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5 Transformation (SXSLT)

(pre-post-order tree bindings)  sxml?
  tree : sxml?
  bindings : (listof binding)
Pre-Post-order traversal of a tree and creation of a new tree.

  binding = (trigger-symbol *preorder* . _handler)
  | (trigger-symbol *macro* . _handler)
  | (trigger-symbol _new-bindings . _handler)
  | (trigger-symbol . _handler)
  trigger-symbol = XMLname
  | *text*
  | *default*
  handler = procedure: (symbol? sxml? ... -> sxml?)

The pre-post-order function visits the nodes and nodelists pre-post-order (depth-first). For each node of the form (name node ...) it looks up an association with the given name among its bindings. If it fails, pre-post-order tries to locate a default binding. It’s an error if the latter attempt fails as well. Having found a binding, the pre-post-order function first checks to see if the binding is of the form

(trigger-symbol *preorder* . handler)

If it is, the handler is applied to the current node. Otherwise, the pre-post-order function first calls itself recursively for each child of the current node, with new-bindings prepended to the bindings in effect. The result of these calls is passed to the handler (along with the head of the current node). To be more precise, the handler is applied to the head of the current node and its processed children. The result of the handler, which should also be a tree, replaces the current node. If the current node is a text string or other atom, a special binding with a symbol *text* is looked up.

A binding can also be of a form

(trigger-symbol *macro* . handler)

This is equivalent to *preorder* described above. However, the result is re-processed again, with the current stylesheet.

A tiny example:

> (define sample-doc
      (html (title "the title")
            (body (p "paragraph 1")
                  (p "paragraph 2")))))
> (define italicizer
    `((p . ,(lambda (tag . content)
              (cons tag (cons "PARAGRAPH BEGINS: " content))))
      (*text* . ,(lambda (tag content)
                  `(i ,content)))
      (*default* . ,(lambda args args))))
> (pre-post-order sample-doc italicizer)



   (title (i "the title"))


    (p "PARAGRAPH BEGINS: " (i "paragraph 1"))

    (p "PARAGRAPH BEGINS: " (i "paragraph 2")))))