[Tcl-bugs] [ tcl-Feature Requests-784359 ] Tcl'2003: Unknown Array Element Callback

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Feature Requests item #784359, was opened at 2003-08-06 15:32
Message generated for change (Comment added) made by kennykb
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Category: 07. Variables
Group: None
>Status: Closed
>Resolution: Rejected
Priority: 5
Submitted By: Donal K. Fellows (dkf)
Assigned to: Kevin B KENNY (kennykb)
Summary: Tcl'2003: Unknown Array Element Callback

Initial Comment:
Yet another idea that cropped up at Tcl'2003:

It should be possible to specify a callback for an
array that is called whenever an attempt to access
(read/write) a non-existant element is made. This would
enable all sorts of cool things, according to KBK.

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>Comment By: Kevin B KENNY (kennykb)
Date: 2004-10-30 14:49

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Was I sober at the time ?

I certainly don't recall why I suggested such a thing, and
Miguel's read-trace idea appears workable. 

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Comment By: Donal K. Fellows (dkf)
Date: 2004-03-29 04:02

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Handing this off to kbk; it was his idea originally

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Comment By: miguel sofer (msofer)
Date: 2004-03-26 11:53

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dkf: comments?

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Comment By: miguel sofer (msofer)
Date: 2003-08-06 15:51

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I guess I didn't understand: in what respect is the
trace-based solution short of what's requested? Oris it just
a matter of performance?

% proc readTrace {name1 name2 op} {
        upvar 1 $name1 ary
        if {![info exists ary($name2)]} {
            puts "Non-existent element: set to \"z\""
            set ary($name2) z
        }
    }
% array set a {1 a 2 b}
% trace add variable a read readTrace
% set a(1)
a
% set a(2)
b
% set a(3)
Non-existent element: set to "z"
z



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