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A list is a monad

(alexyorke.github.io)
153 points polygot | 1 comments | 29 Jun 25 17:53 UTC | HN request time: 0.202s | source
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nemo1618 ◴[02 Jul 25 18:20 UTC] No.44447106[source]▶
I think this adds more confusion than it removes.

A list is not a monad. A list is a data structure; a monad is more like a "trait" or "interface." So you can define a List type that "implements" the monad interface, but this is not an inherent property of lists themselves. That's the sense in which a list "is a" monad: the OOP sense.

Haskell's List monad provides a model for nondeterminism. But that certainly isn't the only way List could satisfy the monad interface! It was a deliberate choice -- a good choice, possibly the best choice, but a choice nonetheless.

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1. instig007 ◴[03 Jul 25 03:32 UTC] No.44451386[source]▶
> A list is not a monad. A list is a data structure; a monad is more like a "trait" or "interface."

You operate with things that are bound to PL notions of specific languages. Instead, consider that list isn't a data structure, it's an abstraction that defines a multitude of position-ordered values. The multitude of position-ordered values called "list" is a presented entity of "monad", because it can be used as a valid input for a monadic computation defined consistently (in terms of the monad laws).