jchon258/Result

Swift type modelling the success/failure of arbitrary operations.

Result

This is a Swift µframework providing Result<Value, Error>.

Result<Value, Error> values are either successful (wrapping Value) or failed (wrapping Error). This is similar to Swift’s native Optional type: Success is like Some, and Failure is like None except with an associated ErrorType value. The addition of an associated ErrorType allows errors to be passed along for logging or displaying to the user.

Using this µframework instead of rolling your own Result type allows you to easily interface with other frameworks that also use Result.

Use

Use Result whenever an operation has the possibility of failure. Consider the following example of a function that tries to extract a String for a given key from a JSON Dictionary.

typealias JSONObject = [String:AnyObject]

enum JSONError : ErrorType {
    case NoSuchKey(String)
    case TypeMismatch
}

func stringForKey(json: JSONObject, key: String) -> Result<String, JSONError> {
    guard let value = json[key] else {
        return .Failure(.NoSuchKey(key))
    }
    
    if let value = value as? String {
        return .Success(value)
    }
    else {
        return .Failure(.TypeMismatch)
    }
}

This function provides a more robust wrapper around the default subscripting provided by Dictionary. Rather than return AnyObject?, it returns a Result that either contains the String value for the given key, or an ErrorType detailing what went wrong.

One simple way to handle a Result is to deconstruct it using a switch statement.

switch stringForKey(json, key: "email") {

case let .Success(email):
    print("The email is \(email)")
    
case let .Failure(JSONError.NoSuchKey(key)):
    print("\(key) is not a valid key")
    
case .Failure(JSONError.TypeMismatch):
    print("Didn't have the right type")
}

Using a switch statement allows powerful pattern matching, and ensures all possible results are covered. Swift 2.0 offers new ways to deconstruct enums like the if-case statement, but be wary as such methods do not ensure errors are handled.

Other methods available for processing Result are detailed in the API documentation.

Result vs. Throws

Swift 2.0 introduces error handling via throwing and catching ErrorType. Result accomplishes the same goal by encapsulating the result instead of hijacking control flow. The Result abstraction enables powerful functionality such as map and flatMap, making Result more composable than throw.

Since dealing with APIs that throw is common, you can convert such functions into a Result by using the materialize method. Conversely, a Result can be used to throw an error by calling dematerialize.

Higher Order Functions

map and flatMap operate the same as Optional.map and Optional.flatMap except they apply to Result.

map transforms a Result into a Result of a new type. It does this by taking a function that transforms the Value type into a new value. This transformation is only applied in the case of a Success. In the case of a Failure, the associated error is re-wrapped in the new Result.

// transforms a Result<Int, JSONError> to a Result<String, JSONError>
let idResult = intForKey(json, key:"id").map { id in String(id) }

Here, the final result is either the id as a String, or carries over the .Failure from the previous result.

flatMap is similar to map in that in transforms the Result into another Result. However, the function passed into flatMap must return a Result.

An in depth discussion of map and flatMap is beyond the scope of this documentation. If you would like a deeper understanding, read about functors and monads. This article is a good place to start.

Integration

1. Add this repository as a submodule and/or add it to your Cartfile if you’re using carthage to manage your dependencies.
2. Drag Result.xcodeproj into your project or workspace.
3. Link your target against Result.framework.
4. Application targets should ensure that the framework gets copied into their application bundle. (Framework targets should instead require the application linking them to include Result.)