What are the core concepts in functional programming?

Question

In object-oriented programming, we might say the core concepts are:

1. encapsulation
2. inheritance,
3. polymorphism

What would that be

Answer 1

Not directly an answer to your question, but I'd like to point out that "object-oriented" and functional programming aren't necessarily at odds. The "core concepts" you cite have more general counterparts which apply just as well to functional programming.

Encapsulation, more generally, is modularisation. All purely functional languages that I know of support modular programming. You might say that those languages implement encapsulation better than the typical "OO" variety, since side-effects break encapsulation, and pure functions have no side-effects.

Inheritance, more generally, is logical implication, which is what a function represents. The canonical subclass -> superclass relation is a kind of implicit function. In functional languages, this is expressed with type classes or implicits (I consider implicits to be the more general of these two).

Polymorphism in the "OO" school is achieved by means of subtyping (inheritance). There is a more general kind of polymorphism known as parametric polymorphism (a.k.a. generics), which you will find to be supported by pure-functional programming languages. Additionally, some support "higher kinds", or higher-order generics (a.k.a. type constructor polymorphism).

What I'm trying to say is that your "core concepts of OO" aren't specific to OO in any way. I, for one, would argue that there aren't any core concepts of OO, in fact.