Lazily evaluated template literal types

Suggestion

Template literal types seem to be resolved immediately, which makes the combinatorics blow up and the whole thing much less useful than it would seem to be at first.

For example I can't even represent a type equals to 5 consecutive digits with the current system without getting a "too complex" error:

type Digit = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9;
type UUID = `${Digit}${Digit}${Digit}${Digit}${Digit}`;

The current system may be more composable than regexes, but if it's not even able to represent something like /\d{5}/ I'd argue that's not a replacement for it at all.

The suggestion is that there's no need to resolve all the possible combinations at all, but a divide and conquer approach should be implemented where each little piece of the template literal does it's job individually and when all little pieces match than the whole thing matches.

🔍 Search Terms

• template literal
• lazy

✅ Viability Checklist

• 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, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Evaluate template literal types lazily to not make the combinatorics blow up. Meaning that each little piece of the template literal should be its own little function that performs some type matching internally, so there's no need to resolve all the possible combinations at all.

📃 Motivating Example

I can't even represent the equivalent of /\d{5}/ currently.

type Digit = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9;
type UUID = `${Digit}${Digit}${Digit}${Digit}${Digit}`;

💻 Use Cases

For example creating a type for UUID strings, which are a fairly common thing.

Bắt đầu với ví dụ TypeScript tạo động lực trong issue: một kiểu literal template gồm năm chữ số được xây dựng từ union Digit. Điều tra việc đánh giá và hành vi matching của kiểu literal template trong compiler; được coi là hoàn thành khi kiểu này có thể được biểu diễn và sử dụng mà không gây ra lỗi "too complex" hiện tại, đồng thời vẫn giữ nguyên matching kiểu literal template.