A list exit  facility will be introduced with version  1.8a to answer the
concerns expressed about  the removal of the .DO  mailform command. While
we expect  this facility to be  extended to all list  management commands
with version 1.8b, only the commands  implemented in PASCAL will make use
of the exit facility in version  1.8a, as appropriate. For now this means
only the  ADD and SUBSCRIBE commands,  although the SET command  might be
included before  final release.  It is unlikely  that SIGNOFF  and DELETE
will  be  included,  because  internals  for  the  X-REQ  synchronization
mechanism are presently  available only in REXX. In version  1.8b we plan
to have  all the list  management commands  in PASCAL, with  the possible
exception of EXPLODE.
 
An exit  is enabled by  adding "Exit= filename"  to the list  header. For
security  reasons, all  exits  must  be explicitly  declared  in the  new
LIST_EXITS configuration variable. This prevents list owners from causing
the  invocation of  arbitrary executable  files  through the  use of  the
"Exit=" keyword. However,  once a particular program  has been identified
as a list exit, any list may refer  to it. If you write exits which could
compromise confidential data if executed on  behalf of the wrong list, it
is  your responsibility  to test  the  'listname' parameter  in the  exit
program and  take appropriate action. Note  that the exit name  itself is
available to  anyone who can  review the list;  make sure to  program the
exit to test the list name, and not to rely on the confidentiality of the
exit name.
 
List exits are regular REXX programs  under VM, which must be prepared to
handle  calls from  an arbitrary  number  of exit  points. The  parameter
string always include the exit point  name, followed by a blank, the name
of the list, another blank, and parameters that depend on the exit point.
In other words:
 
                       Parse arg ep listname more
 
The exit returns a numeric return code,  which may or may not be checked,
depending on the exit  point. The value 0 always has  the meaning "do the
same thing  as if  there had  not been  any exit  for this  list". Always
return the value 0  for exit point names your program  does not handle or
recognize. At the moment, the following exit points are available:
 
SUB_FILTER, one  parameter: originator's e-mail address.  This exit point
gives you a chance to reject a subscription by returning a nonzero value.
This adds to rather than replaces the "Filter=" keyword.
 
SUB_FAIL, one parameter: originator's e-mail  address. This exit point is
called whenever a  subscription is rejected (in particular,  it is called
if you return  a nonzero value from SUB_FILTER). You  may send additional
messages to  the user,  log the  event, and so  on. LISTSERV  ignores the
return value.
 
SUB_REQ, one  parameter: originator's e-mail address.  The user's request
for subscription  is being  forwarded to  the list  owners. You  may send
additional  messages to  the user,  log the  event, and  so on.  LISTSERV
ignores the return value. To implement custom subscription/rejection, use
the SUB_FILTER exit with "Subscription= Open". When SUB_REQ is called, it
is too late to let the user through.
 
SUB_NEW,  one parameter:  originator's  e-mail address.  This exit  point
notifies you that the user has  been successfully subscribed to the list.
You can  send additional  messages, log  the event,  and so  on. LISTSERV
ignores the return value.
 
ADD_NEW, one parameter: subscriber's e-mail address. Same as SUB_NEW, but
for the ADD command.
 
The  return values  from xxx_NEW  and SUB_REQ  may be  checked in  future
versions to override the delivery of  standard mail templates, so be sure
to return zero for now.
 
IMPORTANT: List exits  may not make any change to  the list file, because
the command processor may have changes of its own to make to the file, or
may have kept the file open for further processing. If you really have to
change the file, use CP MSG *  CMS EXEC to call yourself after processing
completes.
 
The new template processor can be called from list exits. The syntax is:
 
        Call LSVTMAIL recipients,template_name,listname,keywords
 
keywords = 'KWD1 value of first keyword'||'15'x'KWD2 second keyword'||...
 
Note the change in calling sequence,  even when no keyword is defined. Do
not use LSVIMAIL as  it will be removed in version  1.8b. Version 1.8a no
longer  uses LSVIMAIL,  but it  will be  kept around  for one  version to
facilitate the migration to new versions of optional packages such as the
line monitor, which call LSVIMAIL.
 
  Eric