TMBASIC Documentation

Optional Type

Normally, a variable is required to have a value. For instance, a variable of type Number will always contain some number, by default zero. Sometimes it is useful to have a variable that may or may not have a value.
dim a as Number ' default value is zero
print a ' prints 0
dim b as Optional Number
print HasValue(b)  ' prints false
b = 5
print HasValue(b)  ' prints true
print Value(b)     ' prints 5
This example demonstrates the usage of an Optional type.
Declare a variable with dim and include the keyword Optional before the type.
The variable will initially contain no value.
Use HasValue to check whether an Optional variable contains a value.
If it does, usa Value to retrieve the value.