4 Search (SXPath)

The W3C "XPath" standard describes a standardized way to perform searches in XML documents. For instance, the XPath string "/A/B/C" (thank you, Wikipedia) describes a search for a C element whose parent is a B element whose parent is an A element that is the root of the document.

The functions in this module perform similar functions. The txpath function accepts the standard XPath syntax, whereas the sxpath function is structured in a more idiomatic (for Racket) way.

This documentation desperately needs more examples.

If we use Neil Van Dyke’s html parser, we might parse this into the following sxml document:

> (define example-doc     '(*TOP*       (aaa "\n" "  "         (bbb "\n" "     "           (ccc) "\n" "     "           (www " www content " (xxx) (www "\n" "     " (zzz) "\n" "  ")))         "\n" "  "         (xxx "\n" "     " (ddd " content in ccc \n" "     ") "\n" "  ")         "\n")))

To extract the xxx’s inside the aaa from the example document:

> ((sxpath '(aaa xxx)) example-doc)

'((xxx "\n" "     " (ddd " content in ccc \n" "     ") "\n" "  "))

To extract all cells from an HTML table:

> (define table     `(*TOP*       (table        (tr (td "a") (td "b"))        (tr (td "c") (td "d")))))

> ((sxpath '(table tr td)) table)

'((td "a") (td "b") (td "c") (td "d"))

To extract all cells anywhere in a document:

> (define table     `(*TOP*       (div        (p (table            (tr (td "a") (td "b"))            (tr (td "c") (td "d"))))        (table         (tr (td "e"))))))

> ((sxpath '(// td)) table)

'((td "a") (td "b") (td "c") (td "d") (td "e"))

One result may be nested in another one:

> (define doc     `(*TOP*       (div        (p (div "3")           (div (div "4"))))))

> ((sxpath '(// div)) doc)

'((div (p (div "3") (div (div "4")))) (div "3") (div (div "4")) (div "4"))

There’s also a string-based syntax, txpath. As shown in the grammar above, sxpath assumes that any strings in the path are expressed using the txpath syntax.

So, for instance, the prior example could be rewritten using a string:

> (define doc     `(*TOP*       (div        (p (div "3")           (div (div "4"))))))

> ((sxpath "//div") doc)

'((div (p (div "3") (div (div "4")))) (div "3") (div (div "4")) (div "4"))

More generally, lists in the s-expression syntax correspond to string concatenation in the txpath syntax.

So, to find all italics that appear at top level within a paragraph:

> (define doc     `(*TOP*       (div        (p (i "3")           (froogy (i "4"))))))

> ((sxpath "//p/i") doc)

'((i "3"))

Handling of namespaces in sxpath is a bit surprising. In particular, it appears to me that sxpath’s model is that namespaces must appear fully expanded in the matched source. For instance:

> ((sxpath "//ns:p" `((ns . "http://example.com")))    '(*TOP* (html (http://example.com:body                   (http://example.com:p "first para")                   (http://example.com:p                    "second para containing"                    (http://example.com:p "third para") "inside it")))))

'((http://example.com:p "first para")   (http://example.com:p    "second para containing"    (http://example.com:p "third para")    "inside it")   (http://example.com:p "third para"))

But the corresponding example where the source document contains a namespace shortcut does not match in the same way. That is:

> ((sxpath "//ns:p" `((ns . "http://example.com")))    '(*TOP* (@ (*NAMESPACES* (ns "http://example.com")))            (html (ns:body (ns:p "first para")                           (ns:p "second para containing"                                 (ns:p "third para") "inside it")))))

'()

It produces the empty list. Instead, you must pretend that the shortcut is actually the namespace. Thus:

> ((sxpath "//ns:p" `((ns . "ns")))    '(*TOP* (@ (*NAMESPACES* (ns "http://example.com")))            (html (ns:body (ns:p "first para")                           (ns:p "second para containing"                                 (ns:p "third para") "inside it")))))

'((ns:p "first para")   (ns:p "second para containing" (ns:p "third para") "inside it")   (ns:p "third para"))

Ah well.

Deprecated; use sxpath instead.

procedure (sxml:filter pred) → sxml-converter

pred : sxml-converter-as-predicate

procedure (sxml:complement pred) → sxml-converter-as-predicate

pred : sxml-converter-as-predicate

procedure (node-join selector) → sxml-converter

selector : sxml-converter

procedure (node-reduce converter) → sxml-converter

converter : sxml-converter

procedure (node-or converter) → sxml-converter

converter : sxml-converter

procedure (node-closure converter) → sxml-converter

converter : sxml-converter

The following procedures depend explicitly on the root node.

Here’s an example:

> (((sxml:preceding (ntype?? 'www)) example-doc) ((sxpath `(aaa xxx)) example-doc))

'((www "\n" "     " (zzz) "\n" "  ")   (www " www content " (xxx) (www "\n" "     " (zzz) "\n" "  ")))

> (define doc '(*TOP* (div (p "foo") (p "bar")                            (img "baz") (p "quux"))))

> (((sxml:preceding-sibling (ntype?? 'p)) doc) ((sxpath '(// img)) doc))

'((p "bar") (p "foo"))