[JonChesterfield]

I've seen it claimed that ML was originally written, in lisp, in order to have a better language to write compilers in.

[lispm]

ML was written as the language used by the theorem prover LCF. It was written in Lisp.

[JonChesterfield]

I didn't know that link, searches seem to confirm. Thank you.

The LCF style provers rely on the host language type system. A theorem instance can only be created by the trusted kernel. Lisp doesn't obviously lend itself to that - you could represent the proof, but I'm not clear how you'd prevent forging one.