Why does a programming language need keywords?

Question

For example (in C):

int break = 1;
int for = 2;

Why will the compiler have any problems at all in deducing that break and fo

Answer 1

Since it's tagged C, the original C language was such that by default any variable was defined as type int.

It means that foo; would declare a variable of type int.

Let's say you do break;. So how does the compiler know whether you want to declare a variable named break or use the keyword break?

Answer 2

It's not necessary -- Fortran didn't reserve any words, so things like:

if if .eq. then then if = else else then = if endif

are complete legal. This not only makes the language hard for the compiler to parse, but often almost impossible for a person to read or spot errors. for example, consider classic Fortran (say, up through Fortran 77 -- I haven't used it recently, but at least hope they've fixed a few things like this in more recent standards). A Fortran DO loop looks like this:

DO 10 I = 1,10

Without them being side-by-side, you can probably see how you'd miss how this was different:

DO 10 I = 1.10

Unfortunately, the latter isn't a DO loop at all -- it's a simple assignment of the value 1.10 to a variable named DO 10 I (yes, it also allows spaces in a name). Since Fortran also supports implicit (undeclared) variables, this is (or was) all perfectly legal, and some compilers would even accept it without a warning!

Answer 3

FWIW, Tcl doesn't have any reserved words. You can have variables and functions named "if", "break", etc. The interpretation of a token is totally dependent on the context. The same token can represent a command in one context, a variable in another, or a literal string in another.

Answer 4

Then what will the computer do when it comes across a statement like:

while(1) {
  ...
  if (condition)
    break;
}

Should it actually break? Or should it treat it as 1;?

The language would become ambiguous in certain cases, or you'd have to create a very smart parser that can infer subtle syntax, and that's just unnecessary extra work.

Answer 5

The compiler would have problems if you write something like this:

while(*s++);
return(5);

Is that a loop or a call to a function named while? Did you want to return the value 5 from the current function, or did you want to call a function named return?

It often simplifies things if constructs with special meaning simply have special names that can be used to unambiguously refer to them.

Answer 6

They don't. PL/1 famously has no keywords; every "keyword" (BEGIN, DO, ...) can also be used a variable name. But allowing this means you can write really obscure code: IF DO>BEGIN THEN PRINT:=CALL-GOTO; Reserving the "statement keywords" as the language isn't usually a loss if that set of names is modest (as it is in every langauge I've ever seen except PL/1 :-).

APL also famously has no keywords. But it has a set of some 200 amazing iconic symbols in which to write complicated operators. (the "domino" operator [don't ask!] is a square box with a calculator divide sign in the middle) In this case, the langauge designers simply used icons instead of keywords. The consequence is that APL has a reputation of being a "write only" language.

Bottom line: not a requirement, but it tends to make programs a lot more readable if the keywords are reserved identifiers from a small set known to the programmers. (Some langauges has insisted that "keywords" start with a special punctuation character like "." to allow all possible identifiers to be used, but this isn't worth the extra trouble to type or the clutter on the page; its pretty easy to stay away from "identifiers" that match keywords when the keyword set is small).