Purpose: describe what Tcl considers a list element - i.e., the basic component of a list.
Given that Tcl considers everything is a string, then a list element is, at its core, just a string. This string can either be 0 length, 1 or more characters, and can be formatted in such a way that the element is also another list.
However, just because every element is a string does NOT mean that every string is a list element.
I beg to differ with that assertion. Every string can be a list element; I've never seen anything to contradict that. The examples below show several incorrect ways to treat a string as a list element, but when done properly (as in the final example using split) it proves that anything -- including a single left or right curly brace -- can indeed be a list element. There is no string that cannot be stored as an element in a list.
Feel free to differ - that's fine. However, I assert that if I read a string from a file, or even get it from a user, but have done nothing to make it list safe, that string will cause problems when used in any command expecting a list as its argument. That is what I mean by saying that not every string is a list element. One cannot take every arbitrary string and use it as if it were a list.
but you're mixing terminology. You claim to be talking about list elements, yet complain a string can't be used as a list. Which is it -- are you talking about lists or list elements? I still see zero evidence that some strings cannot be list _elements_.
For example:
$ tclsh
% set a "This is a test { of a string"
This is a test { of a string
% llength $a
unmatched open brace in listI would say that variable $a contains a string that is not yet a list element - not until something happens to it to make it so.
No, $a contains a string that is not yet a _list_. There's a difference between a list element and a list. list elements are members of a list, and any string may be a member of a list. It is true that not every string can be a full-fledged list, but the stated purpose of this page is to discuss list elements, not lists.
In Tcl, one programatically ensures a string is a list element most safely by usign either the list or the split command to listify the string.
You're getting your terminology mixed up. To make a string into a list element one needs do nothing more than to insert it into a list. You say one must use list or split to listify the string, but that's not the same as making a string into a list element; that's converting a string (split) or list of strings (list) into a list.
Feel free to fix the terminology so that things are stated in a manner you feel is correct - and I will do the same.
But, for example, let us say you want the 5-character literal string
a { b c }turned into a 5 element list.
Simply saying
set l [list "a { b c }" ]does not give you a 5 element list
llength $l 1
nor does calling list with an unquoted argument
set l [list a { b c } ]
llength $l
2but this does:
set l [split "a { b c }"]
llength $l
5The problem in the first list case is that Tcl treats the " character as introducing the beginning and ending of a single element, and in the second list case, list sees the { and } characters and treats its arguments as if they had already been processed by Tcl into elements.
Internally, tcl structures each list element as if it were surrounded by { and }. Split, however, is designed to take as its first argument a string that needs to be quoted properly, so it is able to create what is needed.
See also list, lappend, lindex, linsert, llength, lrange, lreplace, lsearch, lsort, concat, split, foreach, array .
caspian recommends that you also see: join and lset