Ask HN: What makes OCaml so desired for writing compilers?

For all these reasons: http://flint.cs.yale.edu/cs421/case-for-ml.html

Many school courses use ML/OCaml as well in their classes and as you do the course they explain why OCaml/ML is best suited for what's going on https://www.cs.cmu.edu/~fp/courses/15411-f13/schedule.html

The most important factor is that it's easy to define types and use them, especially variants or tagged unions, whatever you want to call them. In languages like Java, defining a new type is a huge exercise in bureaucracy, and it's even worse for a variant. It's also important that it's natively compiled, impure and strictly evaluated, because performance matters. It also has a lot of other features that we take for granted nowadays.

Nowadays there are a lot of options that would be just as good, or better.