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https://github.com/mattpolzin/poly
A light-weight general-purpose library to help represent situations where the type of a value is one of a set of types. An alternative to type erasure.
https://github.com/mattpolzin/poly
library swift type-erasure
Last synced: 5 days ago
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A light-weight general-purpose library to help represent situations where the type of a value is one of a set of types. An alternative to type erasure.
- Host: GitHub
- URL: https://github.com/mattpolzin/poly
- Owner: mattpolzin
- License: mit
- Created: 2019-01-13T02:37:52.000Z (almost 6 years ago)
- Default Branch: main
- Last Pushed: 2024-03-27T22:14:37.000Z (8 months ago)
- Last Synced: 2024-10-01T16:49:45.865Z (about 1 month ago)
- Topics: library, swift, type-erasure
- Language: Swift
- Homepage:
- Size: 71.3 KB
- Stars: 19
- Watchers: 3
- Forks: 5
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- License: LICENSE.txt
Awesome Lists containing this project
README
# Poly
[](http://opensource.org/licenses/MIT) [](https://swift.org) [](https://swift.org) [](https://app.bitrise.io/app/e46602b5a7b267d7)
Poly is a small library to provide an alternative to rolling your own type-erasure when a value has one of a small set of Types. The Poly library contains the Types `Poly1`, `Poly2`, `Poly3`, etc. for representing increasingly larger pools of possible Types. `Poly2` is isomorphic to `Either` (a common generic functional programming Type).
## Dev Environment
### Prerequisites
1. Swift 4.2+
2. Swift Package Manager 5.0 *OR* Cocoapods
### CocoaPods
To use this framework in your project via Cocoapods instead of Swift Package Manager, add the following dependency to your Podfile.
```
pod 'Poly', :git => 'https://github.com/mattpolzin/Poly.git'
```
### Xcode project
To create an Xcode project for Poly, run
`swift package generate-xcodeproj`
## Usage
Usage will be explained by way of an example. Suppose you have some code with three different Types: `Dog`, `Cat`, and `Rat`. You also have a protocol, `Animal`, that they all belong to.
If you need to store animals of all three Types in one place (maybe an array), that looks like:
```swift
let dog = Dog()
let cat = Cat()
let rat = Rat()
let animals: [Poly3] = [
.init(dog),
.init(cat),
.init(rat)
]
```
To access all animals of a certain type, you can use subscripting like:
```swift
let dogs = animals[Dog.self]
let cats = animals[Cat.self]
let rats = animals[Rat.self]
```
You can get the `Dog`, `Cat`, or `Rat` value back out again, but you won't get any guarantees of which Type is being stored in a given `Poly`:
```swift
let animal = Poly3(Dog())
let maybeDog: Dog? = animal.a
let maybeCat: Cat? = animal.b
let maybeRat: Rat? = animal.c
```
Or use the subscript operator to make accessing one of the possible values of a `Poly` a bit more intuitive:
```swift
let maybeDog2 = animal[Dog.self]
let maybeCat2 = animal[Cat.self]
let maybeRat2 = animal[Rat.self]
```
Or switch over the possible values:
```swift
switch animal {
case .a(let dog):
print(dog)
case .b(let cat):
print(cat)
case .c(let rat):
print(rat)
}
```
Or access a type-erased value:
```swift
let someAnimal: Any = animal.value
```
You might consider making a typealias to make your life easier:
```swift
typealias AnyAnimal = Poly3
```
You also might find it worthwhile to go the extra mile and add `Animal` conformance to `Poly`:
```swift
protocol Animal {
var speak: String { get }
}
extension Poly3: Animal where A == Dog, B == Cat, C == Rat {
var speak: String {
switch self {
case .a(let animal as Animal),
.b(let animal as Animal),
.c(let animal as Animal):
return animal.speak
}
}
}
```
So now you can take the array of animals from the first example above and:
```swift
let animalSounds = animals.map { $0.speak }
```
### `Codable`
All of the `Poly`types are `Encodable`/`Decodable` when every generic on which they are specialized is `Ecodable`/`Decodable`. This works by attempting to encode or decode each type and using the first successfuly attempt. That means the behavior only works as expected if all of the types have different encoding/decoding requirements or at least types are ordered with more restrictive rules coming first.
For example, given the following type
```swift
struct Type1: Decodable {
let x: Poly2
}
```
an integer value will never be decoded as an `Int` because `Double` is capable of decoding all `Int` values.
You can fix this by swapping the order to `Poly2`. Now `Poly` will attempt to decode an `Int` and only attempt to decode a `Double` if an `Int` was not found.