Why would I ever want to use Maybe instead of a List?

Question

Seeing as the Maybe type is isomorphic to the set of null and singleton lists, why would anyone ever want to use the Maybe type when I could just use lists to accomodate absence?

Answer 1

Because if you match a list against the patterns [] and [x] that's not an exhaustive match and you'll get a warning about that, forcing you to either add another case that'll never get called or to ignore the warning.

Matching a Maybe against Nothing and Just x however is exhaustive. So you'll only get a warning if you fail to match one of those cases.

If you choose your types such that they can only represent values that you may actually produce, you can rely on non-exhaustiveness warnings to tell you about bugs in your code where you forget to check for a given a case. If you choose more "permissive" types, you'll always have to think about whether a warning represents an actual bug or just an impossible case.