Scott Wlaschin:

But now we have something much more abstract, a set of generalized requirements that can apply to all sorts of things:

• You start with a bunch of things, and some way of combining them two at a time.
• Rule 1 (Closure): The result of combining two things is always another one of the things.
• Rule 2 (Associativity): When combining more than two things, which pairwise combination you do first doesn’t matter.
• Rule 3 (Identity element): There is a special thing called “zero” such that when you combine any thing with “zero” you get the original thing back.

With these rules in place, we can come back to the definition of a monoid. A “monoid” is just a system that obeys all three rules. Simple!

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To sum up, a monoid is basically a way to describe an aggregation pattern – we have a list of things, we have some way of combining them, and we get a single aggregated object back at the end.

The first of three posts that give a simple definition of what monoids are and the benefits they provide.