Optional values

An optional value holds a value or holds nothing. The compiler does not let the program read the value directly, because the value can be empty.

Declaration

def name@(optional):string

The annotation @(optional) is after the name and before the type.

Read an optional value

Operator	Function
??	Give the value, or give the second argument when the value is empty.
!!	Give the value. The program stops when the value is empty.
unwrap	The same as !!.
null?	Give true when the value is empty.
!null?	Give true when the value is not empty.
wrap	Make an optional value from a value.
nullify	Make the optional value empty.

def name@(optional):string
def shown (?? name "unknown")
print ("name " + shown)

if (null? name) {
    print "the name is empty"
}

The operators are in prefix form: the operator is first and the arguments are after it.

What the compiler writes

Each target language has its own way to hold an empty value. The compiler writes the correct one:

Target	Empty value
JavaScript	undefined
Go	A structure with a has_value field
Rust	Option<T>
Swift	An optional type
Java	null

The generic operators page shows the generated code of each operator for each target.

Strict mode

The flag -strict stops the automatic read of an optional value outside of a try block. Use the flag when the program must handle each empty value.