I think that the type signatures that have "Any" in them can be a bit confusing and may be less confusing if some sort of generic type system were added such that the type signature of a function called append might be something like List -> T -> List.

This change might extend such that when you acquire concrete Lists they would have initialised types, such as the list of items that you can make a variable for in Inventory Readers would be a List.

It seems that there is already some sort of internal type system as you already can't append items that don't match the type of the list to the list.

In some cases, such a generic T type could inded be added, might require some hacking though. But in most cases where Any is used, the type will only be known during evaluation time.