[xquery-talk] Implementing algorithms involving state across iterations in XQuery

[Leonard Wörteler]

Charles,

Am 02.01.2014 22:36, schrieb Charles Duffy:
> While I know how to do this in a few other functional languages (for
> instance, in Clojure with its atoms), I'm a bit lost in XQuery. Could I
> ask for a bit of clue?

I think you are looking for `fn:fold-left(...)` and/or 
`fn:fold-right(...)` [1]. They can be used to thread some kind of state 
through while iterating over a sequence. `fn:fold-left($seq, $z, $f)` 
behaves similar to the following imperative snippet:

> $accum <- $z;
> foreach($item in $seq) {
>   $accum <- $f($accum, $item);
> }
> return $accum;

Your algorithm could be written like this (untested):

> fn:fold-left(
>   $inputs,
>   map:new(),
>   function($already-seen, $element) {
>     if(local:map-contains-prefix-of($already-seen, $element))
>     then $already-seen
>     else (
>       let $result := expensive-operation($element)
>       return map:new((
>         $already-seen,
>         map:entry(get-key($element), $result)
>       ))
>     )
>   }
> )

Hope that helps,
   Leo

[1] http://www.w3.org/TR/xpath-functions-30/#func-fold-left