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 `Error` value. The addition of an associated `Error` 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`.
This function provides a more robust wrapper around the default subscripting provided by `Dictionary`. Rather than return `Any?`, 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.
```swift
switch stringForKey(json, key: "email") {
case let .success(email):
print("The email is \(email)")
case let .failure(.noSuchKey(key)):
print("\(key) is not a valid key")
case .failure(.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](http://cocoadocs.org/docsets/Result/).
## Result vs. Throws
Swift 2.0 introduces error handling via throwing and catching `Error`. `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`.
```swift
// 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 it 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](http://www.javiersoto.me/post/106875422394).
## Integration
### Carthage
1. Add this repository as a submodule and/or [add it to your Cartfile](https://github.com/Carthage/Carthage/blob/master/Documentation/Artifacts.md#cartfile) if you’re using [carthage](https://github.com/Carthage/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.)
