[KPGv2]

> All asynchronous interfaces form a monad whether you decide to follow the Haskell monad type class or decide to do something els

JS's then is categorically not a monad because it doesn't follow the monad laws.

fn1 : a -> Promise<b>

fn2 : b -> c

fn3 : b -> Promise<c>

With JavaScript, composing fn1 and fn2 with then gives you a -> Promise<c>. So then is isomorphic to map.

With JavaScript, composing fn1 and fn3 with then gives you a -> Promise<c>. So then is isomorphic to flatmap.

Therefore, with JavaScript, map is isomorphic to flatmap. Which obviously violates monad laws.

There's a rather famous Github issue where someone points this out in the issue tracker for `then` development, and one of the devs in charge of then...leaves responses for posterity.

[anon291]

I mean, JavaScript is weakly typed and then always lifts its argument. Speaking of isomorphism doesn't make sense unless you're talking about a typed portion of the language.