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          The InterPal Programming Language
          =================================

        In this post we present a new language for concurrent 
        programming based in objects. The language is an extension
        of Intercal.

1. Introduction.

InterPal is a object-disoriented version of intercal. It is designed for
bringing Intercal-like programming to the realm of MIMD machines. Specially,
there will a version for the CM-5 real soon now.

InterPal add few news commands to Intercal. The goal of this commands is
to control concurrence and message passing. 

2. Objects

Each InterPal object is described as a InterPal program. They are called
netters. Netters can be in one of 4 states: active, waiting for mail, 
sleeping and dead (or ghost). Active netters are running and can be doing calculations,
i/o or sending messages. A message is called a mail. The mechanism for
sending mails is described later in this post.

A netter that is waiting for mail check its mailbox every tick. A tick
is a unit of time designed by the user.

A netter that is sleeping waits for being awaked. There are two methods
of making a sleeping object active: the system have a alarm clock that
"sound" at a requested time and another netter can wake them.

A dead netter just hangs around in the system. If active netter find
a dead netter in the same processor, the active netter is scared and
faints, becoming a slepping process. If an active netter finds more
than 10 dead netters in the same processor, it is scared to death and
becomes a dead netter. If more than 100 dead netters are in the same 
processor this processor is denominated a cemetery. Active netters
are not allowed to enter in a cemetery unless their religious bit is set.

There is a special object called GOD. The GOD process goes around the
processors (at random) and turn dead netters in active netters (in batches
of at least 100 and at most 4623).

Each netter is identified by an unique number.

3. Interfaces

There are no explicity interface in InterPal objects. When a netter
receives a mail, this mail has an message number.This is the number
of the line the netter should do a GOTO before start executing.


4.The new commands:

a)SENDMAIL(object,messagenumber,parameter).

   Send a mail to netter object to GOTO line number message number
  and one parameter.

b)WAIT MAIL

   Waits for mail with the smallest possible tick in the system

c)WAIT MAIL tick

  Waits for mail is a specified tick

d) SLEEP

  Sleeps

e)SLEEP AND WAKE UP AT X

  Sleep and set up alarm clock for X ticks in the future

f) WAKE UP X

  Wake up netter number X

g) SUICIDE

  Becomes a ghost process

h) KILL n

  Kill process n, that becomes a ghost.

i) BECOME RELIGIOUS

  Processes that are religious have special characteristics:

    1)They don't SUICIDE. They just don't care about this command.
    2)If killed, they don't become a ghost, but disappear from the system
    3)If they find the special process GOD, they become saints. Saints
      are allowed to use the private instruction REINCARNATE.

j) REINCARNATE X

  Turn a dead process X in an active process X.

k) NAME Y

   Gives the name of a process

l)READ MAIL Y

   Read the parameter of the last received mail and put in variable Y

5. The mail distribution process.

  When a netter X send a mail, this mail goes to the post-office. Every
processor has two special processes: a postoffice and a postman. 
The sequence of message delivering is the following.

  a) The postoffice receives mail and put in a mail-bag.
  b) at every tick, it ramdomly select P mails from the mail-bags and
     distribute to the postman.
  c) The postman read the object name and find wich netters belong
     to this processor. Then, the postman distribute the mail to
     the local processor netters. The remaining mails are sent
     back to the post office.

   d) The postoffice exchange its mail-bag to a ramdonly selected post
      office in the system.

6. Example

  Here is a  valid Interpal Program that prints the second
  number read if the first number read is 243:

in netter 1

   NAME 1
2  PLEASE DO  WRITE IN .2
   DO WRITE IN .3
3  GIVE UP
   COME FROM 2 PLEASE SEND MAIL (.2,10,.3)
   SUICIDE  

in netter 243

   NAME 243
   WAIT MAIL
   GIVEUP
10 READ MAIL .4
   PLEASE ABSTAIN FROM SUICIDE
   DO READ OUT  .4     
20 REINSTATE SUICIDE
   SLEEP

   

  
    
  

7. Conclusion

  The language is powerful enough to allow concurrent programming and
  gives enough rope to the user.

  The mail system is designed to simulate real-life mailing and appears
  to be very good to support simulations of enterprises. Its non-determinism
  guarantees the possibilities of various different sequences of
  actions.

  The states of the objects open a totally new perspective of research in
  the areas of deadlock detection, programmers mental sanity and religious
  issues.

                Cheers,

                               Jerry 


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