$haystack must of type string error

Thanks for the report. Can you try the dev version of the module to confirm?

It now enables without error, but I still get

Drupal\libraries\ExternalLibrary\Exception\LibraryDefinitionNotFoundException: The library definition for the library 'caret.js' could not be found. in Drupal\libraries\ExternalLibrary\Definition\ChainDefinitionDiscovery->getDefinition() (line 56 of modules/libraries/src/ExternalLibrary/Definition/ChainDefinitionDiscovery.php).
Drupal\libraries\ExternalLibrary\LibraryManager->load('caret.js') (Line: 293)
atjs_requirements('runtime') (Line: 52)
Drupal\hook_event_dispatcher\HookEventDispatcherModuleHandler::Drupal\hook_event_dispatcher\{closure}(Object, 'atjs') (Line: 405)
Drupal\Core\Extension\ModuleHandler->invokeAllWith('requirements', Object) (Line: 67)
Drupal\hook_event_dispatcher\HookEventDispatcherModuleHandler->invokeAllWith('requirements', Object) (Line: 59)
Drupal\hook_event_dispatcher\HookEventDispatcherModuleHandler->invokeAll('requirements', Array) (Line: 109)
. . . more

I am trying to figure out how to use a library from the local filesystem as the readme says

Currently the module implements a plugin for the JsDelivr CDN
and another for local libraries in the `/libraries` folder.

I have this in my module.libraries.yml

drupal.caret.js
remote: http://ichord.github.com/Caret.js
repo: ichord/Caret.js
description: "Add Github like mentions autocomplete to your application."
version: 0.3.1
license:
name: MIT
gpl-compatible: true
js:
/libraries/caret.js/dist/js/jquery.caret.min.js: {}
/libraries/caret.js/dist/js/jquery.caret.js: { minified: true }

libraries_provider:
enabled: true
source: LibrarySourceBase

dependencies:
- core/jquery
- core/drupal
- core/drupalSettings
- core/once

Do I have something wrong in the source syntax or call or way off base elsewhere ??

That seems like an exception for the "libraries" module, not this one.

I don't understand the line where you specify LibrarySourceBase as the source.
For an example you can look at bulmajs library definition:
https://git.drupalcode.org/project/drulma/-/blob/2.x/drulma.libraries.ym...

LibrarySourceBase.php appeared to be the plugin for library discovery

Going to cdn. jsdelivr.net goes to jsdelivr.com reveals the format for github is gh instead of npm like this

https://cdn.jsdelivr.net/gh/@ichord/At.js/tree/master/dist/js/jquery.atw... {}

or something like this

after several tries of various configurations nothing worked so I will have to wait and see if the libraries maintainers answer

Automatically closed - issue fixed for 2 weeks with no activity.