I have been playing with your example, making slight changes to understand what each function does. Could you explain me why does this code doesn’t work?

(get-value xml :friends :person :name)

Thank you

I have updated the urls.

Nice post! You may like to replace in the URL the text “richhickey.github.com” with “clojure.github.com” as the former is not being maintained anymore. I know it’s easy to overlook – just a gentle reminder.

Regards,
Shantanu

zf/xml1 takes a location and set of predicates.

From xml1 doc,

“The loc is passed to the first predicate. If the predicate returns
a collection, each value of the collection is passed to the next
predicate. If it returns a location, the location is passed to the
next predicate. If it returns true, the input location is passed to
the next predicate. If it returns false or nil, the next predicate
is not called.

This process is repeated, passing the processed results of each
predicate to the next predicate. xml-> returns the final sequence.
The entire chain is evaluated lazily.

There are also special predicates: keywords are converted to tag=,
strings to text=, and vectors to sub-queries that return true if
they match.”

In get-value function, we are creating location by
(zip/xml-zip (get-struct-map xml)))

and a set of predicates

(conj (vec tags) zf/text))

When we do a conj we will have a vector of

[:person :name text-fn]

xml1-> call processes keywords as a special predicate by converting them to tag= fn. tag=
Usage: (tag= tagname)
Returns a query predicate that matches a node when its is a tag named tagname.

So when we pass the ROOT location, first (tag= :person) is called , which will return a PERSON location

Then PERSON location is called with (tag= :name) predicate, which will return NAME location.

To get the text from NAME location we use a build in predicate (text).

In get-value fn if you take out zf/text, you can see that it will return

([{:tag :name, :attrs nil, :content ["Siva"]} {:l [], :pnodes [{:tag :friends, :attrs nil, :content [{:tag :person, :attrs nil, :content [{:tag :name, :attrs nil, :content ["Siva"]}]}]} {:tag :person, :attrs nil, :content [{:tag :name, :attrs nil, :content ["Siva"]}]}], :ppath {:l [], :pnodes [{:tag :friends, :attrs nil, :content [{:tag :person, :attrs nil, :content [{:tag :name, :attrs nil, :content ["Siva"]}]}]}], :ppath nil, :r nil}, :r nil}])

I hope I answered your question. Let me know if you it is still unclear.

regards,

– Siva Jagadeesan