It is used in The definition of Standard ML to define the syntax and semantics of the language. That is the most "in the wild" usage I'm familiar with in the s

It is used in The definition of Standard ML to define the syntax and semantics of the language. That is the most "in the wild" usage I'm familiar with in the sense that it gives a formal specification of the language, which has been used across multiple implementations of the language successfully... https://smlfamily.github.io/sml90-defn.pdf https://smlfamily.github.io/sml90-defn.pdf