Defining Functional Data Structures
Defining Functional Data Structures
by Paul Chiusano and Rúnar Bjarnason, authors of Functional Programming in Scala
Functional programs do not update variables or modify data structures. This raises pressing questions—what sort of data structures we use in functional programming, how do we define them can in Scala, and how do we operate over these data structures? This article, based on chapter 3 of Functional Programming in Scala, explains the concept of a functional data structure and how to define and work with such structures.
A functional data structure is (not surprisingly!) operated on using only pure functions. A pure function may only accept some values as input and yield a value as output. It may not change data in place or perform other side effects. Therefore, functional data structures are immutable. For example, the empty list, (denoted List() or Nil in Scala) is as eternal and immutable as the integer values 3 or 4. And just as evaluating 3 + 4 results in a new number 7 without modifying either 3 or 4, concatenating two lists together (the syntax for this is a + b for two lists a and b) yields a new list and leaves the two inputs unmodified.
Doesn't this mean we end up doing a lot of extra copying of the data? Somewhat surprisingly, the answer is no. We will return to this issue after examining the definition of what is perhaps the most ubiquitous of functional data structures, the singlylinked list. The definition here is identical in spirit to (though simpler than) the data type defined in Scala's standard List library. This code listing makes use of a lot of new syntax and concepts, so don't worry if not everything makes sense at first—we will talk through it in detail. [1]
Listing 1 Singlylinked lists
package fpinscala.datastructures
sealed trait List[+A]
case object Nil extends List[Nothing]
case class Cons[+A](head: A, tail: List[A]) extends List[A]
object List {
def sum(ints: List[Int]): Int = ints match {
case Nil => 0
case Cons(x,xs) => x + sum(xs)
}
def product(ds: List[Double]): Double = ds match {
case Nil => 1.0
case Cons(0.0, _) => 0.0
case Cons(x,xs) => x * product(xs)
}
def apply[A](as: A*): List[A] =
if (as.isEmpty) Nil
else Cons(as.head, apply(as.tail: _*))
val example = Cons(1, Cons(2, Cons(3, Nil)))
val example2 = List(1,2,3)
val total = sum(example)
}
Let's look first at the definition of the data type, which begins with the keywords sealed trait. In general, we introduce a data type with the trait keyword. A trait is an abstract interface that may optionally contain implementations of some methods. Here we are declaring a trait, called List, with no methods on it. Adding sealed in front means that all implementations of our trait must be declared in this file. [2]
There are two such implementations or data constructors of List (each introduced with the keyword case) declared next, to represent each of the two possible forms a List can take—it can be empty, denoted by the data constructor Nil, or it can be nonempty (the data constructor Cons, traditionally short for construct), in which case it consists of an initial element, head, followed by a List (possibly empty) of remaining elements (the tail).
Listing 2 The data constructors of List
case object Nil extends List[Nothing]
case class Cons[+A](head: A, tail: List[A]) extends List[A]
Just as functions can be polymorphic, data types can be as well, and by adding the type parameter [+A] after sealed trait List and then using that A parameter inside of the Cons data constructor, we have declared the List data type to be polymorphic in the type of elements it contains, which means we can use this same definition for a list of Int elements (denoted List[Int]), Double elements (denoted List[Double]), String elements (List[String]), and so on (the + indicates that the type parameter, A is covariant—see sidebar "More about variance" for more info).
A data constructor declaration gives us a function to construct that form of the data type (the case object Nil lets us write Nil to construct an empty List, and the case class Cons lets us write Cons(1, Nil), Cons(1, Cons(2, Nil)), and so on for nonempty lists), but also introduces a pattern that can be used for pattern matching, as in the functions sum and product.
More about variance
In the declaration, the in trait List[+A], the + in front of the type parameter A is a variance annotation that signals A is a covariant or positive parameter of List. This means that, for instance, List[Dog] is considered a subtype of List[Animal], assuming Dog is a subtype of Animal. (More generally, for all types X and Y, if X is a subtype of Y, then List[X] is a subtype of List[Y]). We could leave out the + in front of the A, which would make List invariant in that type parameter.
But notice now that Nil extends List[Nothing]. Nothing is a subtype of all types, which means that, in conjunction with the variance annotation, Nil can be considered a List[Int], a List[Double], and so on, exactly as we want.
These concerns about variance are not very important for the present discussion and are more of an artifact of how Scala encodes data constructors via subtyping. [3]
Pattern matching
Let's look in detail at the functions sum and product, which we place in the object List, sometimes called the companion object to List (see the sidebar). Both of these definitions make use of pattern matching.
def sum(ints: List[Int]): Int = ints match {
case Nil => 0
case Cons(x,xs) => x + sum(xs)
}
def product(ds: List[Double]): Double = ds match {
case Nil => 1.0
case Cons(0.0, _) => 0.0
case Cons(x, xs) => x * product(xs)
}
As you might expect, the sum function states that the sum of an empty list is 0, and the sum of a nonempty list is the first element, x, plus the sum of the remaining elements, xs. [4] Likewise, the product definition states that the product of an empty list is 1.0, the product of any list starting with 0.0 is 0.0, and the product of any other nonempty list is the first element multiplied by the product of the remaining elements. Notice these are recursive definitions, which are quite common when writing functions that operate over recursive data types like List (which refers to itself recursively in its Cons data constructor).
This isn't the most robust test—pattern matching on 0.0 will match only the exact value 0.0, not 1e102 or any other value very close to 0.
Pattern matching works a bit like a fancy switch statement that may descend into the structure of the expression it examines and extract subexpressions of that structure (we'll explain this shortly). It is introduced with an expression (the target or scrutinee), like ds followed by the keyword match, and a {}wrapped sequence of cases. Each case in the match consists of a pattern (like Cons(x,xs)) to the left of the => and a result (like x * product(xs)) to the right of the =>. If the target matches the pattern in a case (see below), the result of that case becomes the result of the entire match expression. If multiple patterns match the target, Scala chooses the first matching case.
Companion objects in Scala
We will often declare a companion object in addition to our data type and its data constructors. This is just an object with the same name as the data type (in this case List) where we put various convenience functions for creating or working with values of the data type.
If, for instance, we wanted a function def fill[A](n: Int, a: A): List[A], that created a List with n copies of the element a, the List companion object would be a good place for it. Companion objects are more of a convention in Scala. [5] We could have called this module Foo if we wanted, but calling it List makes it clear that the module contains functions relevant to working with lists, and also gives us the nice List(1,2,3) syntax when we define a variadic apply function (see sidebar "Variadic functions in Scala").
Let's look at a few more examples of pattern matching:
 List(1,2,3) match { case _ => 42 } results in 42. Here we are using a variable pattern, _, which matches any expression. We could say x or foo instead of _ but we usually use _ to indicate a variable whose value we ignore in the result of the case. [6]
 List(1,2,3) match { case Cons(h,t) => h } results in 1. Here we are using a data constructor pattern in conjunction with variables to capture or bind a subexpression of the target.
 List(1,2,3) match { case Cons(_,t) => t } results in List(2,3).
 List(1,2,3) match { case Nil => 42 } results in a MatchError at runtime. A MatchError indicates that none of the cases in a match expression matched the target.
What determines if a pattern matches an expression? A pattern may contain literals, like 0.0 or "hi", variables like x and xs which match anything, indicated by an identifier starting with a lowercase letter or underscore, and data constructors like Cons(x,xs) or Nil, which match only values of the corresponding form (Nil as a pattern matches only the value Nil, and Cons(h,t) or Cons(x,xs) as a pattern only match Cons values). These components of a pattern may be nested arbitrarily—Cons(x1, Cons(x2, Nil)) and Cons(y1, Cons(y2, Cons(y3, _))) are valid patterns. A pattern matches the target if there exists an assignment of variables in the pattern to subexpressions of the target that make it structurally equivalent to the target. The result expression for a matching case will then have access to these variable assignments in its local scope.
EXERCISE 1: What will the result of the following match expression be?
val x = List(1,2,3,4,5) match {
case Cons(x, Cons(2, Cons(4, _))) => x
case Nil => 42
case Cons(x, Cons(y, Cons(3, Cons(4, _)))) => x + y
case Cons(h, t) => h + sum(t)
case _ => 101
}
You are strongly encouraged to try experimenting with pattern matching in the REPL to get a sense for how it behaves.
Variadic functions in Scala
The function List.apply in the listing above is a variadic function, meaning it accepts zero or more arguments of type A. For data types, it is a common idiom to have a variadic apply method in the companion object to conveniently construct instances of the data type. By calling this function apply and placing it in the companion object, we can invoke it with syntax like List(1,2,3,4) or List("hi","bye"), with as many values as we want separated by commas (we sometimes call this the list literal or just literal syntax).
Variadic functions are just providing a little syntax sugar for creating and passing a Seq of elements explicitly. Seq is the interface in Scala implemented by sequencelike data structures like lists, queues, vectors, and so forth. Inside apply, as will be bound to a Seq[A] (see the documentation), which has the function's head (returns the first element) and tail (returns all elements but the first).
We can convert a Seq[A], x, back into something that can be passed to a variadic function using the syntax x: _*, where x can be any expression, for instance: List(x: _*) or even List(List(1,2,3):_*).
Summary
In this article, we covered a number of important functional programming concepts related to Scala. We introduced algebraic data types and pattern matching.
1. Note—the implementations of sum and product here are not tail recursive.
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2. We could also say abstract class here instead of trait. Technically, an abstract class can contain constructors, in the OO sense, which is what separates it from a trait, which cannot contain constructors. This distinction is not really relevant for our purposes right now.
↩
3. It is certainly possible to write code without using variance annotations at all, and function signatures are sometimes simpler (while type inference often gets worse).
↩
4. We could call x and xs anything there, but it is a common convention to use xs, ys , as, bs as variable names for a sequence of some sort, and x , y , z , a, or b as the name for a single element of a sequence. Another common naming convention is h for the first element of a list (the "head" of the list), t for the remaining elements (the "tail"), and l for an entire list.
↩
5. There is some special support for them in the language that isn't really relevant for our purposes.
↩
6. The _ variable pattern is treated somewhat specially in that it may be mentioned multiple times in the pattern to ignore multiple parts of the target.↩
Here are some other Manning titles you might be interested in:
The RealWorld Functional Programming 

Scala in Action 

Play for Scala 
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