A '''deep [list]''' is a [data format] for representing an
[tree%|%ordered tree]:  Each item in the list is either a simple value or a
sequence of values.



** See Also **

   [list] and [Depth of a List]:   [escargo] ruminates about differentiating between a single value and a sequence of values in a list.

   [ycl%|%ycl list deep], by [PYK]:   An implementation of a deep list.  Provides `index`, `insert`, `is struct`, `range`, and `set`.



** Description **

Built-in Tcl `[list]` routines provide no grammar to distinguish between a list
item that is a simple value and a list item that is itself a list.  The caller
of `[lindex]` or `[lset]` dictates the desired interpetation by passing index
arguments.  Sometimes, however, the caller would like to discover the structure
of the data rather than dictating it.

A self-describing data format provides information about the intepretation of
the represented data.  In a deep list, a simple value is encoded as a list
containing only one item.  All other values are considered to be sequences.
The following example is a list containing two items:

======
one
{
    {{two three}}
    {two three}
}
======

The first item is the simple value `one`.  The The second item is a sequence
where the first item is the simple value `two three` and whose second item is
itself a sequence containing the values `two` and `three`.


Looking again at that second item:

======
{{two three}}
{two three}
======

It is clear that although the value of the first item, `two three` could itself
be interpreted as a list, it is tagged as a single value, not a sequence.






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