Comment by tome

The problem with monads is their composition. Ie. questions like how is the do notation supposed to work if I want to return:

* an Option<Async> or Async<Option>?

* both an Option and an Async (a product, or tuple type)?

* either an Option or an Async (a sum, or tagged union type)?

Monad transformers can be written for wrapping monads into other monads (as a simple example, an Option is equivalent a List with exactly zero or one elements), but they're something of an ad hoc solution and do not generalize well.

This is fundamentally an issue similar to the "function color" problem, or the fact that exceptions in most languages are a limited, ad hoc effect system and do not compose well. Java gave checked exceptions a bad name largely because of their lack of compositionality, but it's more that the particular implementation is poor.

To be fair, that was in 1994 and nobody had worked out these things yet even in the academia. Algebraic effects are the attempt to do just that.