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Srikumar K. S edited this page May 16, 2016 · 1 revision

Introduction

These days, a lot of data exchanged between computer programs gets done using an XML structure. Be it SOAP, XML/RPC, Jabber, whatever, there is no escaping having to deal with simple XML data structures, even though they can all be mapped perfectly well to equivalent s-expressions.

Enter read-xml - a function that does exactly that - read a single XML node from any port and presents it as an equivalent s-expression.

An example

The xml expression below gets transformed to an s-expression as shown -

<article id='Hudak92'>
    <author> P. R. Hudak </author>
    <author> J. H. Fasel </author>
    <title> A Gentle Introduction To Haskell </title>
    <journalref name="ACM SIGPLAN Notices" volume="27" number="5" pages="1--53"/>
    <date year="1992"/>
    <keyword>functional</keyword>
    <keyword>tutorial</keyword>    
</article>

becomes

(article ((id . "Hudak92"))
    (author () "P. R. Hudak")
    (author () "J. H. Fasel")
    (title () "A Gentle Introduction To Haskell")
    (journalref ((name . "ACM SIGPLAN Notices")
                 (volume . "27")
                 (number . "5")
                 (pages . "1--53")))
    (date ((year . "1992")))
    (keyword () "functional")
    (keyword () "tutorial"))

Though in its current incarnation it is only useful for data represented as XML as opposed to marked up text, it can handle a broad enough subset to be useful in a variety of contexts.

read-xml and write-xml are duals.

Querying for data

Simply having XML data represented as an s-expression is rather useless if you have to write complicated programs to traverse the tree to extract data useful to your case at hand. It is good to have a collection of functions that you can compose in order to express the portion of the xml node that you're interested in.

xml.scm defines an expressive collection of predicates and combinators that for this purpose. The basic design of the predicates and combinators is described below. The xml.scm file defines the XML module containing the operator bindings. To bring in the bindings exported by the XML module into the current work scope, you can write -

(import (load "xml.scm"))

Data extractors

The functions tag=, attr= and body= are used to extract the tag name, the value of an attribute and the body of a node respectively.

Predicates

A predicate is a function that takes an XML node (as an s-expression) and evaluates to a list of XML nodes that satisfy the condition of the predicate.

fn (node) -> list-of-nodes

For example, the expression (:tag article) evaluates to a predicate that evaluates to (node) if the tag of the node is the symbol article and to () if it isn't. The expression (:attr year (= year "1992")) evaluates to a predicate that evaluates to (node) if it contains an attribute year with the value 1992.

The list-of-nodes monadic representation was chosen because it lends itself to composition using map and join functions.

Combinators

These are functions that take other predicates and create a new predicate.

For example, the :and combinator takes a number of predicates and evaluates to a predicate that will succeed with those nodes that satisfy all of the given predicates. The expression

(:and (:tag journalref) (:attr name (= name "ACM SIGPLAN Notices")))

is a predicate that will match the journalref tag as shown above.

Here are some other combinators -

  • (:or ..preds..) succeeds with a list of nodes each of which satisfy at least one of the argument predicates.
  • (:not pred) succeeds with a list of nodes that don't satisfy the given predicate.
  • (:child pred) succeeds with the list of child nodes that satisfy the given predicate.
  • (:descendant pred) succeeds with the list of descendant nodes - including the root node itself - that satisfy the given predicate.
  • (:select pred1 pred2 ..) the results of pred1 are refined by pred2 which are further refined by the remaining predicates.
  • (:path pred1 pred2 ... predN) matches the deepest node predN such that pred1 is pred2's parent, pred2 is pred3's parent, and so on.

Examples

Here is a new predicate that matches a node if the first element of its body is the string "functional". It shows how you can define your own predicates.

(define kw-functional 
    (fn (node) 
        (if (= (first (body= node)) "functional") 
            (list node)
            ())))

A query for all articles after 1990 that match the keyword "functional".

(define functional-articles-after-1990
    (:and (:tag article) 
          (:child (:and (:tag date) (:attr year (> (number year) 1990))))
          (:child (:and (:tag keyword) kw-functional))))

A query to extract all the "keyword" tags.

(define all-keywords (:descendant (:tag keyword)))

A query to extract all articles between 1990 and 1995.

(define articles-between-1990-1995
    (:and (:tag article)
          (:child (:and (:tag date) 
                        (:attr year (and (>= (number year) 1990)
                                         (<= (number year) 1995)))))))
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