java 2024-06-03 | Slack Archive

Ben Sless06:06:18

Trying to implement category theory things in Java (why not), I get tripped up on java syntax and generics Thought I'd start with interfaces for Functor, Applicative and Monad, then go from there Starting off, Functor:

public interface Functor<A> {
    // which is the correct signature for map?
    <B extends Functor<B>> Functor<B>map(Function<A,B> f);
    <B> Functor<B>map(Function<A,B> f);
}

The comment says it. In theory, we can map over every functor, but do I need to express somehow that the receiver and return value can be anything that extends Functor? It gets more complicated when Applicative provides a default implementation for functor:

@Override
    default <B> Functor<B> map(Function<A, B> f) {
        return this.apply(pure(f));
    }

Is this right? Does map over Applicative return Functor? Can I express somehow that it returns anything which extends Functor<B>? I have the same problem with Monad but figuring out the syntax / correct typing for applicative will probably carry over

Ben Sless07:06:13

Do I need to change the implementation in Applicative to return an Applicative?

littleli13:06:14

I'm not really good at this. But take a look at https://typelevel.org/cats/typeclasses/applicative.html it may cast some light on what is happening there with map and why you need ap (or product as an alternative reasoning) as a supportive construct https://typelevel.org/cats/typeclasses/applicative.html#what-is-ap So the way I see it is that Applicative is a Functor where you teach your map an additional superpower to be able to handle things like Optional types. I'm not sure if Java type system is capable to express this.

2024-06-03

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