Question: please clarify equals/hashCode/toString philosophy as stated in README about derive4j HOT 4 CLOSED

Essentially it is because those methods (all object methods) breaks parametricity which is a very important property to be able to reason about types and code. To quote @tel:

If I have a type BiFunction<A,A,A> then with parametricity it’s either an infinite loop or one of the following

(x, y) -> x

or

(x, y) -> y

that’s a trivial example, but it scales very well and can eliminate huge swaths of potential implementations and vastly improve type-driven reasoning.

On the other hand, if you account for Object#hashcode/equals, BiFunction<A,A,A> has more inhabitants, even without considering impurity, for instance:

 (x, y) -> if (x.hashCode > 1000) x else y

Other pitfalls of Object#equals include:

• it let you compare Apples and Oranges.
• there is usually no guarantees that it is not the default implementation inherited from Object which never what you want.
• implementing it correctly in presence of subtyping is almost impossible.

For further reading/viewing on why you would want parametricity, I recommend Tony Morris' talks on the topic.

Hope it helps!

Edit (thanks @TomasMikula): the Bifunction<A, A, A> is to be read in a context where A is a universally quantified (unrestricted) type variable. Eg:

<A> BiFunction<A, A, A> foo() {
  // either:
  return (x, y) -> x
  // or
  return (x, y) -> y
}

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Thank you! I kept glossing over this part of the README and finally just had to ask :). I don't think I have the necessary background to actually understand your answer, but this gives me plenty of direction for further exploration. <3

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@jbgi I get your point, but BiFunction<A,A,A> is a somewhat confusing type, because it doesn't make sense without an enclosing scope where A is introduced. In that scope, A might be an unrestricted type parameter (which is what you intended) as in

class Foo<A> {
    BiFunction<A, A, A> f = ??? // this has to be one of
                                //   (x, y) -> x
                                //   (x, y) -> y
                                //   infinite recursion
}

but there might also be more information available about A in the enclosing scope, as in

class Foo<A extends Bar> {
    BiFunction<A, A, A> f = ??? // many more options
}

abstract class Bar {
    public int x;
}

So maybe instead of BiFunction, it is better to use a generic method to illustrate the point:

static <A> A foo(A a1, A a2) {
    // now my only options are
    //   return a1;
    //   return a2;
    //   infinite recursion, e.g. return foo(a1, foo(a2, a1));
}

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@TomasMikula totally, thanks for the clarification; I fail to mention that A in my example need to be a universally quantified type variable for it to make sense.

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