Appian Community and Appian Academy are being upgraded. As a part of the upgrade, Appian Community is currently in read-only mode, and user registration is disabled until August 3. We apologize for any inconvenience this may cause, but a more secure, stable, and performant Community experience is coming soon!
The new Appian Community launches August 3, followed by Appian Academy on August 7. During the migration, Appian Community Edition, Appian Academy, Documentation, Certifications, Instructor-led Customer Training, Partner Sales Training & Accreditation, and Forum (for Appian Partners and Customers only) will remain available.
what might be my mistake
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The underlying issue here is, that index() returns a copy of the date, but not a reference which you would need to store data to. Use dot-notation or angular brackets "[ ]" to get a reference you can use in a saveInto.