data-composition.xqy

xquery version "3.0";

(: the power of algebraic data types in xquery

   This example shows how we can composite up data models
   which 'carry' their own operations along with them.

 :)

(: using John Snelson's inspirational https://github.com/jpcs/data.xq :)
import module namespace data="http://snelson.org.uk/functions/data" at "data.xq";

(: one day we create a person data model :)
declare variable $person := data:declare(
<Person>
  <name><data:Sequence/></name>
</Person>);

(: some time later we realise we need an address :)
declare variable $address := data:declare(
<Address>
  <street><data:Sequence/></street>
  <city><data:Sequence/></city>
  <postcode><data:Sequence/></postcode>
</Address>);

(: example of providing custom input validation :)
declare function local:set-city(
   $city
){
 if($city ne "London")
 then error(xs:QName('BADCITY'))
 else $address[2]($city)
};

(: now for the magic, algebraic data type lets us easily compose data models :)
let $record := ($person,$address)

(: data:declare has already provided us the accessors :)
let $me := (
        $record[1]("Jim Fuller"),
        $record[2]("Station road"),
        local:set-city("London"), (:as an example of providing custom validation:)
        $record[4]("N13 9PR")
        )

return
     (
       (: direct access values:)
       $me[1](function($v){"Full Name: " || $v}), (: custom output :)

       $me[2]((function($v){$v},function($v){$v},function($v){$v})),
       $me[3]((function($v){$v},function($v){$v},function($v){$v})),
       $me[4]((function($v){$v},function($v){$v},function($v){$v})),

       (: useful data: functions for describing type and structure:)
       $me ! (data:type(.),data:describe(.)))