TIP 436: Improve TclOO isa Introspection

State:		Final
Type:		Project
Tcl-Version:	8.6.5
Vote:		Done
Author:		Donal Fellows <[email protected]>
Created:	30-Jun-2015
Implementation-URL: https://core.tcl-lang.org/tcloo/info/5fa1374aa026d4c7


The various info object isa introspectors should not produce errors when given a non-object; the set membership tests should simply return boolean false in those cases.


The info object isa command is intended to be used to allow asking whether some object is a member of a general set of entities; for example, info object isa object allows querying whether you actually have a handle to an object at all. However, the other membership sets all throw an error when given a non-object. This complicates the use of the API when all that is really needed is to return a false value in those cases.

Motivating example (with thanks to Will Duquette): is the proc a class? No. It's not even an object, so it clearly cannot be a class and so the following command should produce false (or 0) and not an error:

 info object isa class proc

Proposed Change

Where one of the info object isa introspectors:

Would produce an error due to either the object or (where appropriate) the class object not passing some critical precondition to the test, the result of the command will be 0 (i.e., boolean false). Errors will be still generated when the wrong number of arguments are supplied.

Note that this rule is already followed by info object isa object.


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