Method Concurrent.Future()->on_failure()
- Method on_failure
 this_programon_failure(function(mixed,mixed... :void)cb,mixed...extra)- Description
 Register a callback that is to be called on failure.
- Parameter 
cb Function to be called. The first argument will be the failure result of the Future.
- Parameter 
extra Any extra context needed for
cb. They will be provided as arguments two and onwards whencbis called.- Note
 cbwill always be called from the main backend.- See also