Probabilistic Types

Search Terms

Probabilistic, union, supertypes

Suggestion

If a union type contains a member which is a supertype of one or more other members, the supertype subsumes those other members, but the information isn't lost. Tooling can then use that information as a hint about the expected distribution of possible values within that type.

type MetaSyntacticVar = 'foo' | 'bar' | 'baz' | string;

For the purposes of type safety, the above type is simply string. However, tools may consider 'foo' | 'bar' | 'baz' to be the most likely values for MetaSyntacticVar. An IDE might use them for autocomplete. A linter (i.e. not tsc) might flag other values as a warning. Etc.

Use Cases

A backend service offers a range of options, and I want to statically model them as a union type. Sometimes, the service adds a new option. I must either generalize the type to a string, do a type cast and tolerate some type unsafety, or release code updates in sync with all backend updates.

Examples

type MetaSyntacticVar = 'foo' | 'bar' | 'baz' | string;

const name: MetaSyntacticVar = 'qux';
================================^ IDE suggests foo, bar, or baz as I type this
================================^^^ eslint flags this as a warning (if I opt in)

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.

Start by reviewing TypeScript's union-type checking and the tooling behavior described for autocomplete and linting. Determine how literal members could remain available as hints while the containing type retains its current type-safety behavior. Done means the proposal has an agreed design and covers the type checker, IDE tooling, and optional linter behavior.