Great project but very weird choice of language. How probable is it (pun intended) that the person coming to learn about probabilistic programming would already know functional programming?

Isn't functional programming a standard part of any computer science curriculum? Why would you expect programmers not to know it?

Even the places where it is a part of the curriculum, it is often such a small part that unless people specifically take courses related to functional programming you can't expect them to be able to actually use it. Or remember much of it for that matter.

Heck, I spent months on a binge reading functional programming research papers, and it still doesn't mean I know any functional languages other than very superficially.

That's true of any language though. If you know Java, you don't automatically know C#, but the core concepts will still mostly transfer. You'll still know Object-Oriented Programming, even if you've never used C# and can't write HelloWorld without looking something up.

I was very surprised to learn from two Stanford CS alums that functional programming was not a requirement in that program.

Not everyone does a computer science degree.

No it's not, in most of the world.

I actually considered that book as good as any ressource for functional programming introduction.