On: The visitor pattern is essentially the same thing ...

[h_anna_h]

What is the point of doing forall T but not forall A and B?

[Twisol → h_anna_h]

A and B are free to be chosen by a producer of the generic Sum type. However, T can be chosen "late", by a consumer of Sum; a value of type Sum A B must work for any T you decide to use down the line.

If it helps, the equivalent Java signature is:

    interface Sum<A, B> {
      <T> T visit(Function<A, T> onA, Function<B, T> onB);
    }

Hopefully this makes it more clear that A and B are fixed when you receive a value of type Sum A B, but you get to pick a T when consuming that value.

[h_anna_h → Twisol]

Thanks