easy_logging | Ruby utility that lets you include a unique logger

You're omitting a lot of context here, and I had to remove references to missing modules in order to reproduce, which means I've made several assumptions that may be wrong.

Given that, my guess is that you've copied code that originally used the Base or Core standard library replacement, where the List.nth function returns an 'a option, as opposed to the standard OCaml implementation which will raise an exception instead if the given index is out of bounds.

The reason I think so is that the parent field of dec has type dec option and is assigned last_scope directly, which means last_scope must have the same type. And if List.nth has type 'a list -> int -> 'a, then 'a must have type dec option, which means table must have type dec option list. And you can't prepend a dec to a dec option list, hence the error.

Lastly, a good way to find the cause of issues like this is to make your assumptions explicit by annotating the type of variables. For example, annotating the type of table here will give you a different error, narrowing it down last_scope having the type dec but being expected to have type dec option when being assigned to parent.