🚀 go-pugleaf

Tim Walker (tsw@netcom.com) wrote:
: 
:  Does anyone know if Occam3 will be recursive? This is a big drawback
: in Occam2 when trying to implement things like parallel searchs.
: 
:  -- Tim Walker  (tsw@netcom.com)

The short answer is `no'.

There is, however, a way to get fixed depth recursion.  Suppose the
definition you want to achieve is:

  PROC foo (formals)
    body  --  contains call to foo
  :

Then you can achieve recursion to depth n by repeating the code thus:

  PROC foo (formals)
    STOP
  :
  PROC foo (formals)
    body  --  contains call to foo
  :
  ...  repeat n-2 times
  PROC foo (formals)
    body  --  contains call to foo
  :

This might be regarded as a dirty hack.

More seriously, it does illustrate an important property of occam
processes, namely that their workspace requirements can be determined
statically.  If you can't do this, then the implementation of parallel
processes becomes much more difficult.  In current implementations of
occam, it is possible to allocate a fixed amount of workspace for each
parallel process.  If you allow recursive processes you cannot
determine statically how much workspace each process will need.  This
means that you have to know what to do with stack overflow when the
obvious place to extend it has been allocated to something else
already.  If you think you have a solution to this, and it does not
have an adverse affect on the context switch time, then I will be
interested to hear it.  In general, it is more important for an
embedded application to be sure of enough space in which to run.  After
all, you don't want to run out of memory just as you reach a critical
section of control code.

Geoff Barrett
INMOS Limited