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.

El issue no menciona archivos ni pruebas. Empieza reproduciendo los ejemplos de FunctionPair y EffectDescriptor en un caso de prueba de TypeScript y, después, investiga el tipado contextual y el comportamiento de la inferencia genérica del compilador. Se considera terminado cuando los alias o interfaces no parametrizados infieren sus parámetros en las formas de asignación y aserción demostradas sin hacer retroceder la inferencia existente.