help with writing a monad typeclass (associated type classes?)

Question

I want to write a monad type class which will specify some basic actions for a DSL (domain-specific language) monad.

class Monad 𝔪 => MyDSLMonad 𝔪 where
    type ExprTyp 𝔪 :: * -> *
    var :: String -> ExprTyp 𝔪 α -> 𝔪 (ExprTyp 𝔪 α)

where ExprType indicates the expression type, and var will introduce a new variable declaration.

The idea is that I'll have a base monad Base implementing MyDSLMonad, which maybe just works on things like Int's and Bool's, and then higher-level abstractions will be monad transformers, which can work on higher level types.

However, the implementation for Base's var function might rely on the fact that it only works on Int's and Bool's, and require some kind of type class constraint on the var function. So, I want something like

class Monad 𝔪 => MyDSLMonad 𝔪 where
    type ExprTyp 𝔪 :: * -> *
    class ValidTypes 𝔪
    var :: ValidTypes 𝔪 α => String -> ExprTyp 𝔪 α -> 𝔪 (ExprTyp 𝔪 α)

instance MyDSLMonad Base where
    class ValidTypes Base = MyClass

I'm aware rmonads do this via some type caseing, but is there a nicer way? Also, I kinda like having monad transformers like StateT in the regular monads package ... :)

Answer 1

There's no nicer way, yet. Max Bolingbroke is building this in GHC, hopefully it will arrive in 7.4. See https://twitter.com/mbolingbroke, here's an example: http://hpaste.org/50576