From 754bbf7a25a8dda49b5d08ef0d0443bbf5af0e36 Mon Sep 17 00:00:00 2001 From: Craig Jennings Date: Sun, 7 Apr 2024 13:41:34 -0500 Subject: new repository --- devdocs/elisp/box-diagrams.html | 54 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 54 insertions(+) create mode 100644 devdocs/elisp/box-diagrams.html (limited to 'devdocs/elisp/box-diagrams.html') diff --git a/devdocs/elisp/box-diagrams.html b/devdocs/elisp/box-diagrams.html new file mode 100644 index 00000000..f30af8eb --- /dev/null +++ b/devdocs/elisp/box-diagrams.html @@ -0,0 +1,54 @@ +

Drawing Lists as Box Diagrams

A list can be illustrated by a diagram in which the cons cells are shown as pairs of boxes, like dominoes. (The Lisp reader cannot read such an illustration; unlike the textual notation, which can be understood by both humans and computers, the box illustrations can be understood only by humans.) This picture represents the three-element list (rose violet buttercup): 

    --- ---      --- ---      --- ---
+   |   |   |--> |   |   |--> |   |   |--> nil
+    --- ---      --- ---      --- ---
+     |            |            |
+     |            |            |
+      --> rose     --> violet   --> buttercup
+
+

In this diagram, each box represents a slot that can hold or refer to any Lisp object. Each pair of boxes represents a cons cell. Each arrow represents a reference to a Lisp object, either an atom or another cons cell.

In this example, the first box, which holds the CAR of the first cons cell, refers to or holds rose (a symbol). The second box, holding the CDR of the first cons cell, refers to the next pair of boxes, the second cons cell. The CAR of the second cons cell is violet, and its CDR is the third cons cell. The CDR of the third (and last) cons cell is nil.

Here is another diagram of the same list, (rose violet +buttercup), sketched in a different manner: 

 ---------------       ----------------       -------------------
+| car   | cdr   |     | car    | cdr   |     | car       | cdr   |
+| rose  |   o-------->| violet |   o-------->| buttercup |  nil  |
+|       |       |     |        |       |     |           |       |
+ ---------------       ----------------       -------------------
+
+

A list with no elements in it is the empty list; it is identical to the symbol nil. In other words, nil is both a symbol and a list.

Here is the list (A ()), or equivalently (A nil), depicted with boxes and arrows: 

    --- ---      --- ---
+   |   |   |--> |   |   |--> nil
+    --- ---      --- ---
+     |            |
+     |            |
+      --> A        --> nil
+
+

Here is a more complex illustration, showing the three-element list, ((pine needles) oak maple), the first element of which is a two-element list: 

    --- ---      --- ---      --- ---
+   |   |   |--> |   |   |--> |   |   |--> nil
+    --- ---      --- ---      --- ---
+     |            |            |
+     |            |            |
+     |             --> oak      --> maple
+     |
+     |     --- ---      --- ---
+      --> |   |   |--> |   |   |--> nil
+           --- ---      --- ---
+            |            |
+            |            |
+             --> pine     --> needles
+
+

The same list represented in the second box notation looks like this: 

 --------------       --------------       --------------
+| car   | cdr  |     | car   | cdr  |     | car   | cdr  |
+|   o   |   o------->| oak   |   o------->| maple |  nil |
+|   |   |      |     |       |      |     |       |      |
+ -- | ---------       --------------       --------------
+    |
+    |
+    |        --------------       ----------------
+    |       | car   | cdr  |     | car     | cdr  |
+     ------>| pine  |   o------->| needles |  nil |
+            |       |      |     |         |      |
+             --------------       ----------------
+
+
+

+ Copyright © 1990-1996, 1998-2022 Free Software Foundation, Inc.
Licensed under the GNU GPL license.
+ https://www.gnu.org/software/emacs/manual/html_node/elisp/Box-Diagrams.html +

+
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