Why Scala?
You want to write a program that handles data. Which language should you choose?
There are a few different options. You might choose a dynamic language such as Python or R or a more traditional object-oriented language such as Java. In this section, we will explore how Scala differs from these languages and when it might make sense to use it.
When choosing a language, the architect's trade-off lies in a balance of provable correctness versus development speed. Which of these aspects you need to emphasize will depend on the application requirements and where on the permanence spectrum your program lies. Is this a short script that will be used by a few people who can easily fix any problems that arise? If so, you can probably permit a certain number of bugs in rarely used code paths: when a developer hits a snag, they can just fix the problem as it arises. By contrast, if you are developing a database engine that you plan on releasing to the wider world, you will, in all likelihood, favor correctness over rapid development. The SQLite database engine, for instance, is famous for its extensive test suite, with 800 times as much testing code as application code (https://www.sqlite.org/testing.html).
What matters, when estimating the correctness of a program, is not the perceived absence of bugs, it is the degree to which you can prove that certain bugs are absent.
There are several ways of proving the absence of bugs before the code has even run:
- Static type checking occurs at compile time in statically typed languages, but this can also be used in strongly typed dynamic languages that support type annotations or type hints. Type checking helps verify that we are using functions and classes as intended.
- Static analyzers and linters that check for undefined variables or suspicious behavior (such as parts of the code that can never be reached).
- Declaring some attributes as immutable or constant in compiled languages.
- Unit testing to demonstrate the absence of bugs along particular code paths.
There are several more ways of checking for the absence of some bugs at runtime:
- Dynamic type checking in both statically typed and dynamic languages
- Assertions verifying supposed program invariants or expected contracts
In the next sections, we will examine how Scala compares to other languages in data science.
Static typing and type inference
Scala's static typing system is very versatile. A lot of information as to the program's behavior can be encoded in types, allowing the compiler to guarantee a certain level of correctness. This is particularly useful for code paths that are rarely used. A dynamic language cannot catch errors until a particular branch of execution runs, so a bug can persist for a long time until the program runs into it. In a statically typed language, any bug that can be caught by the compiler will be caught at compile time, before the program has even started running.
Statically typed object-oriented languages have often been criticized for being needlessly verbose. Consider the initialization of an instance of the Example class in Java:
Example myInstance = new Example() ;
We have to repeat the class name twice—once to define the compile-time type of the myInstance variable and once to construct the instance itself. This feels like unnecessary work: the compiler knows that the type of myInstance is Example (or a superclass of Example) as we are binding a value of the Example type.
Scala, like most functional languages, uses type inference to allow the compiler to infer the type of variables from the instances bound to them. We would write the equivalent line in Scala as follows:
val myInstance = new Example()
The Scala compiler infers that myInstance has the Example type at compile time. A lot of the time, it is enough to specify the types of the arguments and of the return value of a function. The compiler can then infer types for all the variables defined in the body of the function. Scala code is usually much more concise and readable than the equivalent Java code, without compromising any of the type safety.
Scala encourages immutability
Scala encourages the use of immutable objects. In Scala, it is very easy to define an attribute as immutable:
val amountSpent = 200
The default collections are immutable:
val clientIds = List("123", "456") // List is immutable
clientIds(1) = "589" // Compile-time errorHaving immutable objects removes a common source of bugs. Knowing that some objects cannot be changed once instantiated reduces the number of places bugs can creep in. Instead of considering the lifetime of the object, we can narrow in on the constructor.
Scala and functional programs
Scala encourages functional code. A lot of Scala code consists of using higher-order functions to transform collections. You, as a programmer, do not have to deal with the details of iterating over the collection. Let's write an occurrencesOf function that returns the indices at which an element occurs in a list:
def occurrencesOf[A](elem:A, collection:List[A]):List[Int] = {
for {
(currentElem, index) <- collection.zipWithIndex
if (currentElem == elem)
} yield index
}How does this work? We first declare a new list, collection.zipWithIndex, whose elements are (collection(0), 0), (collection(1), 1), and so on: pairs of the collection's elements and their indexes.
We then tell Scala that we want to iterate over this collection, binding the currentElem variable to the current element and index to the index. We apply a filter on the iteration, selecting only those elements for which currentElem == elem. We then tell Scala to just return the index variable.
We did not need to deal with the details of the iteration process in Scala. The syntax is very declarative: we tell the compiler that we want the index of every element equal to elem in collection and let the compiler worry about how to iterate over collection.
Consider the equivalent in Java:
static <T> List<Integer> occurrencesOf(T elem, List<T> collection) {
List<Integer> occurrences = new ArrayList<Integer>() ;
for (int i=0; i<collection.size(); i++) {
if (collection.get(i).equals(elem)) {
occurrences.add(i) ;
}
}
return occurrences ;
}In Java, you start by defining a (mutable) list in which to put occurrences as you find them. You then iterate over the collection by defining a counter, considering each element in turn and adding its index to the list of occurrences, if need be. There are many more moving parts that we need to get right for this method to work. These moving parts exist because we must tell Java how to iterate over the collection, and they represent a common source of bugs.
Furthermore, as a lot of code is taken up by the iteration mechanism, the line that defines the logic of the function is harder to find:
static <T> List<Integer> occurrencesOf(T elem, List<T> collection) {
List<Integer> occurences = new ArrayList<Integer>() ;
for (int i=0; i<collection.size(); i++) {
if (collection.get(i).equals(elem)) {
occurrences.add(i) ;
}
}
return occurrences ;
}Note that this is not meant as an attack on Java. In fact, Java 8 adds a slew of functional constructs, such as lambda expressions, the Optional type that mirrors Scala's Option, or stream processing. Rather, it is meant to demonstrate the benefit of functional approaches in minimizing the potential for errors and maximizing clarity.
Null pointer uncertainty
We often need to represent the possible absence of a value. For instance, imagine that we are reading a list of usernames from a CSV file. The CSV file contains name and e-mail information. However, some users have declined to enter their e-mail into the system, so this information is absent. In Java, one would typically represent the e-mail as a string or an Email class and represent the absence of e-mail information for a particular user by setting that reference to null. Similarly, in Python, we might use None to demonstrate the absence of a value.
This approach is dangerous because we are not encoding the possible absence of e-mail information. In any nontrivial program, deciding whether an instance attribute can be null requires considering every occasion in which this instance is defined. This quickly becomes impractical, so programmers either assume that a variable is not null or code too defensively.
Scala (following the lead of other functional languages) introduces the Option[T] type to represent an attribute that might be absent. We might then write the following:
class User {
...
val email:Option[Email]
...
}We have now encoded the possible absence of e-mail in the type information. It is obvious to any programmer using the User class that e-mail information is possibly absent. Even better, the compiler knows that the email field can be absent, forcing us to deal with the problem rather than recklessly ignoring it to have the application burn at runtime in a conflagration of null pointer exceptions.
All this goes back to achieving a certain level of provable correctness. Never using null, we know that we will never run into null pointer exceptions. Achieving the same level of correctness in languages without Option[T] requires writing unit tests on the client code to verify that it behaves correctly when the e-mail attribute is null.
Note that it is possible to achieve this in Java using, for instance, Google's Guava library (https://code.google.com/p/guava-libraries/wiki/UsingAndAvoidingNullExplained) or the Optional class in Java 8. It is more a matter of convention: using null in Java to denote the absence of a value has long been the norm.
Easier parallelism
Writing programs that take advantage of parallel architectures is challenging. It is nevertheless necessary to tackle all but the simplest data science problems.
Parallel programming is difficult because we, as programmers, tend to think sequentially. Reasoning about the order in which different events can happen in a concurrent program is very challenging.
Scala provides several abstractions that greatly facilitate the writing of parallel code. These abstractions work by imposing constraints on the way parallelism is achieved. For instance, parallel collections force the user to phrase the computation as a sequence of operations (such as map, reduce, and filter) on collections. Actor systems require the developer to think in terms of actors that encapsulate the application state and communicate by passing messages.
It might seem paradoxical that restricting the programmer's freedom to write parallel code as they please avoids many of the problems associated with concurrency. However, limiting the number of ways in which a program behaves facilitates thinking about its behavior. For instance, if an actor is misbehaving, we know that the problem lies either in the code for this actor or in one of the messages that the actor receives.
As an example of the power afforded by having coherent, restrictive abstractions, let's use parallel collections to solve a simple probability problem. We will calculate the probability of getting at least 60 heads out of 100 coin tosses. We can estimate this using Monte Carlo: we simulate 100 coin tosses by drawing 100 random Boolean values and check whether the number of true values is at least 60. We repeat this until results have converged to the required accuracy, or we get bored of waiting.
Let's run through this in a Scala console:
scala> val nTosses = 100 nTosses: Int = 100 scala> def trial = (0 until nTosses).count { i => util.Random.nextBoolean() // count the number of heads } trial: Int
The trial function runs a single set of 100 throws, returning the number of heads:
scala> trial Int = 51
To get our answer, we just need to repeat trial as many times as we can and aggregate the results. Repeating the same set of operations is ideally suited to parallel collections:
scala> val nTrials = 100000 nTrials: Int = 100000 scala> (0 until nTrials).par.count { i => trial >= 60 } Int = 2745
The probability is thus approximately 2.5% to 3%. All we had to do to distribute the calculation over every CPU in our computer is use the par method to parallelize the range (0 until nTrials). This demonstrates the benefits of having a coherent abstraction: parallel collections let us trivially parallelize any computation that can be phrased in terms of higher-order functions on collections.
Clearly, not every problem is as easy to parallelize as a simple Monte Carlo problem. However, by offering a rich set of intuitive abstractions, Scala makes writing parallel applications manageable.
Interoperability with Java
Scala runs on the Java virtual machine. The Scala compiler compiles programs to Java byte code. Thus, Scala developers have access to Java libraries natively. Given the phenomenal number of applications written in Java, both open source and as part of the legacy code in organizations, the interoperability of Scala and Java helps explain the rapid uptake of Scala.
Interoperability has not just been unidirectional: some Scala libraries, such as the Play framework, are becoming increasingly popular among Java developers.
