Why `Concurrently` is not a monad in Haskell?

Question

I'm reading the doc of package async , and trying to find something similar to JavaScript's Promise, and I find Concurrently , which is the most close concept that implemented Functor , Applicative (Promise.all), Alternative (Promise.race). But it doesn't implement Monad (Promise.then), I'm wondering why.

I think it maybe because (>>=) is a sequential operation, which conflict with the name Concurrently , but is this the only reason? is there some more important reason here?

Answer 1

The Monad typeclass states that (<*>) and ap should be equivalent. (<*>) for Concurrently evaluates both the LHS and the RHS at the same time. ap can't evaluate the RHS until the LHS is done, since (>>=) needs to finish evaluating the LHS before it can call the function that gives it the RHS.