On Nov 23, 2022, at 12:23 PM, Tobias Pape <Das.Linux@gmx.de> wrote:

Yet, nil is only seldom a good domain object.

Precisely. Being disjoint from any domain it is the ideal “I am not a domain object” marker. So when one wants a variable to range over a domain and the singleton “not a member of the domain” nil is a great choice. And that’s exactly how I use it below.

There is another excellent marker of a non-domain object, and that is a newly instantiated object. That object is known to not be any other object, since objects are unique. So if code is searching for something (eg applying a block to every literal in the system), having the newly instantiated object that implements the search use itself as the “I’m not in the domain of all pre-existing objects” is a sensible choice. This is the pattern InstructionStream uses when scanning for selectors.

-t

On 23. Nov 2022, at 19:34, tim Rowledge <tim@rowledge.org> wrote:

I won't quote it all again but what Eliot wrote is important. There are good solid reasons why Smalltalk has a rigorously defined UndefinedObject. We demand rigorously defined areas of doubt and uncertainty!

tim