[bcrosby95]

Depending upon language, and what you're using to hold the array, inheritance.

A 'NotEmpty a' is just a subclass of a potentially empty 'a'. You also get the desirable behavior, in this scenario, of automatic upcasting of a 'NotEmpty a' into a regular old 'a'.

[secdeal]

Not quite, 'a' is the type of the elements 'NonEmpty a' contains.

It is rather the subclass of some kind of 'Iterable a'.