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Comment by 0DHm2CxO7Lb3 | Hacker News Reader

> I don't understand why having a formal model for that commonality is so important.

So generic libraries can be built and abstracting frequently used operations over many different data types. I've never implemented a monad myself outside of a toy project and am by no means an expert, but it's quite nice to be able to transfer certain knowledge on lists to options or futures. Similar to generic interfaces like a collection interface, so whether you are working on a set or a list you know that 'get' or 'exists' can be used.

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Sure, but what generic libraries can you meaningfully built above optional, list, either, futures? Do-notation is one, but what else?

Essentially any library function that takes any kind of callback benefits from being able to take a generically-effectful callback. There's plenty of mileage in basic things like database row mappers.

If you want fancy examples, things like generic data structure traversals (I use a cataM like https://github.com/ekmett/recursion-schemes/issues/3 all the time). Another good example is iteratees - you can define an effectful source, an effectful sink, or an effectful transformation stage in a stream pipeline, which makes it very practical to have very small reusable pieces. Since it's using the generic monad interface you can define an intermediate stream transformer even for some custom effect that you wrote yourself, but be confident that everything will be plumbed together correctly.

There are a lot of simple examples right in `Control.Monad`. As soon as I know that something provides the `Monad` interface, I know that I can use things like `mapM`, `foldM`, `replicateM`, `filterM`, `forever`, `sequence`, `zipWithM`, the list goes on...

Certainly not all of these are useful in every case - eg. `forever` needs your action to do something or you're just hanging forever.

But it's a toolbox that's easy to reach for, and which applies to a large pile of things. And when you can structure a new function only in terms of things in that toolbox, you've added another thing to the toolbox. See, for instance, `monad-loops` for more.

Exploring pairs of utility function and Monad instance can be interesting, asking (eg.) "what does `unfoldM` mean when I use it with `State`?"

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