[Smartfrog-developer] NULL data

[Goldsack, P. <Pat...@hp...>]

There has been a request to provide a NULL data value in the smartfrog
language. It is impossible to use java "null" as this cannot be manipulated
by the underlying classes, such as hashtable.

However, on the surface this is easy: define a new class (say SFNull) and
use the keyword NULL such that any attribute definition
    x NULL;
gives the attribute x the value which is an instance of SFNull. (NULL can
also used to indicate no extension, so we might decide to use a different
keyword, or simply use the syntactic form of comment (1) below.)

There are a number of possible refinements...

1) should the definition
    aName;
change its semantics to be the equivalent to NULL? Currently this is
equiavalent to 
    aName "aName";
However, I know of no uses of this used to define the string. Normally it is
used to indicate either a null value or more often simply attribute
existance as a boolean, so we should probably change this.

2) what should be the result of an sfResolve? There are three choices:
    - return the instance of SFNull;
    - throw the exception "SmartFrogNullResolutionException", a subclass of
"SmartFrogResolutionException", though in this case the difference in result
when testing for attribute existance is minimal and awkward to program to;
    - trap the return of SFNull, and return the java "null" instead

Any comments as to the desired semantics would be appreciated.

Patrick