"Data is not abstracted from the language". This phrase from [About Tcl and popularity] has been echoing on my mind. I sort of understand what it means. But only so much. I can't say I understand it because I still can't see the possibilities it creates. More explanation, examples and lucubrations welcomed.
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[RS]: The language manifests itself in scripts, which are [string]s. 

It is brought to life in commands, which are [list]s (which are a view on strings). 

In data, [everything is a string], and many strings can be [eval]ed if they're in the (current) language. Every proc can be reconstructed to be a string with [info args], [info default], [info body]. In short: Code is data. Data is code. Everything is possible :)

[LES]: I understand that. But that still is as far as I could go before I asked my question. I still don't see what ocean of possibilities that wondrous trait could open to me. A little example, perhaps? Anyone?
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[Category Concept]  |  [Category Example]  |  

[Arts and crafts of Tcl-Tk programming] - is that a category? [RS]: No - it was the earliest cross-link, categories came later. I still often use when I don't want to think about categories :)