const TemplateStringsArray for TaggedTemplateExpression

Search Terms

TemplateStringsArray, TaggedTemplateExpression

Suggestion

There shouldn't be a type TemplateStringsArray, it should be a const string tuple type, so we can write code:

interface SQL<TSA, VS> {
  texts: TSA;
  values: VS;
}

function sql<TSA extends readonly string[], VS extends any[]>(texts:TSA, ...values: VS): SQL<TSA, VS> {
  return { texts, values };
}

// then
let s: SQL<['select * from person where a=', ' and b=', ''], [number, Date]> = sql`select * from person where a=${1} and b=${new Date()}`;

Use Cases

Just like in the above code, we can test the sql at compile time or use a TypeScript Language Service Plugin (in fact, I'm writing itts-sql-plugin and then come across this problem).

Examples

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

There is another issue is alike. https://github.com/microsoft/TypeScript/issues/16552#issuecomment-492919476

Besides, TemplateStringsArray is ReadonlyArray<string>, it should be const anyway.

Start by reading the proposal and its related discussion in TypeScript issue #16552, then inspect how TaggedTemplateExpression and TemplateStringsArray are represented in the compiler. Done means the type system supports the requested const string tuple behavior without changing emitted JavaScript, with appropriate compiler coverage.