I actually initially had it as sort of a callback:
.otherwise(panic(msg))
(although I assume a rust panic! isn't really a function call).
But isn't the way to get the default, simply to use unwrap()?
In a simple script, failing to open a configuration file for reading is likely a show stopper, and you probably want to log/print an error (no such file, wrong permission, etc).
But in, say, a paint program, you'd normally not want to panic and crash if the image file a user selected to open in a file dialog is invalid or went away between the click-to-select and the click-to-open. In such a program you'd want to handle most file errors much more defensively.
