What is the purpose of null?

问题:

I am in a compilers class and we are tasked with creating our own language, from scratch. Currently our dilemma is whether to include a 'null' type or not. What purpose does null provide? Some of our team is arguing that it is not strictly necessary, while others are pro-null just for the extra flexibility it can provide.

Do you have any thoughts, especially for or against null? Have you ever created functionality that required null?

回答1:

Null: The Billion Dollar Mistake. Tony Hoare:

I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years. In recent years, a number of program analysers like PREfix and PREfast in Microsoft have been used to check references, and give warnings if there is a risk they may be non-null. More recent programming languages like Spec# have introduced declarations for non-null references. This is the solution, which I rejected in 1965.

回答2:

null is a sentinel value that is not an integer, not a string, not a boolean - not anything really, except something to hold and be a "not there" value. Don't treat it as or expect it to be a 0, or an empty string or an empty list. Those are all valid values and can be geniunely valid values in many circumstances - the idea of a null instead means there is no value there.

Perhaps it's a little bit like a function throwing an exception instead of returning a value. Except instead of manufacturing and returning an ordinary value with a special meaning, it returns a special value that already has a special meaning. If a language expects you to work with null, then you can't really ignore it.

回答3:

Oh no, I feel the philosophy major coming out of me....

The notion of NULL comes from the notion of the empty set in set theory. Nearly everyone agrees that the empty set is not equal to zero. Mathematicians and philosophers have been battling about the value of set theory for decades.

In programming languages, I think it is very helpful to understand object references that do not refer to anything in memory. Google about set theory and you will see similarities between the formal symbolic systems (notation) that set theorists use and symbols we use in many computer languages.

Regards, Sam

回答4:

What's null for you ask?

Well,

Nothing.

回答5:

I usually think of 'null' in the C/C++ aspect of 'memory address 0'. It's not strictly needed, but if it didn't exist, then people would just use something else (if myNumber == -1, or if myString == "").

All I know is, I can't think of a day I've spent coding that I haven't typed the word "null", so I think that makes it pretty important.

In the .NET world, MS recently added nullable types for int, long, etc that never used to be nullable, so I guess they think its pretty important too.

If I was designing a lanaguage, I would keep it. However I wouldnt avoid using a language that didn't have null either. It would just take a little getting used too.

回答6:

the concept of null is not strictly necessary in exactly the same sense that the concept of zero is not strictly necessary.

回答7:

I don't think it's helpful to talk about null outside the context of the whole language design. First point of confusion: is the null type empty, or does it include a single, distinguished value (often called "nil")? A completely empty type is not very useful---although C uses the empty return type void to mark a procedure that is executed only for side effect, many other languages use a singleton type (usually the empty tuple) for this purpose.

I find that a nil value is used most effectively in dynamically typed languages. In Smalltalk it is the value used when you need a value but you don't have any information. In Lua it is used even more effectively: the nil value is the only value that cannot be a key or a value in a Lua table. In Lua, nil is also used as the value of missing parameters or results.

Overall I would say that a nil value can be useful in a dynamically typed setting, but in a statically typed setting, a null type is useful only for talking about functions (or procedures or methods) that are executed for side effect.

At all costs, avoid the NULL pointer used in C and Java. These are artifacts inherent in the implementations of pointers and objects, and in a well designed lanugage they should not be allowed. By all means give your users a way to extend an existing type with a null value, but make them do it explicitly, on purpose---don't force every type to have one by accident. (As an example of explicit use, I recently implemented Bentley and Sedgewick's ternary search trees in Haskell, and I needed to extend the character type with one additional value meaning 'not a character'. For this purpose Haskell provides the Maybe type.)

Finally, if you are writing a compiler, it is good to remember that the easiest parts of the language to compile, and the parts that cause the fewest bugs, are the parts that aren't there :-)

回答8:

It seems useful to have a way to indicate a reference or pointer that isn't currently pointing at anything, whether you call it null, nil, None, etc. If for no other reason to let people know when they're about to fall off the end of a linked list.

回答9:

In C NULL was (void*(0)), so it was a type with value(?). But that didn't work with C++ templates so C++ made NULL 0, it dropped the type and became a pure value.

However it was found that having a specific NULL type would be better so they (the C++ committee) decided that NULL will once again become a type (in C++0x).

Also almost every language besides C++ has NULL as a type, or an equivalent unique value not the same as 0 (it might be equal to it or not, but its not the same value).

So now even C++ will use NULL as a type, basically closing the discussions on the matter, since now everyone (almost) will have a NULL type

Edit: Thinking about it Haskell's maybe is another solution to NULL types, but its not as easy to grasp or implement.

回答10:

You can think of any type as a set along with a collection of operations. There are many cases where it's convenient to have a value with isn't a "normal" value; for example, consider an "EOF" value. for C's getline(). You can handle that in one of several ways: you can have a NULL value outside the set, you can distinguish a particular value as null (in C, ((void *)0) can serve that purpose) or you can have a way of creating a new type, so that for type T, you create a type T' =def { T ∪ NULL }, which is the way Haskell does it (a "Maybe" type).

Which one is better is good for lots of enjoyable argument.

回答11:

Null is not a mistake. Null means "I don't know yet"

For primitives you don't really need a null (I have to say that strings (in .NET) shouldn't get it IMHO)

But for composite entities it definitely serves a purpose.

回答12:

Null is only useful in situations where there are variables with unassigned values. If every variable has a value, then there is no need for null values.

回答13:

Null is a sentinel value. It's a value that cannot possibly be real data and instead provides meta-data about the variable in use.

Null assigned to a pointer indicates that the pointer is uninitialized. This gives you the ability to detect misuse of uninitialized pointers by detecting dereferences of null valued pointers. If you instead leave the value of a pointer equal to whatever happened to be in memory then you would have crazily irregular program behavior that would be much more difficult to debug.

Also, the null character in a C-style variable length string is used to mark the end of the string.

The use of null in these ways, especially for pointer values, has become so popular that the metaphor has been imported into other systems, even when the "null" sentinel value is implemented entirely differently and has no relation to the number 0.

回答14:

Null is not the problem - everyone treating, and interpreting null differently is the problem.

I like null. If there was no null, null would only be replaced with some other way for the code to say "I have no clue, dude!" (which some would write "I have no clue, man!", or "I have not a clue, old bean!" etc. and so, we'd have the exact same problems again).

I generalize, I know.

回答15:

Consider the examples of C and of Java, for example. In C, the convention is that a null pointer is the numeric value zero. Of course, that's really just a convention: nothing about the language treats that value as anything special. In Java, however, null is a distinct concept that you can detect and know that, yes, this is in fact a bad reference and I shouldn't try to open that door to see what's on the other side.

Even so, I hate nulls almost worse than anything else.

CLARIFICATION based on comments: I hate the defacto null pointer value of zero worse than I hate null.

Any time I see an assignment to null, I think, "oh good, someone has just put a landmine in the code. Someday, we're going to be walking down a related execution path and BOOM! NullPointerException!"

What I would prefer is for someone to specify a useful default or NullObject that lets me know that "this parameter has not been set to anything useful." A bald null by itself is just trouble waiting to happen.

That said, it's still better than a raw zero wandering around loose.

回答16:

A practical example of null is when you ask a yes/no question and don't get a response. You don't want to default to no because it might be important to know that the question wasn't answered in situations where the answer is very important.

回答17:

That decision depends on the objective of the programing language.

Who are you designing the programing language for? Are you designing it for people who are familiar with c-derived languages? If so, then you should probably add support for null.

In general, I would say that you should avoid violating people's expectations unless it serves a particular purpose.

Take switch-blocks in C# as an example. All case labels in C# must have an explicit control-flow expression in every branch. That is they must all end with either a "break" statement or an explicit goto. That means that while this code is legal:

switch(x)
{
    case 1:
    case 2:
        foo;
        break;
}

That this code would not be legal:

switch (x)
{
    case 1:
        foo();
    case 2:
        bar();
        break;
}

In order to create a "fall through" from case 1 to case 2, it's necessary to insert a goto, like this:

switch (x)
{
    case 1:
        foo();
        goto case 2;
    case 2:
        bar();
        break;
}

This is arguably something that would violate the expectations of C++ programmers who are leaning C#. However, adding that restriction serves a purpose. It eliminates the possibility of an entire class of common C++ bugs. It adds to the learning curve of the language slightly, but the result is a net benefit to the programmer.

If your goal is to design a language targeted at C++ programmers, then removing null would probably violate their expectations. That will cause confusion, and make your language more difficult to learn. The key question is then, "what benefit do they get"? Or, alternatively, "what detriment does this cause".

If you are simply trying to design a "super small language" that can be implemented in the course of a single semester, then the story is different. In that case your objective isn't to be build a useful language targeted at a particular segment of the population. Instead, it's just to learn how to create a compiler. In that scenario, having a smaller language is a big benefit, and so it's worth eliminating null.

So, to recap, I would say that you should:

1. Identify your goals in creating the language. Who is the language designed for, and what are their needs.
2. Make the decision based on what helps the target users meet their goals in the best way.

Usually this will make the desired result pretty clear.

Of course, if you don't explicitly articulate your design goals, or you can't agree on what they are, then you are still going to argue. In that case, however, you are pretty much doomed anyways.

回答18:

One other way to look at null is that it's a performance issue. If you have a complex object containing other complex objects and so on, then it is more efficient to allow for all properties to initially become null instead of creating some kind of empty objects that won't be good for nothing and soon to be replaced.

That's just one perspective that I can't see mentioned before.

回答19:

What purpose does null provide?

I believe there are two concepts of null at work here.

The first (null the logical indicator) is a conventional program language mechanism that provides runtime indication of a non-initialized memory reference in program logic.

The second (null the value) is a base data value that can be used in logical expressions to detect the logical null indicator (the previous definition) and make logical decisions in program code.

Do you have any thoughts, especially for or against null?

While null has been the bane of many programmers and the source of many application faults over the years, the null concept has validity. If you and your team create a language that uses memory references that can be potentially misused because the reference was not initialized, you will likely need a mechanism to detect that eventuality. It is always an option to create an alternative, but null is a widely known alternative.

Bottom line, it all depends upon the goals of your language:

1. target programming audience
2. robustness
3. performance
4. etc...

If robustness and program correctness are high on your priority list AND you allow programmatic memory references, you will want to consider null.

BB

回答20:

If you are creating a statically typed language, I imagine that null could add a good deal of complexity to your compiler.

If you are creating a dynamically typed language, NULL can come in quite handy, as it is just another "type" without any variations.

回答21:

Null is a placeholder that means that no value (append "of the correct type" for a static-typed language) can be assigned to that variable.

There is cognitive dissonance here. I heard somewhere else that humans cannot comprehend negation, because they must suppose a value and then imagine its unfitness.

回答22:

My suggestion to your team is: come up with some examples programs that need to be written in your language, and see how they would look if you left out null, versus if you included it.

回答23:

Use a null object pattern!

If you language is object oriented, let it have an UndefinedValue class of which only one singleton instance exists. Then use this instance wherever null is used. This has the advantage that your null will respond to messages such as #toString and #equals. You will never run into a null pointer exception as in Java. (Of course, this requires that your language is dynamically typed).

回答24:

Null provides an easy way out for programmers who haven't completely thought through the logic and domains needed by their program, or the future maintenance implications of using a value with essentially no clear and agreed upon definition.

It may seem obvious at first that it must mean "no value", but what that ACTUALLY means depends on context. If, for instance LastName === null, does that mean that person doesn't have a last name, or that we don't know what their last name is, or that it hasn't be entered into the system yet? Does null equal itself, or doesn't it? In SQL it does not. In many languages it does. But if we don't know the value of personA.lastName, or personB.lastName, how can we know that personA.lastName === personB.lastName, eh? Should the result be false, or .. . null?

It depends on what you're doing, which is why it's dangerous and silly to have some kind of system wide value that can be used for any kind of situation that kind of looks like "nothing", since how other parts of your program and external libraries or modules can't really be depended upon to correctly interpret what you meant by "null".

You're much better off clearly defining the DOMAIN of possible values of lastName, and exactly what every possible value actually means, rather than depending on some vague systemwide notion of null, which may or may not have any relevance to what you're doing, depending on which language you're using, and what you're trying to do. A value, which may in fact, behave in exactly the wrong way when you begin to operate on your data.

回答25:

Null is to objects what 0 is to numbers.