How to transform a Tree (see above) into an x expression
(see How to Build a Context-Free Grammar with Racket's unique Recursive Contract System)
This converts the beautiful tree notation possible in Racket into the ugly, verbose representation necessary to send hierarchical structures to other computational environments with no respect for beauty, symmetry nor notational elegance.
(require xml)
(define tree/c
(flat-rec-contract
tree
(or/c symbol?
(listof symbol?)
(cons/c symbol? (listof tree)))))
(define atom?
(lambda (x)
(and (not (pair? x)) (not (null? x)))))
(define/contract (tree->xexpr tree)
(tree/c . -> . xexpr/c)
(letrec ((parent-proc (λ (subtree)
(cond
[(atom? (car subtree)) `(div ((id ,(symbol->string (car subtree)))) ,@(child-proc (cdr subtree)))])))
(child-proc (λ (subtree)
(cond
[(null? subtree) '("")]
[(symbol? (car subtree)) (cons `(div ((id ,(symbol->string (car subtree)))) "") (child-proc (cdr subtree)))]
[(cons? (car subtree)) (cons (parent-proc (car subtree)) (child-proc (cdr subtree)))]))))
(parent-proc tree)))
#|
USAGE
(display (xexpr->string
(tree->xexpr '(S1 S2 (S3 (S3-1 S3-1-1 S3-1-2) S3-2) S4))))
RETURNS
<div id="S1">
<div id="S2"></div>
<div id="S3">
<div id="S3-1">
<div id="S3-1-1"></div>
<div id="S3-1-2"></div>
</div>
<div id="S3-2"> </div>
</div>
<div id="S4"></div>
</div>
|#
