This page presents some thoughts on a feature which is not currently implemented in Tcl -JCW
The unknown command is a powerful mechanism in Tcl to take over when an undefined command is called. It can do shell-like execs in an interactive Tcl environment, and lazy package loading through auto_index, to name a few useful applications.
The question is: could as similar approach be used for variables and arrays?
Here's an example of what I have in mind: suppose you want to implement something which looks, walks, and quacks like an array, but isn't. One such use would be for arrays which are persistent, and not (yet) fully loaded in memory. Another would be to make remote data appear to be local.
Traces go a long way, as the "global shared arrays" of Tequila illustrate, but it only works with complete copies in memory for now.
The "trace add variable <blah> array <cmd>" which is available in recent Tcl versions seems to be yet another step in this direction. But from a brief test, I can't seem to get at what array request was being made, let alone taking over its function.
A good example is "array names foo bar*". It would be nice if it were possible to catch that call for array foo, and return a list which matches the intended result, without actually having to set up the array with all the necessary name-value pairs.
What about extending the current trace syntax as follows. The code:
trace add variable foo array trigger set x [array names foo bar*]
leads to a call to trigger of the form:
trigger foo {names bar*} arrayAnd the return of this call is then used to set the value of "x".
Or maybe some variation - if the above breaks existing code.
Is this worth discussing further? Are there other ways to virtualize?