[seek-dev] Re: extending the type system

[Bertram Ludaescher]

Steve, Chad:

Here is a related question (I had discussed this earlier today with
Shawn) on "how to do things properly" in PTII:

The type extension mentioned by Chad, let's call it "ndouble" for
"nullable double" can (and probably should) be modeled as a
*supertype* to "double" since its instances are precisely "double U
null" (all double numbers plus null). The problem with this, however,
is that any code that works for double (say an actor computing the
running average of its stream of double values) now won't work for
"ndouble" since the latter is a supertype of the former. 
If it were the other way round, i.e., ndouble being a subtype of
double, we could assume that all code still works for the subtype. Of
course this doesn't work as-is, since the double actors don't really
know what to do with a null value.

Bottom line (not surprising): for existing actors to work on a new
type such as ndouble, something needs to be changed to take into
account the new behavior (for nulls in this case). The question is how 
to best engineer this in general, and in PTII in particular. 

Any suggestions most welcome.

It also seems that the following article by Castagna is relevant to
this discussion:
	Covariance And Contravariance:conflict Without A Cause (1994)  
	Giuseppe Castagna
	http://citeseer.nj.nec.com/24009.html

Bertram


>>>>> "SAN" == Stephen Andrew Neuendorffer <neuendor at eecs.berkeley.edu> writes:
SAN> 
SAN> Modifying the type hierarchy in this way is difficult: We haven't yet 
SAN> implemented a good way to make it extensible in this way.
SAN> Part of the problem is that the type lattice is somewhat hardcoded into 
SAN> operations on the token classes.  Modifying the type hierarchy in 
SAN> significant ways is not currently possible without modifying the existing 
SAN> token classes at the same time.  I have some ideas on improving the 
SAN> situation, but I haven't had time to implement them yet.
SAN> 
SAN> The one way that the type hierarchy *can* be simply extended is that it 
SAN> allows the addition of new data types that are incomparable with
SAN> the existing types (other than UNKNOWN and GENERAL, of course).  If you 
SAN> define a new Token class, and a corresponding type class, then this type 
SAN> will be correctly type checked.
SAN> 
SAN> There are other ways to get the behavior you desire..  One is through an 
SAN> (unimplemented) structured type called "Union" which operates similar 
SAN> Unions in C...  This should be relatively easy to implement, we just 
SAN> haven't gotten around to it.  Your type is then Union(double, event) or 
SAN> something similar.
SAN> Another way is to use side information (a boolean, for instance) to 
SAN> determin whether or not the "double" is valid.
SAN> Another way is to use a less constrained Model of Computation (like SR or 
SAN> DE) which has a built-in notion of absent.
SAN> Any one of these would probably be significantly simpler than modifying the 
SAN> existing double token....
SAN> 
SAN> Steve
SAN> 
SAN> At 11:08 AM 10/13/2003 -0700, Chad Berkley wrote:
>> Hello,
>> 
>> We have a question about the extensibility of the Ptolemy II type system. 
>> In our project, we need to extend the type system by adding new
>> data types and modifying the type hierarchy.  As an example, we need to
>> incorporate a new double type (as a superclass of double) that accepts
>> null, or missing, values.
>> 
>> We would like to add new data types without altering the Ptolemy codebase. 
>> It seems the way to do this is by overriding the TypeLattice class (to add 
>> new edges for our new extended types).  We had the impression (e.g., as 
>> discussed in Chapter 12 of the manual) that this extensibility was 
>> possible.  However, it appears that the TypeLattice is hard coded, e.g., 
>> it is not a parameter to the BaseType class, and instead, a default 
>> instance of TypeLattice is hard coded into BaseType.
>> 
>> So, the questions are:  Are our ideas using the Ptolemy type system code
>> in the way in which it was intended?  Are there other ways to extend types 
>> without altering the Ptolemy code?  Do you have any suggestions as to our 
>> approach?
>> 
>> thanks,
>> chad
>> 
>> 
>> --
>> -----------------------
>> Chad Berkley
>> National Center for
>> Ecological Analysis
>> and Synthesis (NCEAS)
>> berkley at nceas.ucsb.edu
>> -----------------------
>> 
>> 
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SAN> 
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