Church was the original probabilistic programming language used for the book, and as per another comment in this thread and a draft PhD dissertation[1], WebPPL

Church was the original probabilistic programming language used for the book, and as per another comment in this thread and a draft PhD dissertation[1], WebPPL is the successor to Church. "Niche" is probably in the eyes of the beholder if the audience is deliberately diving into probabilistic programming languages and probabilistic cognitive models. [1] http://library.meritology.com/fundamentals/chapters/1a-In_a_nutshell http://library.meritology.com/fundamentals/chapters/1a-In_a_...