mirror of
https://github.com/mas-cli/mas
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113 lines
5.1 KiB
Markdown
113 lines
5.1 KiB
Markdown
# Result
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[](https://travis-ci.org/antitypical/Result)
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[](https://github.com/Carthage/Carthage)
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[](https://cocoapods.org/)
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[](https://www.versioneye.com/objective-c/result/references)
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This is a Swift µframework providing `Result<Value, Error>`.
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`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.
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Using this µframework instead of rolling your own `Result` type allows you to easily interface with other frameworks that also use `Result`.
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## Use
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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`.
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```swift
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typealias JSONObject = [String: Any]
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enum JSONError: Error {
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case noSuchKey(String)
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case typeMismatch
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}
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func stringForKey(json: JSONObject, key: String) -> Result<String, JSONError> {
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guard let value = json[key] else {
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return .failure(.noSuchKey(key))
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}
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if let value = value as? String {
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return .success(value)
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}
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else {
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return .failure(.typeMismatch)
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}
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}
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```
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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.
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One simple way to handle a `Result` is to deconstruct it using a `switch` statement.
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```swift
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switch stringForKey(json, key: "email") {
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case let .success(email):
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print("The email is \(email)")
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case let .failure(.noSuchKey(key)):
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print("\(key) is not a valid key")
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case .failure(.typeMismatch):
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print("Didn't have the right type")
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}
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```
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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.
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Other methods available for processing `Result` are detailed in the [API documentation](http://cocoadocs.org/docsets/Result/).
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## Result vs. Throws
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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`.
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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`.
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## Higher Order Functions
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`map` and `flatMap` operate the same as `Optional.map` and `Optional.flatMap` except they apply to `Result`.
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`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`.
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```swift
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// transforms a Result<Int, JSONError> to a Result<String, JSONError>
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let idResult = intForKey(json, key:"id").map { id in String(id) }
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```
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Here, the final result is either the id as a `String`, or carries over the `failure` from the previous result.
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`flatMap` is similar to `map` in that it transforms the `Result` into another `Result`. However, the function passed into `flatMap` must return a `Result`.
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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).
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## Integration
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### Carthage
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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.
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2. Drag `Result.xcodeproj` into your project or workspace.
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3. Link your target against `Result.framework`.
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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.)
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### Cocoapods
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```ruby
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pod 'Result', '~> 3.0.0'
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```
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### Swift Package Manager
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```swift
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import PackageDescription
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let package = Package(
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name: "MyProject",
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targets: [],
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dependencies: [
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.Package(url: "https://github.com/antitypical/Result.git",
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majorVersion: 3)
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]
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)
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```
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