"Historically, ML was conceived to develop proof tactics in the LCF theorem prover (whose language, pplambda, a combination of the first-order predicate calculu

"Historically, ML was conceived to develop proof tactics in the LCF theorem prover (whose language, pplambda, a combination of the first-order predicate calculus and the simply-typed polymorphic lambda calculus, had ML as its metalanguage)." -- https://en.wikipedia.org/wiki/ML_(programming_language) https://en.wikipedia.org/wiki/ML_(programming_language)