My 2 cents: Algebraic data types (tagged unions) are very useful for representing syntax trees and other things of the sort. I'm not sold on the idea of going

My 2 cents: Algebraic data types (tagged unions) are very useful for representing syntax trees and other things of the sort. I'm not sold on the idea of going 100% purely functional (a-la haskell) though. Even a compiler written in a nonfunctional language is already going to be architected as a series of passes. The thing that makes it hard is not the presence of mutable global state. It's more the actual algorithms and optimizations.