CODEWITHSUNDEEP
genericstypescript
Mike Ball
Mike Ball

Reputation: 68

Generic class without type parameter in generic function

Is there any way in Typescript to capture a generic class without its type parameter? I'm essentially trying to make a procedural style map function that operates on collection classes in my program like

interface Collection<T> {
    ...
}

function map<T, U, V extends Collection>(collection:V<T>, fn:(e:T) => U):V<U> {
    ....
}

but this doesn't work since the compiler wants me to put a type parameter on the Collection in map's generic list.

Essentially I want the type checker to know that if I call it on a List<T>, I get back a List<U>; call it on a Set<T>, I get a Set<U>; etc. Is this possible?

Thanks

Upvotes: 0

Views: 1106

Answers (1)

Fenton
Fenton

Reputation: 251242

Here is my take on this...

function map<T, U, V extends Collection<any>>(collection: V, fn:(e:T) => U) : Collection<U> {
    return null;
}

Compromise one: You can't re-use the type T in V extends Collection<any>. Ideally, you would have used T not any.

Compromise two: We are returning the less specific Collection<U> from the function. Ideally, you accept a general type and return a specific one, so this is a compromise.

No compromise version. You need an in type and an out type and you need a generic class so you can re-use the class types within the method type arguments:

class Mapper<T, U> {
    map<TIn extends Collection<T>, TOut extends Collection<U>>(collection: TIn, fn:(e:T) => U) : TOut {
        return null;
    }
}

Upvotes: 1

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