Why can't variable names start with numbers?

Question

I was working with a new C++ developer a while back when he asked the question: "Why can't variable names start with numbers?"

I couldn't come up with an answer except that some numbers can have text in them (123456L, 123456U) and that wouldn't be possible if the compilers were thinking everything with some amount of alpha characters was a variable name.

Was that the right answer? Are there any more reasons?

string 2BeOrNot2Be = "that is the question"; // Why won't this compile?

Answer 1

One of the key problems about relaxing syntactic conventions is that it introduces cognitive dissonance into the coding process. How you think about your code could be deeply influenced by the lack of clarity this would introduce.

Wasn't it Dykstra who said that the "most important aspect of any tool is its effect on its user"?

Answer 2

Probably because it makes it easier for the human to tell whether it's a number or an identifier, and because of tradition. Having identifiers that could begin with a digit wouldn't complicate the lexical scans all that much.

Not all languages have forbidden identifiers beginning with a digit. In Forth, they could be numbers, and small integers were normally defined as Forth words (essentially identifiers), since it was faster to read "2" as a routine to push a 2 onto the stack than to recognize "2" as a number whose value was 2. (In processing input from the programmer or the disk block, the Forth system would split up the input according to spaces. It would try to look the token up in the dictionary to see if it was a defined word, and if not would attempt to translate it into a number, and if not would flag an error.)

Answer 3

The restriction is arbitrary. Various Lisps permit symbol names to begin with numerals.

Answer 4

I agree it would be handy to allow identifiers to begin with a digit. One or two people have mentioned that you can get around this restriction by prepending an underscore to your identifier, but that's really ugly.

I think part of the problem comes from number literals such as 0xdeadbeef, which make it hard to come up with easy to remember rules for identifiers that can start with a digit. One way to do it might be to allow anything matching [A-Za-z_]+ that is NOT a keyword or number literal. The problem is that it would lead to weird things like 0xdeadpork being allowed, but not 0xdeadbeef. Ultimately, I think we should be fair to all meats :P.

When I was first learning C, I remember feeling the rules for variable names were arbitrary and restrictive. Worst of all, they were hard to remember, so I gave up trying to learn them. I just did what felt right, and it worked pretty well. Now that I've learned alot more, it doesn't seem so bad, and I finally got around to learning it right.

Answer 5

As several people have noticed, there is a lot of historical baggage about valid formats for variable names. And language designers are always influenced by what they know when they create new languages.

That said, pretty much all of the time a language doesn't allow variable names to begin with numbers is because those are the rules of the language design. Often it is because such a simple rule makes the parsing and lexing of the language vastly easier. Not all language designers know this is the real reason, though. Modern lexing tools help, because if you tried to define it as permissible, they will give you parsing conflicts.

OTOH, if your language has a uniquely identifiable character to herald variable names, it is possible to set it up for them to begin with a number. Similar rule variations can also be used to allow spaces in variable names. But the resulting language is likely to not to resemble any popular conventional language very much, if at all.

For an example of a fairly simple HTML templating language that does permit variables to begin with numbers and have embedded spaces, look at Qompose.

Answer 6

COBOL allows variables to begin with a digit.