The ways in which promises do not form a monad (and in fact not even pure values) are actually pretty well understood. In particular, then is not a lawful bind, and join is impossible to express (because simply referring to a value causes effects to start happening).
Moreover you cannot generally express interaction with traversable containers, functor composition, monad transformers, or other general purpose abstractions with respect to promises, because again, they don't actually form a monad.
Regarding "neither is the implementation in this post": I'm not sure you've actually grasped the content of the post if this is the conclusion you've arrived at. The operations given in this post precisely form a monad (including obeying all the relevant laws).
In the construction above, verifyUser is a pure, continuation returning function. In your snippet, async function verifyUser(user, password) { ... } is not even really a function in the functional programming sense of the word.
As a very simple example, the promise produced by mapping a Promise-based implementation of deleteUser over an array of usernames and taking the first element doesn't represent deleting the first user; instead every user in the database would be deleted. Conversely, doing the same thing with a deleteUser based on a lawful asynchronicity monad, as given in the post, would be no different than taking the first element and then applying deleteUser to it. Both would produce a continuation representing deleting the first user (nothing would actually start happening "behind the scenes").
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The ways in which promises do not form a monad (and in fact not even pure values) are actually pretty well understood. In particular,
thenis not a lawfulbind, andjoinis impossible to express (because simply referring to a value causes effects to start happening).Moreover you cannot generally express interaction with traversable containers, functor composition, monad transformers, or other general purpose abstractions with respect to promises, because again, they don't actually form a monad.
Regarding "neither is the implementation in this post": I'm not sure you've actually grasped the content of the post if this is the conclusion you've arrived at. The operations given in this post precisely form a monad (including obeying all the relevant laws).
In the construction above,
verifyUseris a pure, continuation returning function. In your snippet,async function verifyUser(user, password) { ... }is not even really a function in the functional programming sense of the word.As a very simple example, the promise produced by mapping a Promise-based implementation of
deleteUserover an array of usernames and taking the first element doesn't represent deleting the first user; instead every user in the database would be deleted. Conversely, doing the same thing with adeleteUserbased on a lawful asynchronicity monad, as given in the post, would be no different than taking the first element and then applyingdeleteUserto it. Both would produce a continuation representing deleting the first user (nothing would actually start happening "behind the scenes").