http://api.drupal.org/api/drupal/developer!topics!forms_api_reference.ht...

States:

exists: A function name to invoke for checking whether a submitted machine name value already exists. The submitted value is passed as argument.

The full signature for this callback is shown in form.inc where it is called:

    if ($function($element['#value'], $element, $form_state)) {

Comments

jhodgdon’s picture

jhodgdon
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CreditAttribution: jhodgdon commented
Component: Correction/Clarification » API documentation files

Thanks! For future reference, any issues related to things on api.drupal.org (Documentation project git repository files) go into Component = "API Documentation Files".

jhodgdon’s picture

jhodgdon
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CreditAttribution: jhodgdon commented

(That is so that non-programmers looking at the Documentation project issue queue can ignore these issues, because the queue is also/mainly used for issues in the on-line documentation.)