7.2 Annotations for Optional Values🔗ℹ

Sometimes, a function accepts an optional value, and it makes sense to allow #false to indicate “nothing.” A function could annotate such an option with || False.

A drawback of using || is that static information is lost in the case of a non-#false value. In the preceding example, " " ++ last is not statically resolved to string concatenation.

The maybe annotation constructor is a shorthand for || False, but it also cooperates with operators like !!, which returns its argument only if it is non-#false and throws an exception otherwise. If the argument to !! has static information from maybe(annot), then the result from !! has the static information of annot.

The ?. operator is like the . operator, but it produces #false without accessing a field or method if the expression before ?. produces #false. It also recognizes maybe static information for the expression before ?., which enables static dispatch for a non-#false value.

The || operator similarly cooerates with maybe for the left-hand argument to ||. Since a #false result from the left-hand argument is never used as the result, if it has static information from maybe(annot), then it can be treated as having the static information of annot.

Along similar lines, the static information of a && expression corresponds to the static information of its right-hand argument, but adjusted in the same way as by maybe.

The && operator can serve the same role as the elvis operator ?: that is provided some other languages.