The programming world is moving towards functional programming (FP). More developers are using languages with an explicit bias towards FP, such as Scala and Haskell, while object-oriented (OO) languages and their communities adopt FP features and practices. (A striking example of the latter is the rise of Typescript and React in the Javascript community.) So what is FP and what does it mean to write code in a functional style? It's common to view functional programming as a collection of language features, such as first class functions, or to define it as a programming style using immutable data and pure functions. (Pure functions always return the same output given the same input.) This was my view when I started down the FP route, but I now believe the true goals of FP are enabling local reasoning and composition. Language features and programming style are in service of these goals. In this post I attempt to explain the meaning and value of local reasoning and composition.

What Functional Programming Is

I believe that functional programming is a hypothesis about software quality: that software that can be understood before it is run and is built of small reusable components is easier to write and maintain. The first property is known as local reasoning, and the second as composition. Let's address each in turn.

Local reasoning means we can understand pieces of code in isolation. When we see the expression 1 + 1 we know what it means regardless of the weather, the database, or the current status of our Kubernetes cluster. None of these external events can change it. This is a trivial and slightly silly example, but it illustrates a point. A goal of functional programming is to extend this ability across our code base.

It can help to understand local reasoning by looking at what it is not. Shared mutable state is out because relying on shared state means that other code can change what our code does without our knowledge. It means no global mutable configuration, as found in many web frameworks and graphics libraries for example, as any random code can change that configuration. Metaprogramming has to be carefully controlled. No monkey patching, for example, as again it allows other code to change our code in non-obvious ways. As we can see, adapting code to enable local reasoning can mean quite some sweeping changes. However if we work in a language that embraces functional programming this style of programming is the default.

Composition means building big things out of smaller things. Numbers are compositional. We can take any number and add one, giving us a new number. Lego is also compositional. We compose Lego by sticking it together. In the particular sense we're using composition we also require the original elements we combine don't change in any way when they are composed. When we create by 2 by adding 1 and 1 we get a new result but we don't change what 1 means.

We can find compositional ways to model common programming tasks once we start looking for them. React components are one example familiar to many front-end developers: a component can consist of many components. HTTP routes can be modelled in a compositional way. A route is a function from an HTTP request to a handler function or a value indicating the route did not match. We can combine routes as a logical or: try this route or, if it doesn't match, try this other route. Processing pipelines are another example that often use sequential composition: perform this pipeline stage and then this other pipeline stage.

Types

Types are not strictly part of functional programming but statically typed FP is the most popular form of FP and sufficiently important to warrant a mention. Types help compilers generate efficient code but types in FP are as much for the programmer as they are the compiler. Types express properties of programs, and the type checker automatically ensures that these properties hold. They can tell us, for example, what a function accepts and what it returns, or that a value is optional. We can also use types to express our beliefs about a program and the type checker will tell us if those beliefs are incorrect. For example, we can use types to tell the compiler we do not expect an error at a particular point in our code and the type checker will let us know if have made an incorrect assumption. In this way types are another tool for reasoning about code.

Type systems push programs towards particular designs, as to work effectively with the type checker requires designing code in a way the type checker can understand. As modern type systems come to other languages they naturally tend to shift programmers in those languages towards a FP style of coding.

What Functional Programming Isn't

In my view functional programming is not about immutability, or keeping to "the substitution model of evaluation", and so on. These are tools in service of the goals of enabling local reasoning and composition, but they are not the goals themselves. Code that is immutable always allows local reasoning, for example, but it is not necessary to avoid mutation to still have local reasoning. Here is an example of summing a collection of numbers. First we have the code in Typescript:

function sum(numbers: Array<number>): number {
    let total = 0.0;
    numbers.forEach(x => total = total + x);
    return total;
}

Here's the same function in Scala:

def sum(numbers: List[Int]): Int = {
  var total = 0.0
  numbers.foreach(x => total = total + x)
  total
}

In both implementations we mutate total. This is ok though! We cannot tell from the outside that this is done, and therefore all users of sum can still use local reasoning. Inside sum we have to be careful when we reason about total but this block of code is small enough that it shouldn't cause any problems.

In this case we can reason about our code despite the mutation, but neither the Typescript nor the Scala compiler can determine that this is ok. Both languages allow mutation but it's up to us to use it appropriately. A more expressive type system, perhaps with features like Rust's, would be able to tell that sum doesn't allow mutation to be observed by other parts of the system¹. Another approach, which is the one taken by Haskell, is to disallow all mutation and thus guarantee it cannot cause problems.

Mutation also interferes with composition. For example, if a value relies on internal state then composing it may produce unexpected results. Consider generators in Javascript. They maintain internal state that is used to generate the next value. If we have two generators we might want to combine them into one generator that yields values from the two inputs. Here's the code in Typescript:

type Gen<a> = Generator<a, void, never>

function* infinite(): Gen<number> {
    let index = 0;

    while (true) {
        yield index++;
    }
}

function zip<a, b>(left: Gen<a>, right: Gen<b>): Gen<[a, b]> {
    function* zipIt () {
        const l = left.next();
        const r = right.next();
        if (!l.done && !r.done) {
            const result: [a, b] = [l.value, r.value];
            yield result;
        }
    }

    return zipIt();
}

This works if we pass two distinct generators to zip.

zip(infinite(), infinite()).next().value; // [0, 0]

However if we pass the same generator twice we get a surprising result.

const inf = infinite();
zip(inf, inf).next().value; // [0, 1]

The usual functional programming solution is to avoid mutable state but we can envisage other possibilities. For example, an effect tracking system would allow us to avoid combining two generators that use the same memory region. These systems are still research projects, however.

So in my opinion immutability (and purity, referential transparency, and no doubt more fancy words that I have forgotten) have become associated with functional programming because they guarantee local reasoning and composition, and until recently we didn't have the language tools to automatically distinguish safe uses of mutation from those that cause problems. Restricting ourselves to immutability is the easiest way to ensure the desirable properties of functional programming, but as languages evolve this might come to be regarded as a historical artifact.

Why It Matters

I have described local reasoning and composition but have not discussed their benefits. Why are they are desirable? The answer is that they make efficient use of knowledge. Let me expand on this.

We care about local reasoning because it allows our ability to understand code to scale with the size of the code base. We can understand module A and module B in isolation, and our understanding does not change when we bring them together in the same program. By definition if both A and B allow local reasoning there is no way that B (or any other code) can change our understanding of A, and vice versa. If we don't have local reasoning every new line of code can force us to revisit the rest of the code base to understand what has changed. This means it becomes exponentially harder to understand code as it grows in size as the number of interactions (and hence possible behaviours) grows exponentially. We can say that local reasoning is compositional. Our understanding of module A calling module B is just our understanding of A, our understanding of B, and whatever calls A makes to B.

We introduced numbers and Lego as examples of composition. They have an interesting property in common: the operations that we can use to combine them (for example, addition, subtraction, and so on for numbers; for Lego the operation is "sticking bricks together") give us back the same kind of thing. A number multiplied by a number is a number. Two bits of Lego stuck together is still Lego. This property is called closure: when you combine things you end up with the same kind of thing. Closure means you can apply the combining operations (sometimes called combinators) an arbitrary number of times. No matter how many times you add one to a number you still have a number and can still add or subtract or multiply or...you get the idea. If we understand module A, and the combinators that A provides are closed, we can build very complex structures using A without having to learn new concepts! This is also one reason functional programmers tend to like abstractions such a monads (beyond liking fancy words): they allow us to use one mental model in lots of different contexts.

In a sense local reasoning and composition are two sides of the same coin. Local reasoning is compositional; composition allows local reasoning. Both make code easier to understand.

The Evidence for Functional Programming

I've made arguments in favour of functional programming and I admit I am biased---I do believe it is a better way to develop code than imperative programming. However, is there any evidence to back up my claim? There has not been much research on the effectiveness of functional programming, but there has been a reasonable amount done on static typing. I feel static typing, particularly using modern type systems, serves as a good proxy for functional programming so let's look at the evidence there.

In the corners of the Internet I frequent the common refrain is that static typing has neglible effect on productivity. I decided to look into this and was surprised that the majority of the results I found support the claim that static typing increases productivity. For example, the literature review in this dissertation (section 2.3, p16--19) shows a majority of results in favour of static typing, in particular the most recent studies. However the majority of these studies are very small and use relatively inexperienced developers---which is noted in the review by Dan Luu that I linked. My belief is that functional programming comes into its own on larger systems. Furthermore, programming languages, like all tools, require proficiency to use effectively. I'm not convinced very junior developers have sufficient skill to demonstrate a significant difference between languages.

To me the most useful evidence of the effectiveness of functional programming is that industry is adopting functional programming en masse. Consider, say, the widespread and growing adoption of Typescript and React. If we are to argue that FP as embodied by Typescript or React has no value we are also arguing that the thousands of Javascript developers who have switched to using them are deluded. At some point this argument becomes untenable.

This doesn't mean we'll all be using Haskell in five years. More likely we'll see something like the shift to object-oriented programming of the nineties: Smalltalk was the paradigmatic example of OO, but it was more familiar languages like C++ and Java that brought OO to the mainstream. In the case of FP this probably means languages like Scala, Swift, Kotlin, or Rust, and mainstream languages like Javascript and Java continuing to adopt more FP features.

Final Words

I've given my opinion on functional programming---that the real goals are local reasoning and composition, and programming practices like immutability are in service of these. Other people may disagree with this definition, and that's ok. Words are defined by the community that uses them, and meanings change over time.

Functional programming emphasises formal reasoning, and there are some implications that I want to briefly touch on. Later articles may expand on these points.

Firstly, I find that FP is most valuable in the large. For a small system it is possible to keep all the details in our head. It's when a program becomes too large for anyone to understand all of it that local reasoning really shows its value. This is not to say that FP should not be used for small projects, but rather that if you are, say, switching from an imperative style of programming you shouldn't expect to see the benefit when working on toy projects.

The formal models that underlie functional programming allow systematic construction of code. This is in some ways the reverse of reasoning: instead of taking code and deriving properties and start from some properties and derive code. This sounds very academic but is in fact very practical and how I, for example, develop most of my code.

Finally, reasoning is not the only way to understand code. It's valuable to appreciate the limitations of reasoning, other methods for gaining understanding, and using a variety of strategies depending on the situation.