To make it tail-recursive, make the recursive call the last operation in the function.
The else() clause is what keeps it from being tail recursive. Something like this should work:
declare variable $bookstore := <bookstore>
<book>
<name>
story</name>
<price>
50.00</price>
<author>
smith</author>
</book>
<book>
<name>
history</name>
<price>
150.00</price>
<author>
kelly</author>
</book>
<book>
<name>
epic</name>
<price>
300.00</price>
<author>
jones</author>
</book>
</bookstore>
;
declare function local:sum($books, $sum)
{
let $sum := $sum + $books[1]/price
return (
<price>
{ $sum }</price>
,
$books[2] ! local:sum(tail($books), $sum)
)
};
<prices>
{
local:sum($bookstore/book, 0)
}
</prices>
Jonathan