Exceptions are Evil · David Raab

Exceptions are Evil

Most people today agree that null is evil, and they try to get rid of them. One technique that most people prefer is to throw an exception in the case of an error, or if we cannot return a valid value from a function. The problem is, exceptions are not any kind better than null, and they don't solve any problem that null introduced.

In my previous post null is Evil i mentioned seven problems that null introduced. So let's look if exceptions solve one of those problems.

1. You cannot see if a function throws an exception

When you look at the type signature of any function, you just see something like

1: 
PersonId -> Person

The same way that you cannot see that null could be returned. The same way you don't know if an exception could be thrown or not.

2. We are not forced to try/catch

You are not forced to add any kind of try/catch, the same way you are not forced to add a null check. If you forgot to add your null checks, you end up getting NullReferenceException. If you forgot to adding try/catch you end up with XYZException. Replacing a NullReferenceException just with another kind of Exception isn't anyway better.

3. Every function could throw an Exception

The big problem of 1. and 2. combined is that you defensively start to check all return values from a function if it contains null. And you also have to check every argument if it contains null. This leads to numerous null checking throughout your code. But why do you add those checks in the first place? Because you want to prevent NullReferenceException. So how is a function that sometimes returns null that can lead to a NullReferenceException anyhow better as a function that sometimes throws XYZException directly?

Not only does it not solve the problem at all. You still have to add your checkings. But instead of null checks throughout your code. You wrap your code in try/catch blocks. Why is checking for null bad

1: 
2: 
3: 
4: 
5: 
let result = SomeFunction()
if result <> null then
    // Some code if everything was right
else
    // Error condition on null

and suddenly wrapping your code in a try/catch anyhow better?

1: 
2: 
3: 
4: 
5: 
try
    let result = SomeFunction()
    // Some code if everything was right
with
    | exn -> // Error condition on Exception

4. We cannot skip the checking

Probably you would assume that exceptions solve it, but actually, they don't really provide an improvement. Sure, you don't have to wrap a try/catch block around your code. Exactly the same as you don't have to write an explicit null check. So what happens in both cases?

In both cases an exception is thrown either a NullReferenceException or probably you throw some other kind of Exception. Yes, you can catch your Exception further up the stack. The same way as you can catch a NullReferenceException further up the stack. There is no difference at all here.

But overall, this was not meant with skipping. The idea of skipping was that you can do the null check at some later point where it makes sense. That doesn't mean only "further up the stack". The idea is that you pass the whole error as a value around, as you can do with Optional, and additional you are forced to check the condition of your Optional at compile-time.

5. and 6. We can pass functions/objects that throws exception around

At default you don't pass Exception types as values around. You wrap your code in a try/catch and that's it. You also cannot implicitly pass an Exception as a valid value to a function that expects a Person. With null you can do that, that's why we have to also add null checks for function arguments.

So it seems we are not affected if we throw exceptions. But that is wrong. An object itself contains methods. And every method on an object could throw an exception.

With null you have to check every argument if it is null. With exceptions you have to additional add try/catch blocks if you call a method on an object. Because you pass objects around, and objects have methods that could throw exceptions when invoked. You end up with the same problem.

7. Happy Path Coding

It seems Exceptions solve the problem of Happy-Path coding. But it really does not. Yes, you are not forced to add a try/catch around every function directly. You just can use one try/catch around the whole code and catch any exception.

Absolutely, and the thing is. Any exception also includes NullReferenceException. So if you like to have exceptions. No problem, just return null from your functions and don't add any null checks.

Summary

It seems many people forget about why null is bad. null is bad because they throw exceptions when you try to use them. So using exceptions instead of using null makes nothing better at all. The reason why we add all those null checks is to prevent exceptions to happen. So how can we get rid of that problem if we choose to directly throw exceptions?

We can't. Throwing exceptions as a solution of getting rid of null is just a Pyrrhic victory. Not only that. Exceptions in general share the same problems as null.

So what is the alternative? My Post about null is Evil contains solution of getting rid of null. The thing is, the same solutions also works for Exceptions!

Further Reading

type exn = System.Exception

Full name: Microsoft.FSharp.Core.exn