Unknown Values in Spectre

Unknown indicates a value that cannot be computed. Here are some ways to get an unknown value in Spectre:

Uncomputable numeric operations.

For example, `1/0`, `log(-1)`, `arcsin(10)`.
Uncomputable date or period operations.

For example, `day(null)`.
Format or parse errors.

For example, `parse_integer("badnum")`, `date("2015 Jan", "M-D-YYYY")`.
Invalid arguments.

For example, `period("badcalendarname", "2015 Jan")`, `extract(date("2015/01/01"), "badinterval")`.

With very few exceptions, every expression that encounters unknown returns unknown. It is never treated as a normal value because it is impossible to determine which normal value applies.

You can check for unknown with the is_unknown() function and use the catch() function to substitute alternative values for unknown.

Displaying and Diving on Null and Unknown

In summary columns, that is the non-dimension columns, null and unknown both display as blank values.
In dimension columns, null is the blank value and unknown displays as <unknown>.