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
Part 6 – Liskov substitution principle
Part 7 – Four types of experience in software engineering
Part 8 – Testing – is it worth it?
Part 9 – Real life testing
Part 10 – Dogmatic TDD
Part 11 – Principles of good debugging
Part 12 – A word on estimates and Story Points
Part 13 – Long-term planning
Part 14 – Pure functions vs impure ones
Part 15 – Prohibit vs Enable in the software engineering
Part 16 – Encapsulation and making all public
Part 17 – LSP in practice
Part 18 – Your defaults influence the way you think
Part 19 – Senior or Expert or what?
Part 20 – Measuring how hard your work is

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:

class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		System.out.println(foo());
	}
 
	public static int foo(){
		try{
			return 5;
		}finally{
			return 42;
		}
	}
}

class Ideone

{

public static void main (String[] args) throws java.lang.Exception

{

System.out.println(foo());

}

public static int foo(){

try{

return 5;

}finally{

return 42;

}

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:

class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		System.out.println(foo());
	}
	
	public static int foo(){
		try{
			throw new RuntimeException("This disappears");
		}finally{
			return 42;
		}
	}
}

class Ideone

{

public static void main (String[] args) throws java.lang.Exception

{

System.out.println(foo());

}

public static int foo(){

try{

throw new RuntimeException("This disappears");

}finally{

return 42;

}

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

You can see the same in JS:

function foo(){
  try{
    throw "This disappears";
  }finally{
    return 42;
  }
}
console.log(foo());

function foo(){

try{

throw "This disappears";

}finally{

return 42;

}

console.log(foo());

Python:

def foo():
  try:
    raise Exception("This disappears")
  finally:
    return 42

print(foo())

def foo():

try:

raise Exception("This disappears")

finally:

return 42

print(foo())

SEH:

#include "stdafx.h"
#include 

int Filter(){
	return EXCEPTION_EXECUTE_HANDLER;
}

int Foo(){
	__try{
		printf("%d", 1 / divideByZero);
		return 5;
	}
	__finally{
		return 42;
	}
}

int _tmain(int argc, _TCHAR* argv[])
{
	__try{
		printf("%d", Foo());
	}
	__except(Filter()){
	}
	return 0;
}

#include "stdafx.h"

#include

int Filter(){

return EXCEPTION_EXECUTE_HANDLER;

}

int Foo(){

__try{

printf("%d", 1 / divideByZero);

return 5;

}

__finally{

return 42;

}

int _tmain(int argc, _TCHAR* argv[])

{

__try{

printf("%d", Foo());

}

__except(Filter()){

}

return 0;

}

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