It's useless if it's optional.

So is F#.

Assuming you mean “dependent types in F#” rather than “F#”, after reading the article, I concluded that these are, in fact, not dependent types, but just so-called “smart constructors”. Since “dependent types in F#” don't exist, it doesn't make sense to ask whether they're useful or useless.

I meant that F# itself is optional. So are tests, by the way, and therefore also completely useless.

Yeah, F# is kind of problematic since you aren't forced to use F# to typefully use F# libraries. Things would be a lot better in a language like Standard ML, where you can either use a library correctly (assuming it exposes the right interfaces) or not use it at all. Hooray for abstraction!