I want to learn functional programming: where to start?

Elm strikes me as a pretty good place to dive in because it:

• Executes in the browser, similar to other stuff you've used in JS land.
• Is purely functional and pretty hardcore in this sense
• Is designed to be more "mainstream" than Haskell
• Has a friendly community

Since you come from Ruby on Rails land, Elixir might be a good choice because it's sort of an offshoot from that ecosystem and has a lot of community crossover.

This is my answer as someone who tends to start with practical use cases and work down to theoretical.

@xtrasmal is a great choice if you are more of a bottom-up learner.

Hi there. Language apart, I'd like to point you to this resource
Category Theory for Programmers(github.com/hmemcpy/milewski-ctfp-pdf) or buy the book, because it is very good. It talks about composition. Here is a quote from the book:

"Composition is at the very root of category theory — it’s part of the definition of the category itself. And I will argue strongly that composition is the essence of programming. We’ve been composing things forever, long before some great engineer came up with the idea of a subroutine.
Some time ago the principles of structural programming revolutionized programming because they made blocks of code composable. Then came object oriented programming, which is all about composing objects.
Functional programming is not only about composing functions and algebraic data structures — it makes concurrency composable — something that’s virtually impossible with other programming paradigms."

The book uses examples from Haskell and other languages, as illustrations. So you will learn a little of that while you are at it.

When picking a functional language like Elixir, you will find that there is more stuff than just the language that are immensely awesome to tap in to. Like Elixir runs on BEAM, it is fault-tolerant(and other cool things), it has a framework called Phoenix for frontends(for example) and there is a thing called Nerves Project, which allows you to burn your code to an image for embedded systems.

What programming languages do you have the most experience with?

I would start by making sure to understand these basic concepts:

• recursion
• pure function
• closure
• higher-order function

Next I would look at these well-known higher-order functions:

• map
• filter
• fold (aka reduce)

These are ideas that can be used in many programming languages. In particular, if you're familiar with python or javascript, all of the above topics can be understood and applied in a natural way in both of them.

Then you could go into topics that tend to be more relevant in really pure functional languages like Haskell (functors, monads, monoids, combinators...).

To others responding to this question:
I am a python programmer who wishes to learn FP.

In past I have "dabbled" with Elixir - Self learnt via their documentation, did Udemy course and also published a package on hex.pm

Then I "lost touch".

I recently came across coconut Being python programmer, I am intrigued.

My generic question (aside from choice of language) : What are your suggestions to adopt "Functional" mindset ?

The Math-based Grand Unified Programming Theory: The Pure Function Pipeline Data Flow with Principle-based Warehouse/Workshop Model

It is the perfect theory if a theory is based on mathematics and meets the five basic principles of "Simplicity, Unity, Order, Symmetry and Definiteness" in science, industry and aesthetics.

Traditional IT theory (OOP, FP and hardware architecture, etc.) are pseudoscience. they belong to what physicist Wolfgang Pauli said "Not Even Wrong".

Software and hardware are factories that manufacture data, so they have the same "warehouse/workshop model" and management methods as the manufacturing industry.

---- Lin Pengcheng