Void-Unit ¶

Meaning ¶

A serial clause contains one or more units. Of these, the units preceding a completer and the unit appearing last in the clause yield a value which in turn will be the value yielded by the complete serial clause.

Consider the following example:

begin
  int tmp := a;
  a := b;
  if tmp = 0 then divbyzero fi;
  a / temp exit
divbyzero:
  0
end

This particular serial clause contains one declaration, one label and four units.

The units a / tmp, which appears right before a completer, and and 0, which is the unit appearing last in the serial clause, yield an integral value which is the result of the serial clause. These are called int-units or, more generally, mode-units. These are also known as expressions.

On the other hand the units a := b and if tmp = 0 then divbyzero fi also yield values. For example, the assignation yields a of mode ref int. However, the value yielded by these units gets voided and discarded. These are void-units, also known as statements.

Syntax ¶

Simplified [RR 3.2.1.b]:

b) SOID series with PROPSETY:
     strong void unit, go on token, SOID series with PROPSETY ;
     where (PROPSETY) is (DECS DECSETY LABSETY),
       declaration of DECS, go on token,
       SOID series with DECSETY LABSETY ;
     where (PROPSETY) is (LAB LABSETY),
       label definition of LAB,
       SOID series with LABSETY ;
     where (PROPSETY) is (LAB LABSETY) and SOID balances SOID1 and SOID2,
       SOID1 unit, completion token, label definition of LAB,
       SOID2 series with LABSETY ;
     where (PROPSETY) is (EMPTY),
       SOID unit.

In the hyper-rule above the first alternative matches a void-unit. Note that the unit is in a strong context with goal mode void, and therefore is subject to voiding.

See Also ¶

• Statement
• Completer