Sdraw: Cons-Cell Diagrams with Pict

Cons-cell diagrams (also called box-and-pointer diagrams) are often used to visually depict pairs and lists, typically in an educational context. For example,a cons-cell diagram for the list '(1 (2 3) 4) can be seen below:

Sdraw can be used to automatically create these diagrams using Pict. Inspiration for sdraw comes from the SDRAW program included with David S. Touretzky’s Common Lisp: A Gentle Introduction to Symbolic Computation. Like Touretzky’s program, circular lists and other graph-like structures are supported.

1 Usage

There is only one function provided by sdraw:

Pairs are drawn as a box, with arrows going down and right for the car and cdr respectively. If a pict-convertible? is given, it will be drawn directly. Otherwise, it will be converted to a pict? using typeset-code.

Various optional keyword arguments can be given to sdraw to customize the appearance:

• Limiting size: For large objects, you may wish to limit the amount of pairs drawn. Using max-depth and max-width, you can specify how far deep or out will be drawn before pairs are replaced by etc-pict.

  Example:

  > (sdraw (expand '(case a [(b c) #t] [else #f]))          #:max-width 3          #:max-depth 2)

• Null style: By default, null will be represented by an arrow to (). However, other styles are common practice. For example, in SICP, null is represented using a slash thru the containing pair.

  null-style may specify an alternative pict-convertible? to show. For example:

  Examples:

  > (sdraw '(nconc result) #:null-style (text "NIL"))

  > (sdraw '(()) #:null-style (typeset-code #'null))

  Alternatively, null-style may specify one of three in-cell styles:

  Examples:

  > (sdraw '(()) #:null-style '/)

  > (sdraw '(()) #:null-style 'x)

  > (sdraw '(()) #:null-style '|\|)

  Finally, use the null-thickness option to adjust the width of the lines used to draw in-cell styles (the option has no effect when null-style is pict-convertible?):

  Example:

  > (sdraw '(1 2) #:null-style '/ #:null-thickness 0.5)

• Spacing: Use the horizontal-spacing and vertical-spacing options.

  Example:

  > (sdraw '(a b) #:horizontal-spacing 100                 #:vertical-spacing 10)

  To add space around the text and any other drawn pictures, use the object-padding option:
  Examples:

  > (require pict/face)

  > (define happy (scale (face 'happy) 0.2))

  > (sdraw `(1 ,happy (,happy . ,happy) . 2) #:object-padding 15)

• Cell style: Use the cell-border-color, cell-inside-color, cell-inside-size, cell-inside-radius, cell-border-radius, and cell-border-width options.

  Example:

  > (sdraw '(a b) #:cell-inside-color "Green"                 #:cell-border-color "Purple"                 #:cell-inside-size 20                 #:cell-inside-radius 10                 #:cell-border-radius 10                 #:cell-border-width 3                 #:null-style '/)

• Arrow style: arrow-color, arrow-thickness, arrow-head-size, and arrow-point-size can be used to adjust the size of the arrows.

  Example:

  > (sdraw '(+ (* 2 3) 4) #:arrow-color "Purple"                         #:arrow-thickness 1                         #:arrow-head-size 5                         #:arrow-point-size 10)

• Reference labels style: Customization of labels for circular and graph-like structures can be done by passing a function which takes a string? and produces a pict-convertible? for reference-label. By default, a simple black label with white text is used:

  Examples:

  > (require racket)

  > (sdraw (with-input-from-string "#0=(#0# #1=(#0# . #1#) #1#)"                                  read))

  > (sdraw (with-input-from-string "#0=(#0# #1=(#0# . #1#) #1#)"                                  read)          #:reference-label text)