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.

[harrysolovay]

Dear TS team, I’d like to reiterate:

Tagged template library developers will want to…

• Protect their users from supplying incompatibly-typed arguments.
• Produce literal types which can be utilized elsewhere, outside of the tagged template.
• Eliminate the unnecessary right and left parens from their string-accepting fns.

Currently, the above is not possible.

Any thoughts would be greatly appreciated!

[alloy]

+1 for this feature. I have a case where I’m writing typings for a Flow library, where we could have safer typings thanks to template literal types, except the pattern the library uses is to have a tagged template function. I can’t really change their pattern.

The example can be found here, where graphqlTag doesn’t work.