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/*
 This gist contains an analysis of the design space for abstractions in a type system like Swift's.
 It focuses on Monoid as an example but the same set of tradeoffs apply to any abstraction.
 As such, it focuses on a single abstraction and does not explore issues that arise when designing
 an abstraction hierarchy in depth.

 The matrix below describes some of the design important design tradeoffs for various approaches including:
 - OO style protocols such as `Monoid { var identity: Self ... }
    (Haskell also adopts this approach for many abstractions, including for Monoid)
 - ML signature style static protocols with empty enum conformances analagous with ML structures

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// NOTE: This is an experiement that does not actually work (in the 2/8/18 nightly toolchain).
//       The idea is to use Swift's constraint inference to be able to abstract an arbitrary set of constraints.

/// An empty enum serving as an example of the general case of
/// representing a set of constraints that relate multiple types.
/// This specific example provides the constraint that both types are sequences and they have the same Element type.
enum Parallel<S1: Sequence, S2: Sequence> where S1.Element == S2.Element {
    typealias First = S1
    typealias Second = S2
}

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// This example shows how closed protocols can be emulated in Swift today.
// The technique leverages Swift's support for public (but not open) classes.

// First, it's worth observing that there is an almost trivial technique that can be used when
// it is possible to specify a (possibly abstract) superclass for all conforiming types.
// Simply declare a public (not open) base class and a protocol with a Self inheritance constraint.
// Swift does not support open subclasses of a public superclass so no classes outside the module
// will be able to meet the self inheritance constraint.
public class FooBase {}
public protocol Foo where Self: FooBase {}

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// This example shows how higher-kinded types can be emulated in Swift today.
// It acheives correct typing at the cost of some boilerplate, manual lifting and an existential representation.
// The technique below was directly inspired by the paper Lightweight Higher-Kinded Polymorphism
// by Jeremy Yallop and Leo White found at http://ocamllabs.io/higher/lightweight-higher-kinded-polymorphism.pdf

/// `ConstructorTag` represents a type constructor.
/// `Argument` represents an argument to the type constructor.
struct Apply<ConstructorTag, Argument> {
    /// An existential containing a value of `Constructor<Argument>`
    /// Where `Constructor` is the type constructor represented by `ConstructorTag`

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/*
 If you're willing to write a little bit of boilerplate you can have type-safe GADT in Swift today.
 This is accomplished using a wrapper struct with a phantom type parameter that wraps a private enum value.
 Static factory methods on the struct wrap each case returning a value with the phantom type bound as necessary.
 An extension is created for each phantom type binding providng a `switch` method that requires each case
 with a matching type to be covered and uses `fatalError` for cases where values will never be created.
 New members are added in extensions that bind the phantom type and make use of the `switch` method.

 The example below is drawn from https://en.m.wikibooks.org/wiki/Haskell/GADT
*/

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Value semantics and pure functions: referential transparency for Swift

Author: Matthew Johnson

Contents

• Introduction
• Observations
• Motivation
• Referential transparency

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Value Subtypes and Generalized Enums, a manifesto

The goal of this document is to provide a comprehensive view of what value subtyping might look like in Swift and demonstrate how generalized enums play a significant role in this future.

Note: All syntax used in this document that is not currently valid Swift syntax is only intended to serve the purpose of demonstrating ideas and to serve as a point of reference for future proposals. The intent is not to propose that this exact syntax be used.

Acknowledgement: some of the ideas in this document have been inspired by Niko Matsakis' blog post exploring similar ideas in the context of Rust: http://smallcultfollowing.com/babysteps/blog/2015/08/20/virtual-structs-part-3-bringing-enums-and-structs-together/

Definition

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Literal Syntax Protocols

• Proposal: SE-NNNN
• Author: Matthew Johnson
• Status: Awaiting review
• Review manager: TBD

Introduction

This proposal renames the *LiteralConvertible protocols to Syntax.*Literal.

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// Created by Matthew Johnson on 5/28/16.
// Copyright © 2016 Anandabits LLC. All rights reserved.
//
// This is a minimalist implementation of a responder chain in pure Swift.
//
// It is not intended to demonstrate the best way to 
// implement event processing in Swift.
//
// The intent is to show how little code is necessary to acheive behavior 
// similar to Cocoa's responder chain in pure Swift.