[barrkel]

The way I prefer to solve this problem these days is to use a fairly generic tree node structure (fairly close to sexprs) and implement tree grammars which check for correctness of different phases (e.g. verify that nodes are annotated with types, or syntax sugar has been decomposed).

Being playful, I've generally implemented the tree grammars as literal constructions of the tree nodes themselves, so the grammars can self-check.

I've use this approach in Ruby and Java though, not functional languages with pattern matching and destructuring, so I wasn't leaving those powerful tools on the table.

In particular I have given up trying to represent ASTs using classes. I don't think they deliver a lot of value. Behavior generally belongs outside the class and not inside, transformations should be collected together as an algorithm, rather than distributed across multiple definitions. Double dispatch for type-safe tree navigation is unpleasant and you lose control over typing of context. You end up with explicit typecasts and you've lost what little type safety you had, and you still can't enforce constraints around how the tree changes from phase to phase.