Comment by seagreen | Hacker News Reader

Comment by seagreen | Hacker News Reader

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I posted that because I think it's the best you're going to get.

Monads can be two things: (1) a concept in category theory, (2) a rough implementation of that concept in a programming language.

If you want to learn about monads from the category theory perspective, great! This is really hard though, I wouldn't recommend it for most people (and haven't really even succeeded myself).

If you want to learn about a specific programming language's implementation of monads, you will naturally have to know how that language works. This road is far easier than category theory, but still requires knowing (for Haskell) what a typeclass is, what a higher-kinded type is, etc.

It sounds like you want an explanation without prereqs from either category theory or Haskell. If you google for monad tutorials you will find dozens of quick explanations, but I personally don't think this is a good way to actually understand them.

Describe monads to Lisp, Scheme, APL person.

You can do that with the y-combinator but why not monads?

Because these languages lack a way of discussing something at the right "higher order" as Monads. You can implement them, but there's no word for the kind of thing a monad is. There's not even a real word for a type.

You need types. Then you need "higher order types" or parameterized types. Then you need a way to discuss a common set of operations which appear repeatedly for some of these parameterized types and it can't be OO-like.

You can exemplify the operations in, e.g., Lisp, but you have a hard time "talking about" the thing.

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In Scheme, monads are the answer to the following question:

The functions (define (pure a) (list a)) and (define (join ls) (foldl append (list) ls)) are related to lists in the same way that (define (pure a) (lambda (k) (k a))) and (define (join z) (lambda (k) ((z k) k))) is related to CPS-transformed programs. What is the commonality?

Here is monads in JavaScript https://curiosity-driven.org/monads-in-javascript

Here you go, monad explanation in CLOS:

http://www.kylheku.com/cgit/lisp-snippets/tree/monads.lisp

Apologies for there not being a use example for the state transformer monad/comprehension. I can whip one up if needed.

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