In Tcl, '''Everything is a Symbol''', and these symbols are represented by
strings, giving rise to the '''[Everything is a String]''' slogan.  The name of
each [command] is a symbol which resolves to the command itself, and the
arguments that are passed to a command are pure symbols, no strings attached:
The command can interpret them any way it likes.

Each command is an independent domain, and the symbols it initially receives
take on some meaning specific to that domain.  In other words, each command
speaks some [domain-specific language], and the commands in a script are only
connected by the protocol that each must return a single value.

Why is it not sufficient to say that ''everything is a string'', and leave it
at that?  Because the string itself is meaningless without some kind of
interpretation.  From the operational perspective of Tcl itself as it dances
between the commands, everything is just a string, and all it is doing is
[dodekalogue%|%interpolating] values and resolving commands, but the vast
majority of activity within a script takes place inside the commands, where the
strings take on additional meaning, and are treated in particular ways based on
their interpretation.  ''Everything is a string'' and ''everything is a
symbol'' go hand in hand, describing the overal operation of Tcl in a
[http://en.wikipedia.org/wiki/G%C3%B6del,_Escher,_Bach%|%golden braid] of
interleaved processing.

Tcl is the most pure language for
[http://en.wikipedia.org/wiki/Symbolic_computation%|%symbolic computation].
[Lisp] comes close, but its data objects are typed, making it one step less
pure than Tcl in this context.  One consequence of this pure symbolic approach
is that[http://en.wikipedia.org/wiki/Numerical_computation%|%numerical
computation] must be performed by a procedure dedicated to that domain rather
than by the language proper.  The most common tool for this is the venerable
`[expr]`, but it is also possible to pass symbols directly to individual
numerical operators, e.g. `[[+ 2 3]]`, composing more complex expressions
directly in Tcl via [dodekalogue%|%substitution]:

======
* [+ 2 3] [+ 3 5]
======

This is efficient because the numeric interpretation of each `+` command is
cached in the internal metadata for the substituted value, so that the outer
`*` command can re-use it.



** See Also **

   [Symbolic manipulation]:   an experiment by [Arjen Markus] on computing the derivative of a function


** Page Authors **

   [PYK]:   


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