I maintain that the big advantage of the On Lisp approach is that all of that is available without having to write a new compiler.

Granted, I also don't have as heavy an attachment to pure functional as most people seem to build. Don't get me wrong, wanton nonsense is nonsensical. But that is just as true in immutable contexts.

> if you can write compilers anything that involves composing functions is weak beer

> screwing around with functions and macros doesn't hold a candle to what you learn from the Dragon Book.

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So, what is it that you learn from that book that's a revelation for you compared to the weak beer of composable effect systems?

> screwing around with functions and macros doesn't hold a candle to what you learn from the Dragon Book

This depends a lot on what you mean. My first take is that the more you know about macros the more you realize what they can do.

I don’t know what your takeaway from the Dragon Book was, but writing DSLs using macros feels very usefully powerful to me.

I think you are undervaluing modern macros.

But lisp programs are compilers. That's the whole point of lisp and macros. Your functions can happily emit assembly direction.

I don't think you're being fair to Chris Penner. He ends his blog post with: "It may take me another 5 years to finally finish it, but at some point we'll continue this journey and explore how we can sequence effects using the hierarchy of Category classes instead." Emphasis by me.

So while it is true, that what he has described so far is not sufficiently powerful for normal programs, he has clearly stated that there are more abstractions between Applicative and Monad to explore than what he has presented so far.

Paging John Harrop...

https://news.ycombinator.com/item?id=1396763

The important thing to know first is that a monad is not a single thing like "Optional". "monad" is a pattern or "interface" (called a "typeclass" in Haskell), that has many implementations, (Optional, Either, List, State Transormer, IO (Input/Output), Logger, Continuation, etc). Sort of how "Visitor" pattern in C++/Java is not a single thing.

https://hackage.haskell.org/package/base-4.21.0.0/docs/Contr...

https://book.realworldhaskell.org/read/monads.html

A common metaphor for monad is "executable semicolons". They are effectively a way to add (structured) hook computations (that always returns a specific type of value) to run every time a "main" computation (akin to a "statement" in other languages) occurs "in" the monad.

It's sort of like a decorator in Python, but more structured. It lets you write a series of simple computational steps (transforming values), and then "dress them up" / "clean them up" by adding a specific computation to run after each step.

Unfortunately no one can tell you what a monad is. You have to experience it for yourself. - Haskell Morpheus

You should first understand what a typeclass and a Functor is.

This is great imo -- https://www.adit.io/posts/2013-04-17-functors,_applicatives,...

A monad is just a monoid in the category of endofunctors

Forget all the academic definitions, at it's core a monad is a container or wrapper that adds additional functionality to a type.

Just think of it as a design pattern, but a bit more strict than the Gang of Four patterns. Fundamentally it's a relationship between types and other types such that certain operations make sense and follow well-understood rules (the monadic laws). Study the monadic laws, and try playing with the State, IO, and List monads to get a better sense of what those operations are and why they're useful for sequencing in a pure-functional context.

Someone may correct me but - in three levels of conciseness...

A monad is a function that can be combined with other functions.

It's a closure (or functor to the cool kids) that can be bound and arranged into a more complex composite closure without a specification of any actual value to operate on.

It's a lazy operation declaration that can operate over a class of types rather than a specific type (though a type is a class of types with just a single type so this is more a note on potential rather than necessary utility) that can be composed and manipulated in languages like Haskell to easily create large declarative blocks of code that are very easy to understand and lend themselves easily to abstract proofs about execution.

You've probably used them or a pattern like them in your code without realizing it.

Monads are a generalization of Promises. Each type in Monad defines their own `.then` in a different way. For promises, `.then` is defined as "run this function once you have this deferred value from the last promise". For optionals (`Maybe`), `.then` is defined as "run this function if the last optional had an actual value". For Either, `.then` is defined as "run this function if the last Either returned Right, otherwise early-return with the value from Left" (this is functional early-return, basically).

It's an implementation of the typeclass Monad, which happens to come with a special "do" keyword.

Nan-in received a university professor who came to inquire about Monads

Nan-in served tea. He poured his visitor’s cup full, and then kept on pouring.

The professor watched the overflow until he no longer could restrain himself. “It is overfull. No more will go in!”

“Like this cup,” Nan-in said, “you are full of your own opinions and speculations. How can I show you a Monad unless you first empty your cup?”