Contextually infer parameters for type aliases/interfaces

Search Terms

type alias parameter inference contextual

Suggestion

The type parameters of a type alias (or interface) should be able to be inferred when a value is being assigned/cast to that type/interface.

Use Cases/Examples

My program defines a few common types/interfaces to be used as contracts between components. E.g.

// type for an object holding a function + its inverse
type FunctionPair<T, U> = { apply(it: T): U, reverse(it: U): T }; 

Then, throughout the program, I need to make objects of this type. If I have a factory function (or use a class with its constructor), this isn't too bad:

function makeFunctionPair<T, U>(apply: (it: T) => U, reverse: (it: U) => T) {
  return { apply, reverse } as FunctionPair<T, U>;
}

However, I'd like to be able to just write these (simple) domain objects with literals, rather than using a factory function, and then signal the type (with the implied relation between the apply and reverse properties) to the compiler with a type annotation/assertion inline:

const a: FunctionPair<string, () => string> = {
    apply(it: string) { return () => it + "!!!"; },
    reverse(it) { return it().slice(0, -3); }
}

However, the above is a bit verbose, in that I have to add <string, () => string> to the type annotation, whereas it seems like this should be inferrable. I'm proposing to be able to just do:

 /* FunctionPair type param values inferred contextually from the assigned object  */
const a: FunctionPair = {
    apply(it: string) { return () => it + "!!!"; },
    reverse(it) { return it().slice(0, -3); }
}

Here's another example: imagine a runtime that uses the idea of effects as data. The user sends into the runtime an object describing the effect to perform, and a callback to call with any result:

type EffectNamesWithReturnTypes = { 
    x: { status: 0 | 1 },
    y: { changes: string[] }, 
};

type EffectDescriptor<T extends keyof EffectNamesWithReturnTypes> = { 
  name: T; 
  cb: (result: EffectNamesWithReturnTypes[T]) => void
}

It would be nice to be able to write:

const effect = <EffectDescriptor>{ name: "x", cb: (it) => it.status };

Rather than having to write:

const effect = <EffectDescriptor<"x">>{ name: "x", cb: (it) => it.status };

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.

The issue names no files or tests. Start by reproducing the FunctionPair and EffectDescriptor examples in a TypeScript test case, then investigate the compiler's contextual typing and generic inference behavior. Done means unparameterized aliases or interfaces infer their parameters in the demonstrated assignment and assertion forms without regressing existing inference.