pure functions

Scala: What do “effect” and “effectful” mean in functional programming?

When you get started with functional programming (FP) a common question you’ll have is, “What is an effect in functional programming?” You’ll hear advanced FPers use the words effects and effectful, but it can be hard to find a definition of what these terms mean.

Functional programming: Math functions, not programming functions

“There’s only ONE rule, but it’s an important one: all of your values must be functions. Not programming functions, but math functions.”

I think I read that quote in an earlier version of this article. The quote is about functional programming, and it influenced something I wrote in my book, Functional Programming, Simplified: Functional programmers think about themselves as being mathematicians, and think of their code as being a combination of algebraic equations, where each function is a pure function that you can think of in mathematical terms.

This is a page from my book, Functional Programming, Simplified

Pure Function Signatures Tell All

“In Haskell, a function’s type declaration tells you a whole lot about the function, due to the very strong type system.”

Learn You a Haskell for Great Good!

One thing you’ll find in FP is that the signatures of pure functions tell you a lot about what those functions do. In fact, it turns out that the signatures of functions in FP applications are much more important than they are in OOP applications. As you’ll see in this lesson:

This is a page from my book, Functional Programming, Simplified

The Benefits of Pure Functions

When asked, “What are the advantages of writing in a language without side effects?,” Simon Peyton Jones, co-creator of Haskell, replied, “You only have to reason about values and not about state. If you give a function the same input, it’ll give you the same output, every time. This has implications for reasoning, for compiling, for parallelism.”

From the book, Masterminds of Programming

This is a page from my book, Functional Programming, Simplified

Grandma’s Cookies (and Pure Functions)

To help explain pure functions, I’d like to share a little story ...

Once upon a time I was a freshman in college, and my girlfriend’s grandmother sent her a tin full of cookies. I don’t remember if there were different kinds of cookies in the package or not — all I remember is the chocolate chip cookies. Whatever her grandmother did to make those cookies, the dough was somehow more white than any other chocolate chip cookie I had ever seen before. They also tasted terrific, and I ate most of them.