[kepler-dev] Bug 2240: add support for null values to data

[Christopher Brooks]

Ok, I hacked up the following:

* Token.java now has nil() and isNil() methods
  I went with nil because null is a Java keyword.

  The term "missing" is rather appealing since the Token.toString()
  method usually prints out "present".  So, it could be modified to
  print out "missing".  However, I feel that someone is likely to have
  a parameter named "missing" somewhere.  nil seems safer.

  This is not cast in stone, comments are welcome.

  These methods have tests. 
  
* data/expr/Constants.java now defines a "nil" constant which
  is a Token that has the nil() method:
        ptolemy.data.Token nil = new ptolemy.data.Token();
        nil.nil();
        _table.put("nil", nil);
  Thus, one can now create expressions that have nil in them

  The "nil" constant has a test in data/expr/test/PtParser.tcl

* actor/lib/RemoveNilTokens.java:
  A new actor that reads its input and discards any nil tokens
  in the fire() method.  It might be better to do this in prefire()
  This actor is available in the "More Libraries" -> "Esoteric" section.

  Note that this actor should not be used in SDF.
  
  No tests yet.

* domains/pn/demo/RemoveNilTokens/RemoveNilTokens.xml
  A model that uses RemoveNilTokens.
  I think I'm terminating the PN process poorly.  I get 7 outputs
  instead of 5.  I could use some help here.
  Also, I have to explicitly set the type of output of the RemoveNilTokens
  actor.


The model looks like:

          Bool Switch
Ramp----> | 
          |----------------> RemoveNilTokens ---------> Display
Const     |                                      |
that ---> |                                       --> "Code that stops PN"
produces  _
nil       ^
          |
          |
Bernoulli -


So, now we have a straw man in PN to try out.  

Questions:
1) Would something like this meet the needs of the Kepler group?
2) Should RemoveNilTokens do something in prefire()?
   I'm not sure if I can get access to the Token and call Token.isNil()
   in prefire().
3) Is "nil" an ok name?
4) How do I get PN to terminate properly after I see 5 non-nil tokens?

_Christopher

Edward wrote:
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    At 08:16 AM 12/13/2005 -0800, Shawn Bowers wrote:
    >One option for SDF would be to literally "propagate" the missing-valued
    >tokens, instead of "dropping" them (as in PN).   Then, it seems the
    >consumption/production rates at least wouldn't be violated.  Doing this
    >correctly might be a bit tricky, but could probably perform this based on
    >the token production/consumption rates --
    
    In SDF, before any actor is fired, its prefire() method is called.
    For all our actors that require input data, if prefire() will
    return false if there is no input token, and the actor will not
    be fired. Consequently, its outputs will also have no token...
    
    So SDF already does this propagation...
    
    Edward
    
    
    
    ------------
    Edward A. Lee
    Professor, Chair of the EE Division, Associate Chair of EECS
    231 Cory Hall, UC Berkeley, Berkeley, CA 94720
    phone: 510-642-0253 or 510-642-0455, fax: 510-642-2845
    eal at eecs.Berkeley.EDU, http://ptolemy.eecs.berkeley.edu/~eal  
    
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