Tertiaries

Tertiaries are formulæ and NIL. Formulæ were covered in chapter two. All that needs to be said here is that a formula can consist solely of a single secondary or primary or enclosed clause although this is not usual. If a formula, containing at least one operator, is to be used as a primary or a secondary, it must be enclosed in parentheses (or BEGIN and END). For example, in the formula next OF (H declarer), where H = (INT)REF HAND and HAND = STRUCT(...,REF HAND next), the formula must be surrounded by parentheses to make it into a secondary.

The only name having a denotation is NIL. Its mode is REF whatever. In other words, it can have any mode which starts with REF. It does not refer to any value and, although it must only occur in a strong context, it cannot be coerced. Its uses are described in the next chapter.


Sian Mountbatten 2012-01-19