The is operator #

Syntax #

V@[is_expr] ::= V@[spec_expr]  is R@[ident]
          | V@[spec_expr] !is R@[ident]

Typing #

The left hand side should be an expression with some enum type, and the right hand side should be an (unqualified) identifier of a variant of that enum. The V@[is_expr] has type bool.

Semantics #

An is expression returns true if the left hand side matches the named variant. A !is expression returns the negation.

Example #

fn test() {
    let x = Some(5);
    assert(x is Some);
    assert(x !is None);
}