4.22 Exceptional Conditions

When the running program encounters a condition it is not able to handle, it signals an exception. To illustrate the concept, let’s consider the execution of the following code fragment:

  if primitive_hasmx(domainpart($f))
    accept
  fi

The function primitive_hasmx (see primitive_hasmx) tests whether the domain name given as its argument has any ‘MX’ records. It should return a boolean value. However, when querying the Domain Name System, it may fail to get a definite result. For example, the DNS server can be down or temporary unavailable. In other words, primitive_hasmx can be in a situation when, instead of returning ‘yes’ or ‘no’, it has to return ‘don't know’. It has no way of doing so, therefore it signals an exception.

Each exception is identified by exception type, an integer number associated with it.