Why was nil defined as a reserved word in Pascal?

Question

In Pascal, nil (the pointer value to "nothing") is a reserved word. Why wasn't it simply a predefined identifier as true and false are, for example?

This is stated in PASCAL User Manual and Report p. 109, as well in the ISO documents ISO 7185:1990 (Pascal) and ISO 10206:1990 (Extended Pascal) both in section 6.1.2.

Answer 1

The definition of PASCAL is, above all else, intended to be simple. PASCAL was designed as a pedagogical language (with aspirations to be useful for commercial purposes, but that was a secondary concern). For this purpose, the definition had to be small and orthogonal so that it could be explained simply and concisely. For ease of implementation, the number of special cases had to be kept to a minimum.

The boolean type is handled by the system as simply an example of an enumerated type. It is effectively equivalent to having a definition

type boolean = (false, true)

automatically included in the program. Specficially: it can be implemented by entering false and true into the symbol table with associated type of boolean. They can then never be used for any other type, as PASCAL identifiers are associated with a single type only in any scope.

nil, however, could not be defined by an existing language mechanism. The language simply does not provide any means of creating a pointer value other than via new, which creates a value while nil refers to an absence of value. Therefore, a new language feature was required to implement it, so a new keyword was added for that feature.

Also, nil does not behave the same way an identifier does: it does not have a predetermined type. The type of a nil expression is determined by its context -- it may become any type pointer that is required to make the expression type check. If it were implemented as an identifier rather than a keyword, that would have required a special case for polymorphic identifiers, of which only a single instance was required for the language and no way provided of defining new ones: clearly not a useful way of approaching the problem.

Answer 2

Unlike Boolean constants, the value of NIL cannot be assigned a particular type. That's why it has to be parsed in a special way, that is, it has to be a keyword.

Another reason for NIL to be a keyword is to disallow explicitly dereferencing NIL by writing NIL@. If NIL were a predefined constant of any pointer type, even a magic polymorphic pointer, it would have been syntactically allowed.

Answer 3

Because neither 0 nor false is a pointer. Pointers are pointers and not numeric values that can be used in directly in mathematical expressions. Assigning false to a pointer doesn't make any sense so Pascal assigns nil to it.

Talking about "sense" there's a famous example in C++ where previously pointers were generally compared with the literal 0 or macro NULL and that posed several issues. For example NULL*3 would make no sense but it's valid. Using 0 also creates some confusion/feeling that the null pointer always contains 0 value. That's not correct. A null pointer is just a pointer that points to nothing, the binary representation isn't important and there are architectures that use non-zero values to indicate a null pointer.

Since C++11 they created a new keyword for it: nullptr which solves the problem and making pointer comparison a lot more sense.

So Pascal uses nil right from the beginning and there was no similar problem. It also makes Pascal more typesafe. Other languages like Java or C# also have the null keyword for the same reasons.

Answer 4

There are excellent answers in this thread.

The clearest answer is that nil cannot be a value because it has no type. Whereas true and false are understood to be values of enumerated, i.e.,

type boolean = (false, true);

What type of pointer would nil be an instance of? Recall pointer definitions are:

type p = ^integer;

nil, if it is a value, is compatible with all pointer types.

Second, there is no other fixed value that can be assigned to pointers. In the example:

type p = ^integer;

var ip, xp: p;

new(ip);

xp := ip;
ip := nil;

Thus we can see:

1. nil is typeless.
2. nil is the only pointer value that is fixed and not generated at runtime.

So nil does not behave like any other compiler constant, so it is not one, but is a keyword.

Answer 5

null pointer has a value reserved for indicating that the pointer does not refer to a valid object

Wikipedia is having good explanation of it. Thus null, or nil, is a value of pointer, chosen by the compiler, which, by default, is not valid within current application environment, or is not reachable by the user code.

Why this word "nil" ever exists? For convenience - many functions returning pointers sometimes need to return "invalid pointer" to the calling code, and this calling code will check for this making decision on successful execution of the function. Very good example is memory allocation routines. If there's enough RAM to allocate, they will return valid pointer to the area, if no more RAM - it will return value of null (nil), and calling program will know that malloc (for example) failed to find the space requested.

Why wasn't it simply a predefined identifier as true and false are, for example?

Because Boolean logic constant or variable can only be true or false, but pointer is having wider range of values (up to size of addressable CPU/MPU space).

@JeanPierre:

but I don't see how the range of pointer values affects the possibility to predefine an id with one specific value of this range

The exact value is selected at the time of compilation by the compiler according to its settings and architecture of the target system. nil or null exists to inform compiler to apply this value to all pointer operations. Writing code like if(ptr==0) may not be portable and not function properly because some systems may have pointer equal to 0 as valid addressable value.