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Category:Tcl: Difference between revisions
Added more detailed description of Tcl's value model and fundamental types.
(Tcl is a dynamic programming language) |
(Added more detailed description of Tcl's value model and fundamental types.) |
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Its creator, John Ousterhout, wrote about it:
:''“I got the idea for Tcl while on sabbatical leave at DEC's Western Research Laboratory in the fall of 1987. I started actually implementing it when I got back to Berkeley in the spring of 1988; by summer of that year it was in use in some internal applications of ours, but there was no Tk. The first external releases of Tcl were in 1989, I believe. I started implementing Tk in 1989, and the first release of Tk was in 1991.”''
==Language Value Model==
Tcl's value model operates on two levels.
* Classically, it is defined purely on unmodifiable strings over a language of unencoded UNICODE characters.
* Practically, values are polymorphic and hold a cache of the last type-interpretation that they were used with, together with an optional UTF-8 string representation. They are reference counted and are not modifiable (unless the code in question holds the only reference, which is a significant efficiency gain; if the value is shared, it is shallow-copied upon modification). Although only reference counted, they are effectively garbage-collected since circular data structures cannot be constructed (performing such construction requires holding two references to the same object, which forces a copy to be taken and breaks the reference loop). The net effect of this is just like the UNICODE string classical model, except much faster.
The language supports the following basic types, together with many defined by extension packages:
* Unicode strings
* Binary strings
* Integers of unbounded width
* Double-precision IEEE floats
* Booleans
* Lists of values
* Dictionaries mapping values to values
* Assorted "cache" types used to boost performance:
** Command handles (several types)
** Variable handles (several types)
** Compiled regular expressions
** Compiled scripts (several types)
** etc.
Note that all variables can hold values of ''any'' type; the language does not impose type constraints on variables at all.
==External Links==
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