I used SML in my compiler course in college. Our professors loved SML and maintains a pretty popular compiler for it, so he was a wealth of knowledge and helped

I used SML in my compiler course in college. Our professors loved SML and maintains a pretty popular compiler for it, so he was a wealth of knowledge and helped us along the way, which made it a much more approachable language than if I were to embark on learning it myself. Really shows the power for statically typed functional languages, and especially the power of sum types. Representing trees with sum types is so extremely powerful and makes writing compiler oriented code feel very natural. I haven't used it since graduation, but if I had to write a compiler, I'd probably opt for an SML like language, like F#.