Why can't an identifier start with a number?

user_8490848 0
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I have a file named 1_add.rs, and I tried to add it into the lib.rs. Yet, I got the following error during compilation.

error: expected identifier, found `1_add`
 --> src/lib.rs:1:5
  |
1 | mod 1_add;
  |     ^^^^^ expected identifier

It seems the identifier that starts with a digit is invalid. But why would Rust has this restriction? Is there any workaround if I want to indicate the sequence of different rust files (for managing the exercise files).

rust
5 Answers

In your case (you want to name the files like 1_foo.rs) you can write

#[path="1_foo.rs"]
mod mod_1_foo;

Allowing identifies to start with digits can conflict with type annotations. E.g.

let foo = 1_u32;

sets to type to u32. It would be confusing when 1_u256 means another variable.

user_22520870

But why would Rust has this restriction?

Not only rust, but most every language I've written a line of code in has this restriction as well.

Food for thought:

let a = 1_2;

Is 1_2 a variable name or is it a literal for value 12? What if variable 1_2 does not exist now, but you add it later, does this token stop being a number literal?

While rust compiler probably could make it work, it's not worth all the confusion, IMHO.

user_1258160

Allowing identifiers to start with a digit would caus conflicts with many other token types. Here are a few examples:

  • 1e1 is a floating point number.
  • 0x0 is a hexadecimal integer.
  • 8u8 is an integer with explicit type annotation.

Most importantly, though, I believe allowing identifiers starting with digit would hurt readability. Currently everything starting with a digit is some kind of number, which in my opinion helps when reading code.

An incomplete list of programming languages not allowing identifiers to start with a digit: Python, Java, JavaScript, C#, Ruby, C, C++, Pascal. I can't think of a language that does allow this (which most likely does exist).

user_2796270

Rust identifiers are based on Unicode® Standard Annex #31 (see The Rust RFC Book), which standardizes some common rules for identifiers in programming languages. It might make it easier to parse text that could otherwise be ambiguous, like 1e10?

user_1243190

"Why?" cannot be reasoned here but by historical tales, the rules are as such. You cannot play against them.

If you urgently want to start your identifiers with a digit, at least for human readers, prepend an underscore like this: _1_add.

Note: To make sure that sorting works well, use also leading zeroes as many as appropriate (_001_add if you expect more than 99 files).

user_112948310
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