Tuesday, March 3, 2009

Null references

In an interesting abstract, Tony Hoare describes the invention of the null reference a "billion dollar mistake". He goes on to describe the huge number of unforeseeable side-effects that are a result of the null reference.

This brings up an interesting topic of why the null reference is needed. This is actually a question as well. Why are null references needed? I don't think they should exist. There is simply no need for them. A well structured program should never have a need to define a null reference. Null, basically means "I don't know". How is this useful any any application? I suppose it is useful in situations where the developer's best guess is "undefined".

I spend the majority of my time writing code in dynamically-typed languages. I'm not very concerned with null references. However, when I do write in compiled-languages, I find any alternative than using null references. Even if a given reference in the context of some function can be considered dynamic, or polymorphic, it still isn't null.

Having said that, what if we find ourselves in a situation where null references are absolutely unavoidable? Why do null references cause such a massive headache? My guess would be that the null reference is treated differently than the false value. The false value here is anything that would not evaluate to true in a compiled language. Does that not fall into the same category as null? Not necessarily. If we have a null reference and we want to invoke some behaviour on that reference, we simply attempt to invoke it, assuming we do not have a null.

What is needed in these situations is a wrapper that simple checks for "nullness" before invoking any type of behaviour. This way, nulls are treated as booleans. This new overhead checking during run time will no doubt slow things down. This is not a good thing since performance is probably a big part of why we are using a compiled language to begin with. However, I thing stability is slightly higher in the development food chain than raw performance is.