Unset Values

Software Customisation Reference Manual

PML 1 Expressions : Unset Values

A particular class of undefined values are unset values. The concept exists for attributes which are valid for a given element, but for which no value has been assigned. Typically these may be elements of an array, or ’word’ attributes. References of value =0/0 are also treated as unset.

Unset values are propagated as for undefined values (except for Boolean operations- see below). Undefined values take precedence over unset. There is a specific logical function UNSET to test if a values is unset.

Across comparisons, unset values are not propagated, but are treated as follows:


Operator	When Applied to an UNSET
EQ, GT, GE, LT, LE	Results in FALSE.
NE	Results in TRUE.
OR , AND	Values are treated as FALSE.

For example, if DESP(2) and LVAL(3) are unset then:

(DESP(2) GT 99) -> False

(DESP(2) NE 33) -> True

(:LVAL(3) AND TRUE) -> False

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