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type transformation for mapping object properties to function parameters #12265
Description
Activity
I can see this, paired with substraction types, being used for
- ADT constructors
bind/apply
This is really interesting
What determines the order of the parameters?
// somefile.ts interface Data { otherProperty: string; } // otherfile.ts interface Data { name: string; value: number; } // mainfile.ts type Constructor<T> = (P in T) => T; type CreateData = Constructor<Data>; // ?
the order of declaration:
interface One { one: number; } interface Two extends One { two: string; } interface Three { three: boolean; } interface Four extends Three, Two { } type MakeFour = Contructor<Four>; // same as type MakeFour = (three: boolean, one: number, two: string) => Fourin you case it should be a type error since the order cannot be determined
Why not just make object spread types valid in argument positions (so the spread type flattens into part of the argument list)? I think that would solve this elegantly:
type Constructor<T> = (...T) => T; interface Data { name: string; value: number; } type CreateData = Constructor<Data>;
It also makes it obvious how to combine a spread object with normal arguments, and removes a confusingly useless type parameter.
Reacted by Tobias Cudnik, kiara and Leon Adlerwhy not?
because using
(...T)you can only do trivial mapping (property of some type to a parameter of exactly the same type), sure it will support this exact use case, but extrapolating the ideas of mapped type we might support more sophisticated use cases:type Crazify<T> = (P in keyof T: MyCrazyParam<T[P]>) => MyEvenCrazierType<T>;I don't know why we didn't decline this earlier. We can't take a reliance on property ordering.
Reacted by Wesley Wigham and Andrii Kirmas
now that we have map types it would nice being able to map properties to function parameters
would desugar to