Identifiers

An identifier names a binding, function, type, field, module or other declaration.

Identifier ::= IdentStart IdentContinue*
IdentStart ::= 'A'…'Z' | 'a'…'z' | '_'
IdentContinue ::= IdentStart | '0'…'9'

An identifier starts with an ASCII letter or an underscore and continues with letters, digits and underscores. It cannot be one of the 38 keywords.

let count = 1;
let x1 = 2;
let _scratch = 3;
let parsedValue = 4;

Rules

• ASCII only. Letters outside ASCII are not identifier characters: let café = 1; stops with error: unexpected character 'é' (U+00E9). Non-ASCII text belongs in literals and comments.
• Case-sensitive. count, Count and COUNT are three different names.
• No keywords. A keyword in a name's place is a syntax error, such as error: expected a pattern after the binding keyword before 'while' for let while = 1;. For none and fail the message names the problem directly: error: 'none' is a reserved keyword and cannot name a binding.
• A lone _ is not a name. In a binding or pattern, _ discards the value instead of naming it: let _ = Compute();. See Destructuring.

Primitive type names such as int, bool and char8 are not keywords; they are predefined type names, listed in the Primitive Types appendix. A few other words have a meaning only in a particular position; Keywords lists them.

Naming conventions

Rux code uses two spellings, and rux lint checks them:

Neither spelling uses underscores. A name that breaks the convention compiles, and rux lint suggests the fix:

warning: function name 'parse_input' should be PascalCase
  help: rename it to 'ParseInput'
warning: local variable name 'Result' should be camelCase
  help: rename it to 'result'

The primitive types and their built-in aliases (int, float64, byte) are lower case by long tradition. A declaration that has to keep a foreign spelling — a C type name in a binding, say — can opt out with #Allow("naming.type") or #Allow("naming.const").

See also

• Keywords — the words an identifier cannot be
• Modules — qualified names such as Io::PrintLine
• rux lint — the naming checks
• Variable — the lesson on naming values