Hi again, I have a game in which the user can create/write/read variables, using a small DSL. The type of the variable created can be whatever chooses the user, so I'm using existential types to store those variables in a heterogeneous list. This works fine, but the problem is that the "Typeable" class tag leaks into the DSL... The question is, how to get rid of it?
This is literate Haskell {-# LANGUAGE GADTs, ScopedTypeVariables #-} module DSLClass where import Control.Monad import Control.Monad.State import Data.Typeable
This is the (simplified) DSL. With it you can read a variable stored in the game state (creation/writing is not shown). How can we get rid of the "Typeable a" in the ReadFirstVar constructor?
-- first type parameter is used to track effects data Exp a where ReadFirstVar :: (Typeable a) => Exp a <----- Ugly Return :: a -> Exp a Bind :: Exp a -> (a -> Exp b) -> Exp b
This is the definition of a variable. The type is unknow, so I use existantial types.
data Var = forall a . (Typeable a) => Var { v :: a}
This game state. It holds the heterogenous list.
data Game = Game { variables :: [Var]}
The evaluation of "Exp" can be:
eval :: Exp a -> State Game a eval ReadFirstVar = do (Game ((Var v):vs)) <- get case cast v of Just val -> return val Nothing -> error "no cast" eval (Bind exp f) = do a <- eval exp eval (f a)
As you can see, I'm obliged to cast the variable type to match it with the expression's type. Is that the right place to do it? Thanks!! Corentin