问题
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?
回答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.
来源：https://stackoverflow.com/questions/56032669/why-concurrently-is-not-a-monad-in-haskell