When looking up information with the $( and
$) operators it is sometimes desirable to provide
a default return value, should the lookup fail. Default values are
specified with the $: operator, which fits between
the $( and $) operators like
LHS.... $( name key $: default $)
Here, name is the symbolic name you
associated with a dbtype (Section 23.2.2) using the K configuration
command. The key is the value being looked up, and
default is the value to be placed in the
workspace if the lookup fails.
To illustrate, consider the following rule:
R $+ < @ $* . fax > $: $1 < @ $(faxdb $2 $: faxhost $) >
Here, any address that ends in .fax (such as
firstname.lastname@example.org) has the host part
($* or the here) looked up in
the faxdb database (the $2 is
the key). If that host is not found with the lookup, the workspace is
changed to user<@faxhost> (or, for
our example, bob@faxhost).
See Section 23.4.1 for a complete description of the
$: operator as it is used with database maps.