Type Unions for C# 제안서

Chris_Shim · 7월 27, 2024, 10:59오전

dotnet/csharplang/blob/main/proposals/TypeUnions.md

# Type Unions for C#

* [x] Proposed
* [ ] Prototype: [Not Started](pr/1)
* [ ] Implementation: [Not Started](pr/1)
* [ ] Specification: [Not Started](pr/1)

## Summary
[summary]: #summary

A proposal for type unions (aka discriminated unions) in C#.

## Motivation

When developing software you may encounter situations where the values that you want to store in a variable are not always the same kind each time through. While you are usually not concerned about storing strings and numbers in the same spot, you may need to store one of a few related types depending on what that data is meant to represent at that moment.

For example, your application may have both a customer and a supplier definition that share only some of the same properties and you may need to perform a similar operation on both in a fashion that depends on the differences. 

Typically, this is where you might choose to distribute those specialized implementations into the types themselves and expose them through common abstract methods or interfaces. However, this is only good practice when those types exist primarily for the purpose of the operation or it makes sense for the operation to appear as an intrinsic part of the type. If the types have a broader purpose, polluting them with methods like this can be undesirable.

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A proposal for type unions (aka discriminated unions) in C#.

타입이 강조되는 함수형 언어는 대수(Algebraic)적 원리를 도입해 타입을 정의하는데요. 대수에서 ADT(Algebraic Data Type)은 합타입과 곱타입으로 분해됩니다.

C# 7에 튜플이 1급으로 취급되고 C# 8 부터 스위치식이 도입되고 개선되면서 곱타입을 다루는 데 언어적으로 성숙해 졌지요. 다만 합타입을 추가하지 못하는 바람에 ADT를 1급으로 다룰 수 없는 문제가 있습니다.

DU(Discriminated Unions)은 함수형 언어(Scala, haskell, f#)와 최근 개발 언어(Rust, Kotlin)에서 1급으로 제공하는 합타입으로 상속과 비슷해 보이지만 가장 큰 차이점은 상속에 대해 닫혀 있다는 점이지요. FSM(유한상태기계)를 언어에서 1급으로 다룰 수 있다는 장점도 있습니다.

C#이 더욱 저변을 넓힐 수 있는 기회라 생각되네요. (물론 한국 제외)