Effect Polymorphism in Higher-Order Logic (Proof Pearl)

The notion of a monad cannot be expressed within higher-order logic (HOL) due to type system restrictions. I show that if a monad is restricted to values of a fixed type, this notion can be formalised in HOL. Based on this idea, I develop a library of effect specifications and implementations of monads and monad transformers. Hence, I can abstract over the concrete monad in HOL definitions and thus use the same definition for different (combinations of) effects. I illustrate the usefulness of effect polymorphism with a monadic interpreter.