macro boot.janet on line 238, column 1 (try body catch) Try something and catch errors. Body is any expression, and catch should be a form with the first element a tuple. This tuple should contain a binding for errors and an optional binding for the fiber wrapping the body. Returns the result of body if no error, or the result of catch if an error.
# catch and propagate an error with fiber (try (+ 1 nil) ([err fib] (propagate err fib)))