Return one or blow up.
Return one or blow up. You could just use the head method, but the value added here is an error message explaining how what you thought was a unique selection criteria is... not.
parameterized type of list
a list expected to contain a single entry
a single result or an error
Guarantee one or none.
Guarantee one or none.
parameterized type of list
a list expected to contain a single entry or no entries
Some for a single entry list; None for an empty list
Some useful utilities for extracting a single result from a list of results.