The try(...) and can(...) functions are intended to make it more
convenient to work with deep data structures of unknown shape, by allowing
a caller to concisely try a complex traversal operation against a value
without having to guard against each possible failure mode individually.
These rely on the customdecode extension to get access to their argument
expressions directly, rather than only the results of evaluating those
expressions. The expressions can then be evaluated in a controlled manner
so that any resulting errors can be recognized and suppressed as
appropriate.
