Scala best practice: Think “Expression-Oriented Programming”

This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 20.3, “Scala best practice: Think "Expression-Oriented Programming".”

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You’re used to writing statements in another programming language, and want to learn how to write expressions in Scala, and the benefits of the Expression-Oriented Programming (EOP) philosophy.

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To understand EOP, you have to understand the difference between a statement and an expression. Wikipedia provides a concise distinction between the two:

Statements do not return results and are executed solely for their side effects, while expressions always return a result and often do not have side effects at all.”

So statements are like this:


Expressions are like this:

val tax = calculateTax(order)
val price = calculatePrice(order)

On Wikipedia’s EOP page, it also states:

“An expression-oriented programming language is a programming language where every (or nearly every) construction is an expression, and thus yields a value.”

As you might expect, it further states that all pure FP languages are expression-oriented.

The following example helps to demonstrate EOP. This recipe is similar to Recipe 20.1, so it reuses the class from that recipe to show a poor initial design:

// an intentionally bad example
class Stock (var symbol: String,
             var company: String,
             var price: String,
             var volume: String,
             var high: String,
             var low: String) {
    var html: String = _
    def buildUrl(stockSymbol: String): String = { ... }
    def getUrlContent(url: String):String = { ... }
    def setPriceUsingHtml() { this.price = ... }
    def setVolumeUsingHtml() { this.volume = ... }
    def setHighUsingHtml() { this.high = ... }
    def setLowUsingHtml() { this.low = ... }

Although I didn’t show it in that recipe, using this class would result in code like this:

val stock = new Stock("GOOG", "Google", "", "", "", "")
val url = buildUrl(stock.symbol)
stock.html = stock.getUrlContent(url)

// a series of calls on an object ('statements')

Although the implementation code isn’t shown, all of these “set” methods extract data from the HTML that was downloaded from a Yahoo Finance page for a given stock, and then update the fields in the current object.

After the first two lines, this code is not expression-oriented at all; it’s a series of calls on an object to populate (mutate) the class fields, based on other internal data. These are statements, not expressions; they don’t yield values.

Recipe 20.1 showed that by refactoring this class into several different components, you would end up with the following code:

// a series of expressions
val url = StockUtils.buildUrl(symbol)
val html = NetUtils.getUrlContent(url)
val price = StockUtils.getPrice(html)
val volume = StockUtils.getVolume(html)
val high = StockUtils.getHigh(html)
val low = StockUtils.getLow(html)
val date = DateUtils.getDate
val stockInstance = StockInstance(symbol, date, price, volume, high, low)

This code is expression-oriented. It consists of a series of simple expressions that pass values into pure functions (except for getDate), and each function returns a value that’s assigned to a variable. The functions don’t mutate the data they’re given, and they don’t have side effects, so they’re easy to read, easy to reason about, and easy to test.

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In Scala, most expressions are obvious. For instance, the following two expressions both return results, which you expect:

scala> 2 + 2
res0: Int = 4

scala> List(1,2,3,4,5).filter(_ > 2)
res1: List[Int] = List(3, 4, 5)

However, it can be more of a surprise that an if/else expression returns a value:

val greater = if (a > b) a else b

Match expressions also return a result:

val evenOrOdd = i match {
    case 1 | 3 | 5 | 7 | 9 => println("odd")
    case 2 | 4 | 6 | 8 | 10 => println("even")

Even a try/catch block returns a value:

val result = try {
} catch {
    case _ => 0

Writing expressions like this is a feature of functional programming languages, and Scala makes using them feel natural and intuitive, and also results in concise, expressive code.

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Because expressions always return a result, and generally don’t have side effects, there are several benefits to EOP:

  • The code is easier to reason about. Inputs go in, a result is returned, and there are no side effects.
  • The code is easier to test.
  • Combined with Scala’s syntax, EOP also results in concise, expressive code.
  • Although it has only been hinted at in these examples, expressions can often be executed in any order. This subtle feature lets you execute expressions in parallel, which can be a big help when you’re trying to take advantage of modern multicore CPUs.
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See Also

  • The Wikipedia definition of a statement, and the difference between a statement and an expression
  • Expression-Oriented Programming (EOP)
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The Scala Cookbook

This tutorial is sponsored by the Scala Cookbook, which I wrote for O’Reilly:

You can find the Scala Cookbook at these locations:

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