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Infer Subtypes from Template Literal Types #43907
Description
Suggestion
🔍 Search Terms
Inference
Template Literal Types
Subtypes
✅ Viability 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, new syntax sugar for JS, etc.)
- This feature would agree with the rest of TypeScript's Design Goals.
⭐ Suggestion
I'd like to be able to infer a subtype from a template literal type. For example:
const canExtend: `myString.${number}` extends `${infer Key}.${number}` ? true : false = true;📃 Motivating Example
This will allow cross-matching between Values and Paths when traversing an object with dot notation.
💻 Use Cases
See line 35, in this Playground
Activity
johnrom commentedon Apr 30, 2021
I was able to kind of work around this limitation by using the string value "0" instead of
${number}, and then converting that to ${number} later down the road.It seems to randomly cause "Type instantiation is excessively deep and possibly infinite". I'm open to other workarounds that might not cause so much recursion if anyone has any! You can see changes here, I marked the comments with CHANGE:
Playground
johnrom commentedon Apr 30, 2021
For a real world example, here is Formik with the above integration, with strongly typed Fields which enforce valid
name,value, child component, or parse / format function. There are a lot of variants on the core Field component, so testing it is unfortunately very verbose.https://codesandbox.io/s/serene-napier-4wjxs?file=/src/App.tsx
DanielRosenwasser commentedon May 1, 2021
This seems to work in nightlies, right?
DanielRosenwasser commentedon May 1, 2021
Check it out using a nightly release of TypeScript
DanielRosenwasser commentedon May 1, 2021
(Fixed by #43361, duplicate of #43243)
typescript-bot commentedon May 3, 2021
This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.