Purpose: describe what Tcl considers a list element - i.e. the basic
component of a list.
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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.

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.

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
 2
but this does:
 set l [split "a { b c }"]
 llength $l
 5

The 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.

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See also [list], [lappend], [lindex], [linsert], [llength], [lrange], [lreplace], [lsearch], [lsort], [concat], [split], [foreach], [array] .

[caspian] recommends that you also see: [join] and [lset]


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[Tcl syntax help]

[Category Glossary]