Scala Cookbook

20.3. Think “Expression-Oriented Programming”

Problem

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.

Solution

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:

order.calculateTaxes()
order.updatePrices()

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 = ... ...