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

[Charles Duffy]

Howdy --

I'd like to implement an algorithm akin to that described by the following
pseudocode in XQuery 3 (my implementation of choice is BaseX):

declare variable $inputs external;

declare function local:map-contains-prefix-of($map, $element) {
  for $map-key in map:keys($map)
    where fn:string-prefix($map-key, $element/key)
    return map:get($map, $map-key)
}

declare function local:result-collection($inputs) {
  let $already-seen := map:new() (: intended to be "updated" / shadowed
later :)
  for $element in $inputs
    where not(local:map-contains-prefix-of($already-seen, $element))
    let $result := expensive-operation($element)
    let $already-seen := map:new(($already-seen,
map:entry(get-key($element), $result))
    return $already-seen
}

local:result-collections($inputs)[last()]

...to explain in prose:

   - I have a sequence of inputs, and expect to generate a shorter sequence
   of outputs.
   - I have a call, expensive-operation(), which results in (1) the desired
   return value for a given input, and (2) information necessary and
   sufficient to identify which other input elements will have the same output
   relatively cheaply (in this example, done with a string comparison).
   - I'm trying to determine how to minimize the number of calls to
   expensive-operation() in an XQuery statement while still generating the
   desired output.

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?

Thanks!
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