Despite Functional Programming's (FP) growing popularity, for many “normal programmers” the language family seems to be merely an academic exercise that they are neither capable of or willing to engage in. Despite there being many FP languages that are arguably much more accessible than some “normal” programming languages like C++ (e.g. Elixir), FP does at times have some concepts that remain fuzzy even to those who have programmed before in the functional style.

Perhaps no concept breads more fear in the hearts of aspiring functional programmers than that of the monad. In this series of posts, I'll try my best to explain what a monads are and more importantly, why they're awesome.

This won't be a formal attempt at defining monads but rather a practical look at the expressive power they give the programmer using them. In other words, if you're looking for some category theory, look elsewhere.

No PhD Required

In order to get the most out of this article you should be somewhat comfortable with the basic tenants of functional programming (especially first class functions), and you should feel comfortable with static typing. Don't worry though. Even if you've never touched a FP before, you should still be able to get something out of this too.

Our Journey

On our road to understand what the heck a monad is we'll also be getting to know other concepts with seemingly scary names like functors, applicative functors, and monoids.

Accompanying us on our journey will be the Haskell programming language. Don't worry if you've never programmed in Haskell, I'll try to explain everything you need to know along the way. Also, this will not be an academic explanation of what a monad is. If you're looking for your answers based in Category Theory, you'll have to search elsewhere.

Yo Dawg. I Heard You Like Values…

Before we get started, it's best to look ahead to where we are going. Haskell is very statically and strongly typed. "hello" is a String and 1.0 is a Double. When we declare functions we will specify exactly the kinds of types that we will alow to be passed into and returned from our functions. The compiler will make sure we keep this in mind the entire time we program.

Haskell's type system is in fact more pervasive than perhaps you're used to in other languages like Java. Even actions which have side effects (e.g. printing something to the screen or making a network call) have special types.

Let's start with a simple value:

ghci> let myString = "Hello, World!"
"Hello, World!"

If we ask Haskell the type of the value myString:

ghci> :t myString
myString :: String

We see that it is indeed a String. By now you should be saying, “Wow this Haskell thing is easy!". Haskell's type system let's you do more than just have types like Strings, Chars, and Integers. In Haskell you can also have types that are part of the Monad type class. Monads let you have values that are wrapped in a context. This context tells you more about the plain value inside.

Let's take a look at an example to clarify.

Haskell has a type called Maybe which is part of the type class Monad. Maybe wraps another type (like a String or Integer) and gives it a context (or more information) about that plain type inside. What context does Maybe give?

Maybe tells us that a value is either there or it is not. In other words, we maybe have a value.

Here are three instances of the Maybe type:

Just 5
Just ["Hello", "World"]

The first two values tell us we do indeed have values, the Integer 5 in the first case and the list of Strings “Hello” and “World” in the second case. The third value says we have nothing at all.

Scala (another FP style language) has a similar type called option that looks like this:

Some 5
Some ["Hello", "World"]

We either have some of something or none. The Maybe`/Option` monad gives us the ability to say we have some sort of value or we do not.

Get Func-y

Of course, these values in isolation don't do us much good. Of course, if we manually write out the value Just 5, we know that we have the Integer 5. Things become much more interesting when we have functions.

So let's say we have a function (which we'll call myFunction) with the following type signature:

myFunction :: Integer -> Maybe Integer

This says that the function myFunction takes in an Integer as its only argument and returns a Maybe Integer.

So we, and the compiler, know that when we pass in an Integer to myFunction we “maybe” will get an Integer back. Depending on our input we can get back out something like Just 5, or Just 49283 or even Nothing (which, of course, is different than Just 0).

The Maybe monad is often used where in other languages we would use the value nil (or null). However, for reasons that we will explore later, the Maybe monad allows us to do the same sort of things we would do with nils without having to always do nasty if nil checks. The compiler guarantees that you'll never have any undefined method 'foo' for nil type errors again.

Not only can we avoid nil checking, but the compiler will catch any mistakes we make at compile time.

Other Monads

So we now have a monad that gives a wrapped value with the context of either being there or not. Just "Hello" or Nothing. What other “contexts” can we wrap values in?

  • IO - IO wraps a value in a context where side effects (the bane of functional programming) can take place.
  • Writer - Writer Wraps a value in the context of having another value that acts as a “log”. This is often used to log the intermediate steps of a function.
  • List - Yes, List, the data structure backbone of almost all functional programming, is a monad with the context of having many values or having none at all.
  • And many more!

What's Next

Now that we know where we're headed, we'll start next time looking at two concepts central to monads, functors and applicative functors. All monads are both functors and applicative functors so understanding both will get us much closer to understanding monads.