[coolsunglasses]

>2. Tail recursion doesn't really make much of a difference for walking trees, which is recursive, but (mostly) not tail recursive.

Non-strictness helps here more than TCO in a strict language.

>4. ADTs can be good or bad for describing ASTs. Once you enrich ASTs with semantics shared by all variants (such as source coordinates), inheritance can become a better fit than ADTs.

Since this article was written we have better ways of augmenting/annotating ASTs. There's a lot of this out there, but here's one example: https://brianmckenna.org/blog/type_annotation_cofree

There are other alternatives that are like inheritance but with better reasoning properties as well. Finally tagless comes to mind.

>I also generally find it better to explicitly annotate functions with types.

This Haskeller whole-heartedly agrees for all the reasons stated.

[tom_mellior]

> Non-strictness helps here more than TCO in a strict language.

Can you explain? Assume I want to fold a function over a large tree and fully inspect the final result. (For example, to compile a large expression to a piece of code.) If I use non-tail recursion, my stack will be exhausted. How does non-strictness help with stack usage?

[willtim]

More generally, functor sum and products allow composition of recursive data types. The cofree comonad, which Brian describes, is a special case of a functor sum.