What is the use of monads in OCaml?

Question

This might be a stupid question, but since OCaml is not pure and has side effects built-in, what is the use of monads in OCaml?

Answer 1

Monads have nothing to do with purity, except that a pure language without Monads would be almost entirely useless.

In layman's terms, a Monad is just a set of rules that describe how a sequence of steps can be executed. Having a Monad abstraction gives you the ability to define a DSL for executing stuff. A Monad can be built to intelligently handle things like exceptions, ATOMIC rollbacks/commits, retry logic, sleeping between each step, or whatever.

Here are some examples of Monads:

https://wiki.haskell.org/Monad#Interesting_monads

I realize that this list is for Haskell, which is a pure language, but don't let that confuse you.

You don't need to understand category theory to understand what a Monad is, contrary to popular belief. A monad basically has 2 things: (paraphrased from this wikipedia article)

1. A unit function, defined as (a -> M a), called "return" in Haskell, used to put a value into the context of a Monad.

2. A binding operation, defined as (M t -> (t -> M u) -> M u), which looks scary but if you look carefully, this is a function that gets invoked between each step of the process, this is where you inject the good stuff.

Depending on the language, there may be more things, but this is the heart of it.