Types and Programming Languages Part 1 — Do not return in finally

This is the first part of the Types and Programming Languages series. For your convenience you can find other parts using the links below:
Part 1 — Do not return in finally
Part 2 — Exception while handling exception
Part 3 — Finally during termination
Part 4 – Diamond problem
Part 5 – Sleeping and measuring time

Many languages provide exception handling construct, typically in a form of try and catch blocks. While details differ in terms what can be thrown, what can be handled etc, programmers generally assume these constructs work the same across languages. That’s not true, unfortunately, and details are often tricky when it comes to edge cases. We’ll cover that one day.

Some languages support additional block called finally which is supposed to be executed “no matter what” — whether the exception was thrown or not. That’s obviously not true, there are many situations when they may not be called, for instance unhandled exceptions, exiting the application, access violations (or segfaults) etc. I won’t be covering the details now, we’ll get to that some other time. What we’ll cover today is returning in finally.

Some languages let you return value from the finally block. Typical implementation makes the last returned value “win” over others. Let’s take this Java code:

The output is 42, because that’s the last returned value. You can observe the same behavior in Python, JS, Windows SEH, probably other platforms as well. Take your favorite language and check it out. One notable exception here is C# which doesn’t allow to return in finally, just to avoid this confusion.

Seems simple and makes sense. But what happens if you throw exception and then return? Let’s take this Java code:

What’s the output? It’s still 42. Exception was lost.

You can see the same in JS:

Python:

SEH:

As a rule of thumb, never return in finally. It breaks exception handling mechanism.