Object Mentor Blog: A Brief Collection of Convenient Lies about Functional Programming

A Brief Collection of Convenient Lies about Functional Programming

Michael Feathers — Mon, 23 Mar 2009 10:19:00 -0500

A value is the instantaneous state of an object. – In OO languages, we have objects. In FP languages, we throw out the object and instead manipulate the values it would take on over time.
Algebraic data types are classes. – Every case in an ADT is a state that an “object” can be in. data Tree = Empty | Leaf Int | Node Tree Tree When we write functions over ADTs, we are obliged to cover all of the cases. So, for instance, if we define depth for Empty, we have to define depth for the Leaf and Node cases as well. When we do, we can evaluate depth t for any tree value and have a well-defined result.
The functions which we define over an ADT can be considered its public interface. – There’s a school of thought which says that encapsulation doesn’t matter in an functional programming language because values are immutable and corruption can’t happen. Nothing could be further from the truth. If we add or remove a case from an ADT all of the functions which pattern match against it are impacted. While we don’t need to have an encapsulation boundary as tight as we might have in an OO language – it pays to be conscious of how far ADTs travel in a program. Encapsulation is the act of forming a boundary by transforming an ADT into some other form of data.

Each of these statements is a lie, an artful simplification, but they are a convenient and not entirely false way of thinking about functional programming until it becomes second-nature.

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