United modes introduce no new values. A united mode can have any
mode as one of its constituents except a mode which can be firmly
coerced to itself. The uniting coercion is available in firm contexts.
Because the values supplied to print
or read
are united, the context of the parameter of those procedures is firm.
A conformity clause is used to extract the constituent mode or value.
The mode VOID
can be one of the constituents of a united
mode and is useful to signal an exceptional yield from a procedure.
United modes are used in a variety of ways.
CRIB
whose constituent
modes are CHAR
, REAL
, INT
and BOOL
.
AnsUABS
which has a single
operand of mode CRIB
and which yields the absolute value of its
operand. AnsUABS
and a denotation for each
of the constituent modes of CRIB
.
Ans