Xml in Scheme

Our example demonstrates how to work with XML in Scheme using the SXML library. Here’s the code and explanation:

(import (rnrs)
        (sxml simple)
        (sxml transform))

;; Define a structure representing a plant
(define-record-type plant
  (fields id name origin))

;; Convert a plant record to SXML
(define (plant->sxml p)
  `(plant (@ (id ,(number->string (plant-id p))))
          (name ,(plant-name p))
          ,@(map (lambda (o) `(origin ,o)) (plant-origin p))))

;; Convert SXML to a string representation of XML
(define (sxml->xml-string sxml)
  (call-with-string-output-port
   (lambda (out)
     (sxml-display-as-xml sxml out))))

;; Main function
(define (main)
  ;; Create a coffee plant
  (define coffee
    (make-plant 27 "Coffee" '("Ethiopia" "Brazil")))

  ;; Convert to SXML and then to XML string
  (define coffee-xml
    (sxml->xml-string (plant->sxml coffee)))

  ;; Print the XML
  (display coffee-xml)
  (newline)

  ;; Parse XML back into SXML
  (define parsed-sxml
    (call-with-input-string coffee-xml xml->sxml))

  ;; Extract information from parsed SXML
  (define parsed-id
    (car (cdadar parsed-sxml)))
  (define parsed-name
    (cadr (assq 'name (cdr parsed-sxml))))
  (define parsed-origin
    (map cadr (filter (lambda (x) (eq? (car x) 'origin))
                      (cdr parsed-sxml))))

  ;; Print parsed information
  (display (string-append "Parsed: id=" parsed-id
                          ", name=" parsed-name
                          ", origin=" (format "~a" parsed-origin)))
  (newline)

  ;; Create a nested structure
  (define tomato
    (make-plant 81 "Tomato" '("Mexico" "California")))

  (define nesting
    `(nesting
      (parent
       (child
        ,(plant->sxml coffee)
        ,(plant->sxml tomato)))))

  ;; Convert nested structure to XML and print
  (display (sxml->xml-string nesting))
  (newline))

(main)

This program demonstrates XML processing in Scheme using the SXML library. Here’s a breakdown of what it does:

We define a plant record type to represent our data structure.
The plant->sxml function converts a plant record to SXML (S-expression-based XML).
sxml->xml-string converts SXML to a string representation of XML.
In the main function:
- We create a coffee plant and convert it to XML.
- We then parse this XML back into SXML and extract the information.
- We create a nested structure with two plants and convert it to XML.
The program outputs the XML representations and the parsed information.

To run this program, you would need to have Scheme and the SXML library installed. Save the code to a file (e.g., xml_example.scm) and run it using your Scheme interpreter.

This example showcases how to create XML from Scheme data structures, how to parse XML into Scheme data structures, and how to work with nested XML structures. It’s a good starting point for more complex XML processing tasks in Scheme.

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