Pipe Operator instead of conditions and deep extends

まだ誰も着手していません。

### 🔍 Search Terms

- switch case
- switch case type level
- pipe operator ts
- pipe operator typescript
- avoid nested extends
- pipe ts type level
- type level switch
- switch typescript

### ✅ Viability Checklist

- [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
- [x] This wouldn't change the runtime behavior of existing JavaScript code
- [x] This could be implemented without emitting different JS based on the types of the expressions
- [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
- [x] This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
- [x] This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

### ⭐ Suggestion

My suggestion is to add a kind of idiomatic switch case for typescript types, where we could verify a list of `extends` without nesting.

**ERRATUM:** It does more than just adding syntax sugar to syntax, and behaves differently to a switch case. In this case, it determines a set of conditions that computes and returns the first found matching conditions. This way we could have more complex checking, instead of a simple "If Type T matches other Type U".

The example below does this. It not only checks "equality" and also does more things like calling other utility changes and then doing a condition check.

So, instead of a "switch case", it would likely feel closer to a syntax sugar to a sequence of "ifs".

Imagine a "replace" function that takes a text, find placeholders and provide an object with that placeholders as keys. This is how it can be achieved now:

```typescript
type Whitespace = '\n' | '\t' | ' ';

type TrimLeft =
T extends `${Whitespace}${infer S extends string}` ? TrimLeft : T;
type TrimRight =
T extends `${infer S extends string}${Whitespace}` ? TrimRight : T;
type Trim = TrimRight>;

type Equal = [A] extends [B] ? ([B] extends [A] ? true : false) : false;

type IsJustString = Equal;

type Placeholders =
IsJustString extends true
? string
: Equal extends true
? Result
: T extends `${string}{{${infer P extends string}}}${infer Rest extends string}`
? Placeholders>
: Result;

export function withText(text: T) {
return {
replace: (contents: Record, string>) => {
let result: string = text;
for (const [key, value] of Object.entries(contents)) {
result = result.replace(`{{${key}}}`, value as string);
}
return result;
},
};
}

const helloWorld = withText("{{greeting}}, {{person}}").replace({
greeting: "Hello",
person: "World"
})
```

This is how it would look on my proposal:

```
type Whitespace = '\n' | '\t' | ' ';

type TrimLeft =
|> T extends `${Whitespace}${infer S extends string}`: TrimLeft,
|> T;
type TrimRight =
|> T extends `${infer S extends string}${Whitespace}`: TrimRight,
|> T;
type Trim = TrimRight>;

type Equal = [A] extends [B] ? ([B] extends [A] ? true : false) : false;

type IsJustString = Equal;

type Placeholders =
|> IsJustString extends true: string,
|> Equal extends true: Result,
|> T extends `${string}{{${infer P extends string}}}${infer Rest extends string}`: Placeholders>,
|> Result;

export function withText(text: T) {
return {
replace: (contents: Record, string>) => {
let result: string = text;
for (const [key, value] of Object.entries(contents)) {
result = result.replace(`{{${key}}}`, value as string);
}
return result;
},
};
}

const helloWorld = withText("{{greeting}}, {{person}}").replace({
greeting: "Hello",
person: "World"
})
```

### 📃 Motivating Example

Sometimes nested extends can be too complicated to understand:

- The depth is is unmanageable
- Doesn't separate different concerns (are we still verifying possible values of T or are we doing something else?)
- Makes it hard to format
- Makes it hard to read

### 💻 Use Cases

1. What do you want to use this for?

Replace deep extends checks

2. What shortcomings exist with current approaches?

- Split into different types with different concers
- Format so that it looks like a "switch case"

3. What workarounds are you using in the meantime?

I have presented above

この issue では、リポジトリのファイル、テスト、エントリポイントが指定されていません。まず、TypeScript が現在どのように条件型を解析して評価しているかを確認し、そのうえで必要な構文、型チェックの動作、診断、互換性テストを明らかにしてください。完了とするには、局所的な編集だけでなく、承認された設計と実装計画が必要です。

索引モデルが issue の本文から書いたものです。