A ''pure list'' in Tcl is a value that has a list internal representation but no string representation.

Pure lists are significant primarily because the evaluator and bytecode compiler treat them specially.  If confronted with a pure list, these components can recognize immediately that

   * the object in question is a single Tcl command; the words making up the command are precisely the elements of the list.
   * the object requires no substitution.

They can therefore avoid using a parser on the object.  They simply look up the command (element 0 of the list), and construct a parameter vector consisting of the list elements.

Pure lists are preferred for callbacks such as traces, [[eval]] and [[uplevel]], [[after]] calls, Tk commands and bindings, and similar applications.

The [[list]] command always returns a pure list.

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''Is it essential that there be no string rep - isn't a list rep sufficient for this either way? -jcw''

Yes, it is.  Consider these two examples:

 list a set a 1 set b 2

 list a
 set a 1
 set b 2

Yes, I believe so. I cannot recall the exact arguments which were made by Donal (K. Fellows), but there where arguments against using lists having both representations. -- [AK]