Question

Why is it illegal for variables to start with numbers?I know it’s a convention but what’s the reason?

Edit:
I mean variables like “1foo” or “23bar” not only numbers like “3”

Answer 1

In languages such as Prolog, Erlang, and some early versions of Fortran, you very nearly got to do this, for completely different reasons.

Prolog/Erlang don’t have variable assignment, they have unification. IIRC, if X is a variable, then code following 2 = X, or X = 2 is processed if X may have the value 2. So if X is already unified with a value, then that value must be 2, and if not, X becomes 2 from then on. So writing 3 = 3 is fine – it should become a no-op, and 2 = 3 always fails – either a non-match in Prolog or (I think) a runtime error in Erlang. Numbers behave like variables which have already been unified with the value the numbers represent.

In early Fortran ( apologies for not having used fortran in twenty years and forgetting its syntax ), all function arguments were passed by reference, so if you have a function which was equivalent to void foo ( int &x ) { x = 3; } and called it with a number, the compiler would store the number in a static variable and pass that. So calling foo (2) would set that static stored value of 2 to 3. If it happened to use the same static variable for the literal 2 somewhere else, such as calling another function with the literal 2, then the value passed to the second function would be 3 instead.

So you can have variables which are syntactically identical to numbers, as long as they are automatically initialised to the value of the literal. But if you allow them to be mutable rather pure variables, weirdness abounds.