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Lazy Functions

This module contains functions for handling lazy items.

In contrast to standard XQuery items, a lazy item contains a reference to the actual data, and the data itself will only be retrieved if it is processed. Hence, possible errors will be postponed, and no memory will be occupied by a lazy item as long as its content has not been requested yet.

The following BaseX functions return…

Some functions are capable of consuming the contents of lazy items in a streamable fashion: data will not be cached, but instead passed on to another target (file, the calling expression, etc.). The following streaming functions are currently available:

The XQuery expression below serves as an example on how large files can be downloaded and written to a file with constant memory consumption:

file:write-binary('', fetch:binary(''))

If lazy items are serialized, they will be streamed as well.


All functions and errors in this module are assigned to the namespace, which is statically bound to the lazy prefix.



  $input  as item()*,
  $lazy   as xs:boolean?  := false()
) as item()*
SummaryCaches the data of lazy $input items:
  • data of lazy items are retrieved and cached inside the item.
  • non-lazy items, or lazy items with cached data, are simply passed through.
  • If $lazy is set to true(), caching is deferred until the data is eventually requested. Streaming will be disabled: Data will be cached before a stream is returned.

Caching is advisable if an item is processed more than once, or if the data may not be available anymore at a later stage.

let $file := 'data.txt'
let $text := lazy:cache(file:read-text($file))
return (file:delete($file), $text)
In the example, a file is deleted before its content is returned. To avoid a “file not found” error when serializing the result, the content must be cached.


  $item  as item()
) as xs:boolean
SummaryChecks whether the specified $item is lazy.


  $item  as item()
) as xs:boolean
SummaryChecks whether the contents of the specified $item are cached. The function will always return true for non-lazy items.


Version 9.1
  • Updated: lazy:cache: $lazy argument added; support for sequences.
Version 9.0
  • Added: lazy:is-cached
  • Updated: Renamed from Streaming Functions to Lazy Functions.
Version 8.0
  • Updated: stream:materialize extended to sequences.
Version 7.7
  • Added: New module added.

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