Question

In the kaleidoscope parser / AST example at LLVM, the enum is given all negative values. Why the minus sign ?

enum Token {
  tok_eof = -1,
  // commands
  tok_def = -2, tok_extern = -3,
  // primary
  tok_identifier = -4, tok_number = -5
};

Answer 1

A common C idiom with enums is to use negative values to mean one set of conditions and positive values to mean another set. For example, error conditions from the parser might be all positive values, while normal conditions have all negative values, and maybe zero is the “undefined” case. So in your code testing for any error is as simple as tok >= 0.