The good news is you don't have to bookmark this blog because when you're writing scala and googling questions, it's going to come up on page one, because it's one of the best.

Exactly same observation. It's very well written and I have been referring to it quite a lot!

for anyone interested in learning Scala, i highly recommend this set of (~15 or so) blog posts. i started working through them in early 2013 as they were posted. The posts on path-dependent types and the post on type classes are particularly good. The author clearly has a deep understanding of the key parts of the language.

Found this site really useful when I was working with Scala. In particular I remember the descriptions of Option/Either/Try were very clear.

The author writes about “path-dependent types”, but all I can see is plain existentials.

a1.B and a2.B in the example are path-dependent types - you couldn't express them as just existentials (you could express the types of a1 and a2 as existentials, but you couldn't write "a1.b = Some(b1)" in a purely existential typing system - you need some way to "get the type out").

Not with Haskell-style (or, rather, System F-omega-style) existentials, sure. But why not with strong existentials, like what you get with first-class modules in OCaml or 1ML?

At that point this is just semantics. I find the phrase "path-dependent types" useful and intuitive (though admittedly easily confused with "dependent types"); maybe we could use that phrase to describe what OCaml has too?