouter" == 0.5.1  

PKLoginModelPromise.swift

public class PKLoginModelPromise : PKLoginModel
{
    private let reduxStore: PKAppStateStore
    private let networkLoader: PKLoginService
    private let tokenRepo: PKAccessTokenRepo
    
    public init(withReduxStore reduxStore: PKAppStateStore,
                networkLoader: PKLoginService,
                tokenRepo: PKAccessTokenRepo)
    {
        self.reduxStore = reduxStore
        self.networkLoader = networkLoader
        self.tokenRepo = tokenRepo
    }
    
    
    public func loginAsync(
                     _ email: String,
                    password: String)
    {
        let loginFromNetFuture: FetchAccessTokenFuture =
            LoginFutures.login(
                        withService: self.networkLoader,
                              email: email,
                           password: password)
        
        let tokenRepo = self.tokenRepo
        let reduxStore = self.reduxStore
        
        loginFromNetFuture.onSuccess(.background, block:
            {
                tokenFromNet in
                
                let storeLoginFuture = PKAccessTokenRepoFutures.storeToken(tokenFromNet, inRepo: tokenRepo)
                
                return storeLoginFuture
            })
            .onSuccess(.main, block:
            {
                tokenFromDisk in
                
                let action = UpdateAccessTokenAction(accessToken: tokenFromDisk)
                reduxStore.dispatch(action)
                
                return tokenFromDisk
            })
            .onFail(.main, block:
            {
                diskError in
                
                let action = LoginErrorAction(error: diskError)
                self.reduxStore.dispatch(action)
                
                return diskError
            })
    }
}

I'm trying to chain the futures following the example from the FutureKit readme.

So the latest version of the onFail() handler typically does NOT return any values.
It doesn't 're-map' the future into a new Future (unlike onSuccess or onComplete).
This is different than javascript Promises 'catch' which by default 'eats' the error. And then leads to confusion and complexity.

So in FutureKit onFail() will let you handle errors, but it still always returns the same 'Failed' Future. Which will usually lead to more reliable error handling.

onFail() allows you to add 'side' effects when a Future fails, but it always returns the same failed 'Future'. So you usually just return Void.

If you want to 'eat' or modify the Failed Future, use 'onComplete' or 'mapError'.

So try removing the return diskError

.onFail(.main, block:
            {
                diskError in
                
                let action = LoginErrorAction(error: diskError)
                self.reduxStore.dispatch(action)


            })

The Swift type inference machine can be fickle, and when it gets confused it can lead to confusing errors.

if that doesn't help... sometimes it can help to formally declare the return values for onSuccess, since it uses helps the compiler figure out 'which' onSuccess or onComplete method to bind to.

public class PKLoginModelPromise : PKLoginModel
{
    private let reduxStore: PKAppStateStore
    private let networkLoader: PKLoginService
    private let tokenRepo: PKAccessTokenRepo
    
    public init(withReduxStore reduxStore: PKAppStateStore,
                networkLoader: PKLoginService,
                tokenRepo: PKAccessTokenRepo)
    {
        self.reduxStore = reduxStore
        self.networkLoader = networkLoader
        self.tokenRepo = tokenRepo
    }
    
    
    public func loginAsync(
                     _ email: String,
                    password: String)
    {
        let loginFromNetFuture: FetchAccessTokenFuture =
            LoginFutures.login(
                        withService: self.networkLoader,
                              email: email,
                           password: password)
        
        let tokenRepo = self.tokenRepo
        let reduxStore = self.reduxStore
        
        loginFromNetFuture.onSuccess(.background)
            {
                tokenFromNet -> Future<Token> in  // idk what type PKAccessTokenRepoFutures.storeToken returns so I'm guessing it's something.
                
                return PKAccessTokenRepoFutures.storeToken(tokenFromNet, inRepo: tokenRepo)
            }
            .onSuccess(.main) {
                tokenFromDisk -> Token in  // adding return type again
                
                let action = UpdateAccessTokenAction(accessToken: tokenFromDisk)
                reduxStore.dispatch(action)
                
                return tokenFromDisk
            }
            .onFail(.main)
            {
                diskError -> Void in  // onFail works better as Void!
                
                let action = LoginErrorAction(error: diskError)
                self.reduxStore.dispatch(action)
            }

    }
}

So try removing the return diskError

I still had similar errors even without a return statement.

if that doesn't help... sometimes it can help to formally declare the return values for onSuccess

And this has solved my issue. Thanks a lot for responding so quickly.

tokenFromNet -> Future<Token> in
tokenFromDisk -> Token in  // adding return type again