Repository: TypeScript
Issue: Type safety with `TemplateStringsArray` and tag functions
Labels: Suggestion,In Discussion

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Title: Type safety with `TemplateStringsArray` and tag functions
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## Search Terms

`TemplateStringsArray`, `type`, `safety`, `generic`, `tag`, `function`


## Suggestion

Hello, I'd like to know how could I achieve type safety with `TemplateStringsArray` and tag functions or make such scenarios possible.

## Use Cases

I would use `TemplateStringsArray` together with the 

```ts
tagFunction`<type_safe_accessor>`
```

Currently, there's no way to do this, since

1. the definition of `TemplateStringsArray` is not generic

```ts
// source code:
// TypeScript/src/lib/es5.d.ts
// TypeScript/lib/lib.es5.d.ts

// local install:
// /usr/lib/code/extensions/node_modules/typescript/lib/lib.es5.d.ts

interface TemplateStringsArray extends ReadonlyArray<string> {
    readonly raw: ReadonlyArray<string>;
}
```

AND 

2. the usage of

```ts
tagFunction`<type_safe_accessor>`
```

gives the following error, completely preventing the type-safe usage:

```ts
Argument of type '{}' is not assignable to parameter of type keyof TypeAndValueObject | TemplateStringsArray<keyof TypeAndValueObject>'.
  Type '{}' is missing the following properties from type 'TemplateStringsArray<keyof TypeAndValueObject>': raw, length, concat, join, and 19 more. ts(2345)
```

## Examples

### The working case

I have some type-safe `i18n` translations:

```ts
// Dictionary.ts
export interface Dictionary {
	"Hello": string;
	"Click to see more": string;
	"Today is": (day: string) => string;
}

// en.ts
import { Dictionary } from "./Dictionary";

export const en: Dictionary = {
	"Hello": "Hello",
	"Click to see more": "Click to see more",
	"Today is": (day: string) => `Today is ${day}`,
};

// lt.ts
import { Dictionary } from "./Dictionary";

export const lt: Dictionary = {
	"Hello": "Sveiki",
	"Click to see more": "Paspauskite, kad pamatytumėte daugiau",
	"Today is": (day: string) => `Šiandien yra ${day}`,
};
```

```ts
// i18n.ts
import { Dictionary } from "./Dictionary";
import { en } from "./en";
import { lt } from "./lt";

export interface ITranslations {
	en: Dictionary;
	lt: Dictionary;
}

export const translations: ITranslations = {
	en: en,
	lt: lt,
};

// "en" | "lt"
export type ILang = keyof ITranslations;
```

I have a function to get the translations:

```ts
import { ILang, translations } from "./i18n";
import { Dictionary } from "./Dictionary";

const currentLang: ILang = "en";

const dictionary: Dictionary = translations[currentLang];

export const selectTranslation = <K extends keyof Dictionary>(key: K): Dictionary[K] => {
	const translationText: Dictionary[K] = dictionary[key];
	return translationText;
};
```

And I can safely use it with type-safe translations in the following fashion:

```ts
// someFile.ts
import { selectTranslation } from "../selectTranslation.ts";

/**
 * Type-checks, auto-completes etc. the strings from the `Dictionary`
 */
const someText: string = selectTranslation("Hello");
```


### The problem

However, now I'd like to use the template string (template literal) syntax, paired with the tag function to achieve the same functionality, so I improve the `selectTranslation` function:

```ts
// selectTranslation.ts
import { ILang, translations } from "./i18n";
import { Dictionary } from "./Dictionary";

const currentLang: ILang = "en";

const dictionary: Dictionary = translations[currentLang];

export const selectTranslation = <K extends keyof Dictionary>(key: K | TemplateStringArray): Dictionary[K] => {
	let realKey: K;

	if (Array.isArray(key)) {
		realKey = key[0];
	} else {
		realKey = key; // error:
		/** 
		 * Type 'K | TemplateStringsArray<string>' is not assignable to type 'K'.
  		 * Type 'TemplateStringsArray<string>' is not assignable to type 'K'. ts(2322)
		*/
	}

	const translationText: Dictionary[K] = dictionary[realKey];
	return translationText;
};
```

```ts
// anotherFile.ts
import { selectTranslation } from "../selectTranslation.ts";

/**
 * Does NOT type-check and gives the previously mentioned error:
 * 
 * Argument of type '{}' is not assignable to parameter of type K | TemplateStringsArray<K>'.
 * Type '{}' is missing the following properties from type 'TemplateStringsArray<K>': raw, length, concat, join, and 19 more. ts(2345)
 */
const someText: string = selectTranslation`Hello`; // error
```

## Possible solutions

I've tried 3 different solutions, neither of them giving me the results that I want:

### a) Change the `TemplateStringsArray` to be a generic like so

```ts
// source code:
// TypeScript/src/lib/es5.d.ts
// TypeScript/lib/lib.es5.d.ts

// local install:
// /usr/lib/code/extensions/node_modules/typescript/lib/lib.es5.d.ts

interface TemplateStringsArray<T = string> extends ReadonlyArray<T> {
    readonly raw: ReadonlyArray<T>;
}
```

and used it like so

```diff
// selectTranslation.ts
-export const selectTranslation = <K extends keyof Dictionary>(key: K | TemplateStringsArray): Dictionary[K] => {
+export const selectTranslation = <K extends keyof Dictionary>(key: K | TemplateStringsArray<K>): Dictionary[K] => {
```

however, the same problems persisted - the casting from `key` to `realKey` failed
AND the tagged function usage still failed

```ts
// selectTranslation.ts

if (Array.isArray(key)) {
	realKey = key[0];
} else {
	realKey = key; // still errors
}

// anotherFile.ts
const someText: string = selectTranslation`Hello`; // still errors
```

### b) Instead of using `TemplateStringsArray`, just use `Array<K>`

```diff
// selectTranslation.ts
-export const selectTranslation = <K extends keyof Dictionary>(key: K | TemplateStringsArray): Dictionary[K] => {
+export const selectTranslation = <K extends keyof Dictionary>(key: K | Array<K>): Dictionary[K] => {
```

the first problem of casting from `key` to `realKey` disappeared,
BUT the second one still remained

```ts
// selectTranslation.ts

if (Array.isArray(key)) {
	realKey = key[0];
} else {
	realKey = key; // all cool now
}

// anotherFile.ts
const someText: string = selectTranslation`Hello`; // still errors!
```

### c) Just use `any`

```diff
// selectTranslation.ts
-export const selectTranslation = <K extends keyof Dictionary>(key: K | TemplateStringsArray): Dictionary[K] => {
+export const selectTranslation = <K extends keyof Dictionary>(key: K | any): Dictionary[K] => {
```

which then allows me to use the tag function, **BUT** there's NO type-safety, autocompletions etc., making it practically useless.

## TL;DR:

Neither of the solutions helped - I'm still unable to use the `selectTranslation` function as a tag function with type safety.

Is there any way to make it possible?

Reminder - we have 2 problems here:

1. (less important since it can be avoided by using `Array<K>`) `TemplateStringsArray` does not work like it should (or I'm using it wrong), even when used as a generic 
2. (more important) tag functions cannot have type-safe parameters?

## Checklist

My suggestion meets these guidelines:

* [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, etc.)
* [x] This feature would agree with the rest of [TypeScript's Design Goals](https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals).

---

I'm happy to help if you have any further questions.