[dragonwriter]

> His point is that Maybe a should be composed of all the values of a, plus one more value, nil

No, that's a simple union type. There are very good reasons for Maybe to be different than unions (Maybe can nest meaningfully, simple unions can't.)

Maybe Maybe a is valid, and often useful, type.

Of course, if you have a function of type a -> b and find out you need a more general Maybe a -> b, instead of a breaking change, you just write a wrapper function that produces the correct result for Nothing and delegates to the existing function for Some(a) and you're done without breaking existing clients.

(Now, I suppose, if you're u had something like Scala implicits available, having an implicit a -> Maybe a conversion might sometimes be useful, though it does make code less clear.)

[nadagast]

I agree that there are reasons for Maybe a to be a different type from (a | nil) but there are also good reasons to prefer (a | nil). Like most things, it's a set of tradeoffs. What I appreciated about this talk was that he went into the benefits of thinking about types in this way. It's (relatively) common to see the benefits of Maybe a explained, but more rare to see the benefits of full union types explained.