The process of linking a specific type to a generic function behind the scenes

Question

The process of linking a specific type to a generic function behind the scenes

In my particular situation, I have a class (for example "Collection") that is aware of the type it is dealing with (such as "DBType"). The constructor of this class receives a generic function (like "Transform") which takes the subtype of DBType and returns an instance of another class based on this subtype. When a user triggers a method within the collection with certain parameters, the collection can determine which subtype of DBType will be passed to the transform function.

Everything works fine if I separately provide the subtype DBType to the transform function. However, when it comes to implementing this logic within the class, I seem to hit a roadblock. I am struggling to find a solution for this dilemma. It could possibly be a limitation of TypeScript.

I have created a basic example in the playground which showcases the issue. Your assistance would be greatly appreciated.

typescript typescript-generics

Answer 1

Answer №1

The main issue lies in the fact that you are utilizing

ReturnType<typeof transform>

with a transform function that is both generic and has multiple overloads. Unfortunately, ReturnType does not support these scenarios, resulting in incorrect outcomes. The correct approach involves:

type TranslateDoc<T> = T extends TSomeDoc ? SomeDoc<T>
                     : T extends TOtherDoc ? OtherDoc<T>
                     : T

// TEST TRANSFORM
function transform<T extends TSomeDoc | TOtherDoc | Partial<TCountDoc>>(doc: T): TranslateDoc<T> {

It seems impossible to make the TCollection generic in terms of which transform it uses. In this case, you can define the class as follows:

// The second generic parameter is not utilized and solely relies on the transform function.
declare class TCollection<Doc, Fn extends typeof transform = typeof transform> {
    constructor(transform: Fn);

    findOne<T extends Partial<Doc>>(doc: T): TranslateDoc<T>;
}

To my knowledge, there is no method to implement higher-order generics (specifying the argument for TranslateDoc based on the class) in this scenario.

Answer 2

The main issue lies in the fact that you are utilizing

ReturnType<typeof transform>

with a transform function that is both generic and has multiple overloads. Unfortunately, ReturnType does not support these scenarios, resulting in incorrect outcomes. The correct approach involves:

type TranslateDoc<T> = T extends TSomeDoc ? SomeDoc<T>
                     : T extends TOtherDoc ? OtherDoc<T>
                     : T

// TEST TRANSFORM
function transform<T extends TSomeDoc | TOtherDoc | Partial<TCountDoc>>(doc: T): TranslateDoc<T> {

It seems impossible to make the TCollection generic in terms of which transform it uses. In this case, you can define the class as follows:

// The second generic parameter is not utilized and solely relies on the transform function.
declare class TCollection<Doc, Fn extends typeof transform = typeof transform> {
    constructor(transform: Fn);

    findOne<T extends Partial<Doc>>(doc: T): TranslateDoc<T>;
}

To my knowledge, there is no method to implement higher-order generics (specifying the argument for TranslateDoc based on the class) in this scenario.

The process of linking a specific type to a generic function behind the scenes

Answer №1

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