Dhall[1] is a similar concept, though instead of augmenting a particular format, it is an implementation of a simple pure functional programming language (with a very expressive type system which guarantees termination). Dhall programs can then be evaluated to several different formats (currently JSON, YAML and XML). [1]: https://dhall-lang.org/
Dhall is awesome and rooted in the solid ML line. I wish pure functional programming langs were significantly growing in the industry. It would show signs of maturity but it is still currently a high-hanging fruit for many and traction is diluted in the impression of hobbyism or academism.
What can we bring for those dealing with YAML everyday? Hard to tell them "switch all your stuff to X". Easier to tell them "you can iteratively add functional expressions to your YAML". That's the point of Yglu. It's not about proposing a better Dhall or Jsonnet. It is about proposing better YAML-native tooling.