> You might have one author using `[a]` where another uses `PositiveNumeric a, Fin n => NonEmptyList n a` for the same thing. You can always just annotate whatever the library author used (e.g. they return a list of strings, so you use List[str]).
The difference between these two cases is that a function that returns [a] will always return [a] and if you convert it to `NonEmptyList n a` you will be nudged towards handling the case where it didn't actually return n elements, whereas if you annotate someone else's list as returning List[str] then it may well silently not do some of the time, and you won't get an error until a long way away.
I was unclear. I meant the other way around. In the case that the third party code does in fact always return no empty lists of positive numbers, the author might just type it as a list. A third party author in even a language like Haskell won’t necessarily use the bells and whistles of giving very specific types.
And by the List[str] bit, I mean you can annotate it as doing whatever it in fact does. You aren’t protected if you get that wrong, of course, and that’s where things like stubs come in to help.
> In the case that the third party code does in fact always return no empty lists of positive numbers, the author might just type it as a list.
Yes - but as I said, you can be confident that the list type is accurate, and if you want to convert that list to a specific-length list type then you're nudged towards doing it in a way where you handle the case where the list isn't actually the length you specified.
> And by the List[str] bit, I mean you can annotate it as doing whatever it in fact does. You aren’t protected if you get that wrong, of course
Not "of course"; the whole point of a type system is that it's automatically checked and therefore you can have reasonable confidence in the types you're given.
They don't really though? Stubs are just as unchecked as writing the type signature yourself, so even if they're correct at a given point in time, they'll tend to become incorrect for subsequent releases of the libraries they cover.
The difference between these two cases is that a function that returns [a] will always return [a] and if you convert it to `NonEmptyList n a` you will be nudged towards handling the case where it didn't actually return n elements, whereas if you annotate someone else's list as returning List[str] then it may well silently not do some of the time, and you won't get an error until a long way away.