[creata]

> It doesn't compose with other abstractions

But monads don't compose with each other, do they? I thought you needed something more (monad transformers, algebraic effects, etc.) to get things to compose. Or maybe you meant something else by composition.

> I can't use this for my own abstractions

Yep. But Java doesn't even let you overload addition afaik, so being able to overload ? is pretty much a pipe dream.

> Do you mean that, because you don't see people lamenting, it's not a useful feature?

No, it's just that if something "backfired" for Java, I'd expect to hear people complaining about it.

[a1369209993]

> But monads don't compose with each other, do they?

They do if they're[0] traversable (cf literally `Data.Traversable` in Haskell), which almost all specific monads are. You'd have something like:

  newtype Comp f1 f2 a = Comp { unComp :: (f1 (f2 a)) }
  instance (Monad f1,Monad f2,Traversable f2) => Monad (Comp f1 f2) where
    (Comp a) >>= k = Comp $ a >>= (map join . traverse (unComp . k))

Edit: 0: actually, only the inner monad needs to be traversable.

[creata]

Looks like that that's not a valid monad in general.[0] And also, I feel like it's a stretch to say that "almost all specific monads" are traversable.

[0]: https://stackoverflow.com/questions/42284879/is-the-composit...

[a1369209993]

I'm not convinced (Bool->) is a legitimate Traversable[0], but that's at least highly concerning and makes me wish the typechecker could properly handle "Instances should satisfy the following laws" comments as actual code.

0: in vaguely the same way that Int isn't a legitimate Monoid because you get nonsense by switching back and forth between sum and product (and min, max, xor, and, or, etc)