I am writing an (small-step) interpreter for a simple imperative programming language in Haskell. I want to do evaluation outside of the IO monad and I am thus trying to use the ST monad for mutable variables. However I realise that this means introducing a new phantom type variable to my value type, but then my expression type also needs it, and then my statement type also needs it, and so fourth.
My question is, are there any good ways of avoiding this problem. Could I hide the phantom type in some way? I have tried to do this by introducing a forall s. in my AST definitions, but the compiler is never happy with this.