I come from an object oriented background where I've learned that classes are or at least can be used to make a layer of abstraction that allows for easy recycling of code which can then either be used to make objects or be used in inheritance.
Like for example I can have an animal class and then from that inherit cats and dogs and such that all inherit many of the same traits, and from those sub-classes I can then make objects that can specify a breed of animal or even the name of it.
Or I can use classes to specify multiple instances of the same code that handles or contains slightly different things; like nodes in a search-tree or multiple different database connections and what not.
I'm recently moving into functional programming, so I was starting to wonder:
How do purely functional languages handle things like that? That is, languages without any concept of classes and objects.
Best Answer
Many functional languages have a module system. (Many object-oriented languages do too, by the way.) But even in the absence of one, you can use functions as modules.
JavaScript is a good example. In JavaScript, functions are used both to implement modules and even object-oriented encapsulation. In Scheme, which was the major inspiration for JavaScript, there are only functions. Functions are used to implement almost everything: objects, modules (called units in Racket), even data structures.
OTOH, Haskell and the ML family have an explicit module system.
Object Orientation is about data abstraction. That's it. Modularity, Inheritance, Polymorphism, even mutable state are orthogonal concerns.