#|
   The syntax "local" takes a parenthesized sequence of definitions
    (define statements), followed by 1 or more expressions.

   When executed it:
     makes the definitions
     executes the expressions
       the definitions are "in scope": can be referenced in the expressions
     returns the value of the last expression
|#
[require scheme/local]

#| Create and name a very simple counting "object" (as in "object-orientation").
     Represented as a no-argument function.
     State: number of times called (including current call) 1, 2, 3, ....
     Behaviour: when called, return the number of times called.
|#
[define counter
    [local [[define count 0]]
        [lambda []
            [set! count [+ count 1]]
            count]]]

; A true-type font for use by build-type below.
[define font-path "/usr/share/fonts/truetype/msttcorefonts/Arial.ttf"]

[clear]
[opacity 0.4]

#| Pre-order traversal of a tree: self before child (sub) trees.
   Post-order traversal of a tree: child trees before self.
     We tried this in make-tree by moving the build-cube. |#

#| [make-tree levels] shows a binary tree of cubes with levels number of levels.
   Shows pre-order traversal as numbers in the cubes.
   Precondition: levels an integer >= 0. |#
[define make-tree
    [lambda [levels]
        
        [when [> levels 0]

            [with-state [scale 0.5]
                [colour [vector 1 0 0]]
                [translate [vector -0.5 -1 0]]
                [build-type font-path [number->string [counter]]]]
            [build-cube]

            [with-state
                [translate [vector -2 -2 0]]
                [scale 0.5]
                [make-tree [- levels 1]]]

            [with-state
                [translate [vector 2 -2 0]]
                [scale 0.5]
                [make-tree [- levels 1]]]]]]

[make-tree 3]