Preprocess

A # at the start of a line triggers a preprocessing directive: before compilation, source is trimmed by symbol conditions. A good fit for forks like “the same code, with the editor build carrying an extra set of debug functions”.

Conditional Blocks


#if EDITOR
func Mode() -> string => "editor";
#elif SHIPPING
func Mode() -> string => "shipping";
#else
func Mode() -> string => "default";
#endif
 
func main() -> int
{
    print(Mode());
    return 0;
}


default

The host does not define EDITOR, so it evaluates as 0/false and the #else branch applies; Mode() returns “default”. Symbols are injected by the host (DefinedSymbols / SymbolValues); in a direct CLI run none are defined.

The Directive Set

Directive	Kind	Form
`#if expr`	block	starts a conditional block (occupies the whole line)
`#elif expr`	block	a branch
`#else` / `#endif`	block	fallback / end
`#SYM`	prefix	symbol defined → activates the next declaration/statement
`#!SYM`	prefix	negated
`#[expr]`	prefix	parenthesized expression

Prefix forms attach directly in front of a single declaration or statement and trim only that one:


#DEBUG
public val Verbose: bool = true;

Undefined Symbols Always Warn

Every symbol in an #if / #elif / prefix directive that is not in the host’s symbol set raises an MS1214 warning; this warning is what keeps such mistakes visible. Numeric symbols can take part in expression comparisons (#elif DEBUG >= 2).

Notes

# triggers only at the start of a line; a # mid-line is an ordinary character.
Preprocessing runs at the very front of compilation; trimmed-out code never reaches the parser, so each side of a fork only needs to stand on its own syntactically.

Last updated on October 11, 2026