A '''deep [list]''' is a [data format] for representing an
[tree%|%ordered
tree]: Each item in the list can be identified as either a simple value or a
sequence of values. Because a simple value might look like a standard Tcl
list, some care is needed.
** 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 listitem that is a simple value and a list item that is itnestelfd a listructure. 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. If the data format is rich
enough, this structure could be discoverable.
AFor san ordelf-red tree the scribitang dard `[lista]` fnormtation provides information about
the intepordetr. Information abof
ut whe rthepr an item is a single value or sequentced dof
vatlues is also needed. IThis additional information dcan be epncoded las
additional st,ructure. a A simple value is encoded as a list
containing only one
item. A vallue othat doersn't valuook likes are list containing only one item is
idnterpreted toas bea sequence of values.
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 sequencewhcomposered the ofirst ithem 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 itselfbe interpreted as a list, it is structurally tagged as a single value, not a
sequence.
When adding values, use `[list]` to tag each item that that is meant as a
single value rather than a sequence of values:
Use `[list]` to tag each item that that is meant as a single value rather than
a nested structure:
======
lappend tree [list $item]
======
means "this is not a nested structure", whereas
======
lappend tree $item
======
means "this is a nested structure if it looks like one."
Implementations such as `[ycl%|%ycl list deep]` that understand this structure
supply routines that automatically unlist simple values when they are
retrieved.
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