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https://github.com/antitypical/Result

Swift type modelling the success/failure of arbitrary operations.
https://github.com/antitypical/Result

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Swift type modelling the success/failure of arbitrary operations.

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# Result



[![Build Status](https://travis-ci.org/antitypical/Result.svg?branch=master)](https://travis-ci.org/antitypical/Result)

[![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)

[![CocoaPods](https://img.shields.io/cocoapods/v/Result.svg)](https://cocoapods.org/)

[![Reference Status](https://www.versioneye.com/objective-c/result/reference_badge.svg?style=flat)](https://www.versioneye.com/objective-c/result/references)



This is a Swift µframework providing `Result`.



`Result` 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`.



```swift

typealias JSONObject = [String: Any]



enum JSONError: Error {

    case noSuchKey(String)

    case typeMismatch

}



func stringForKey(json: JSONObject, key: String) -> Result {

    guard let value = json[key] else {

        return .failure(.noSuchKey(key))

    }

    

    guard let value = value as? String else {

        return .failure(.typeMismatch)

    }



    return .success(value)

}

```



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 to a Result

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.)



### Cocoapods



```ruby

pod 'Result', '~> 5.0'

```



### Swift Package Manager



```swift

// swift-tools-version:4.0

import PackageDescription



let package = Package(

    name: "MyProject",

    targets: [],

    dependencies: [

        .package(url: "https://github.com/antitypical/Result.git",

                 from: "5.0.0")

    ]

)

```