NoValue

NoValue is a special literal; it does not have a type and is convertible to all types. Any computation with NoValue, other than comparison with NoValue, will return NoValue.

NoValue is used in circumstances where a value is needed that cannot possibly be an expected result.

For example:

Intent>position(1,{3,4,5,1})
--> 4
Intent>position(1,{3,4,5})
--> NoValue
Intent>

NoValue is a legal value. However, with the exception of the "=" operator and a few functions, NoValue "contaminates" any calculation or expression that includes it, such that the result of the calculation is itself NoValue:

Intent>3+NoValue
--> NoValue
Intent>

Note that this does not result in an error. However, some functions will raise an error if you pass in NoValue.

There are exceptional cases where NoValue is allowed, and does not contaminate the result. Most importantly, you can test for NoValue using the ordinary "=" operator and get a Boolean result. For example, "if (foo = NoValue) then..." will not return NoValue merely for referencing NoValue in the equality test.