What would we do without NULL?

Question

I once read that having nullable types is an absolute evil. I believe it was in an article written by the very person who created them(in Ada?) I believe this is the article

Answer 1

We'd create all kinds of strange constructs to convey the message of an object 'being invalid' or 'not being there', as seen in the other answers. A message that null can convey very well.

• The Null Object pattern has its disadvantages, as I explained here.
• Domain-specific nulls. This forces you to check for magic numbers, which is bad.
• Collection wrappers, where an empty collection means 'no value'. Nullable wrappers would be better, but that doesn't differ much from checking for null or using the Null Object pattern.

Personally, I would write some C# preprocessor that allows me to use null. This would then map to some dynamic object, which throws a NullReferenceException whenever a method is invoked on it.

Back in 1965, null references may have looked like a mistake. But nowadays, with all kinds of code analysis tools that warn us about null references, we don't have to worry that much. From a programming perspective null is a very valuable keyword.

Answer 2

Null is not the problem, it is the language allowing you to write code that accesses values that can possibly be null.

If the language would simply require any pointer access to be checked or converted to a non-nullable type first, 99% of null related bugs would go away. E.g. in C++

void fun(foo *f)
{
    f->x;                  // error: possibly null
    if (f)              
    {
        f->x;              // ok
        foo &r = *f;       // ok, convert to non-nullable type
        if (...) f = bar;  // possibly null again
        f->x;              // error
        r.x;               // ok
    }
}

Sadly, this can't be retrofitted to most languages, as it would break a lot of code, but would be quite reasonable for a new language.

Answer 3

You can adopt a simple rule: All variables are initialized (as a default, this can be overridden) to a immutable value, defined by the variable's class. For scalars, this would usually be some form of zero. For references, each class would define what its "null" value is, and references would be initialized with a pointer to this value.

This would be effectively a language-wide implementation of the NullObject pattern: http://en.wikipedia.org/wiki/Null_Object_pattern So it doesn't really get rid of null objects, it just keeps them from being special cases that must be handled as such.

Answer 4

Realistically speaking, in any powerful programming language that allows pointers or object references in the first place, there are going to be situations where code will be able to access pointers which have not had any initialization code run upon them. It may be possible to guarantee that such pointers will be initialized to some static value, but that doesn't seem terribly useful. If a machine has a general means of trapping accesses to uninitialized variables (be they pointers or something else), that's better than special-casing null pointers, but otherwise the biggest null-related mistakes I see occur in implementations that allow arithmetic with null pointers. Adding 5 to a (char*)0 shouldn't yield a character pointer to address 5; it should trigger an error (if it's appropriate to create pointers to absolute addresses, there should be some other means of doing it).

Answer 5

Tcl is one language that not only does not have the concept of null but where the concept of null itself is at odds with the core of the language. In tcl we say: 'everything is a string'. What it really means is tcl has a strict value semantics (which just happens to default to strings).

So what do tcl programmers use to represent "no-data"? Mostly it's the empty string. In some cases where the empty string can represent data then its typically one of:

1. Use empty string anyway - the majority of the time it makes no difference to the end user.

2. Use a value you know won't exist in the data stream - for example the string "_NULL_" or the number 9999999 or my favourite the NUL byte "\0".

3. Use a data structure wrapped around the value - the simplest is a list (what other languages call arrays). A list of one element means the value exist, zero element means null.

4. Test for the existence of the variable - [info exists variable_name].

It is interesting to note that Tcl is not the only language with strict value semantics. C also has strict value semantics but the default semantics of values just happen to be integers rather than strings.

Oh, almost forgot another one:

Some libraries use a variation of number 2 that allows the user to specify what the placeholder for "no data" is. Basically it's allowing you to specify a default value (and if you don't the default value usually defaults to an empty string).