I am quite certain I am not the first to try to do this, but my google-fu is failing me today. How does one go about interpreting a language with mutable objects in Haskell? The best approach I can think of is to represent the language's memory as a `Data.Map.Map RefID LanguageObject` where RefID is some type (probably Int) used as a reference. The LanguageObject structure might contain some values of type RefID to refer to other objects. Mutating an object involves simply replacing the value in the map at a given RefID. I don't like this approach for two reasons: 1. Map lookups aren't very efficient compared with actual references to the value. 2. I have to re-invent garbage collection, removing objects from the map when they no longer have incoming references. (Unlike simple interpreters for languages with immutable values, I can't rely on Haskell's garbage collector to do the work for me.) Is there a better approach? - Jeremy