Lazy Module

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This XQuery 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 requested. 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 lazy items:

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('output.data', fetch:binary('http://files.basex.org/xml/xmark111mb.zip'))

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

Contents

[edit] Conventions

All functions and errors in this module are assigned to the http://basex.org/modules/lazy namespace, which is statically bound to the lazy prefix.

[edit] Functions

[edit] lazy:cache

Updated with Version 9.1: $lazy argument added; support for sequences.

Signatures lazy:cache($items as item()*) as item()*
lazy:cache($items as item()*, $lazy as xs:boolean) as item()*
Summary Caches the data of lazy $items in a sequence:
  • data of lazy items will be retrieved and cached inside the item.
  • non-lazy items, or lazy items with cached data, will simply be passed through.
  • If $lazy is set to true(), caching will be deferred until the data is eventually requested. Streaming will be disabled: Data will always be cached before a stream is returned.

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

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

[edit] lazy:is-lazy

Signatures lazy:is-lazy($item as item()) as xs:boolean
Summary Checks whether the specified $item is lazy.

[edit] lazy:is-cached

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

[edit] Changelog

Version 9.1
Version 9.0
Version 8.0

This module was introduced with Version 7.7.

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