little cubes

The Modern Switch Statement

Zach Posten - 2015 Mar 25

Why do most languages still require breaks in a switch statement? Is Swift the beginning of the end for these break statements?

Context

  • In lab this week, we were tasked researching and experimenting with various features of the Swift language. One feature I particularly liked, was how Swift structures switch statements. Specifically, I think it is fantastic that Swift switch statements do not fall through the bottom of each case and into the next one by default. Instead, the entire switch statement finishes its execution as soon as the first matching case is completed, without requiring an explicit break statement.
  • After seeing Swift's brilliant switch statement design, I began thinking about a question that I've had since I was first introduced to switch statements in SE 1011; why do we require the programmer to insert an implicit break statement inside of every case?

Research

  • The consensus of the community seems to be that we still use break because that's the way it was done in C and the designers of new languages always want their syntax to be easily understood by programmers who are comfortable with this C-like syntax.
  • According to a popular answer on this stack exchange article, C was designed that way likely because of its concept as "portable assembly". The C switch statement is basically an abstraction of a branch table, which has implicit fall-through.
  • The C# switch statement is interesting because it requires that explicit flow control (break, goto, return or throw) occur at the end of a case and will throw a compile time error if that flow control is omitted. Fall-through can be achieved only by using an explicit goto at the end of a case. MSDN says the reason for this is that although "implicit fall-through behavior is often used to reduce the amount of code needed", "as code moves from the initial development phase into a maintenance phase, [implicit fall-through] can lead to subtle errors that are very hard to debug".
  • This Wikipedia article points out that many languages in the Pascal family also only execute the matching block, so this isn't a new idea.
  • In trying to find an answer for the fate of break (or other control flow) in switch statements, I looked into several up and coming languages to see how they implemented switch statements.
    • Dart, Google's new "replacement" for Javascript, took a page out of C#'s book and throws an error if the control flow is omitted.
    • D, which was recently adopted by Facebook as a C++ replacement, also throws an error for any but the final case.
    • Go, another Google backed language written expressly for the cloud, does not require a control flow statement.
    • Although the break in switch statements is not universal, it appears as though it will still be around for years to come.

Status

  • Question is mostly resolved, though a concrete answer is likely not available.