TIP #436: IMPROVE TCLOO ISA INTROSPECTION 
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 Version:      $Revision: 1.3 $
 Author:       Donal Fellows <dkf_at_users.sf.net>
 State:        Final
 Type:         Project
 Tcl-Version:  8.6.5
 Vote:         Done
 Created:      Tuesday, 30 June 2015
 URL:          https://tip.tcl-lang.org436.html
 Post-History: 

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 ABSTRACT 
==========

 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. 

 RATIONALE 
===========

 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: 

     * *info object isa* /class object/ 

     * *info object isa metaclass* /object/ 

     * *info object isa mixin* /object class/ 

     * *info object isa object* /object/ 

     * *info object isa typeof* /object class/ 

 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*. 

 COPYRIGHT 
===========

 This document has been placed in the public domain. 

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