How do you learn and teach monoidal categories and tensor products in functional programming?

1 What are monoidal categories?

A monoidal category is a mathematical structure that consists of objects, morphisms, and a special operation called a tensor product. The tensor product allows you to combine two objects into a new one, and two morphisms into a new one, while preserving some properties. For example, the category of sets and functions is a monoidal category, where the tensor product is the cartesian product of sets and the function composition of functions.

2 What are tensor products?

A tensor product is a way of combining two objects or two morphisms in a monoidal category. It is denoted by a symbol, usually ⊗, and it has some rules that make it compatible with the category structure. For example, in the category of sets and functions, the tensor product of two sets A and B is the set of pairs (a,b) where a ∈ A and b ∈ B, and the tensor product of two functions f and g is the function that maps (a,b) to (f(a),g(b)).

3 Why do they matter in functional programming?

Monoidal categories and tensor products are useful in functional programming because they can express complex computations and data structures in a simple and elegant way. For example, you can use them to model parallelism, concurrency, effects, streams, parsers, and more. They also allow you to reason about your programs using abstract properties and laws, such as associativity, identity, and symmetry.

4 How to learn them in functional programming?

One way to learn monoidal categories and tensor products in functional programming is to study some examples of existing libraries and frameworks that use them. For example, you can look at how Haskell implements monoids, applicative functors, monads, arrows, and profunctors using type classes and operators. You can also explore how Scala uses cats, shapeless, or scalaz to provide similar abstractions. You can also read some books or papers that explain the theory and practice of these concepts, such as Category Theory for Programmers by Bartosz Milewski or The Algebra of Programming by Richard Bird and Oege de Moor.

5 How to teach them in functional programming?

Another way to learn monoidal categories and tensor products in functional programming is to teach them to others. Teaching can help you deepen your understanding and improve your communication skills. You can teach them in different ways, depending on your audience and goals. For example, you can use analogies, diagrams, examples, exercises, or games to illustrate the ideas and motivate the learners. You can also use interactive tools, such as repls, notebooks, or online platforms, to demonstrate the code and test the concepts.

6 How to use them in functional programming?

Finally, the best way to learn monoidal categories and tensor products in functional programming is to use them in your own projects and problems. You can apply them to design and implement solutions that are modular, compositional, and reusable. You can also use them to refactor and optimize your code, or to discover new patterns and abstractions. You can also use them to collaborate and communicate with other functional programmers, or to learn from their code and feedback.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?