Oleg: Very interresting, thanks. I have some questions: - What do you mean by "The type Cont Int a describes an impure computation, which may abort with an Int value, for example". Aborting with an Int value is akin to exceptions? - for me it's not clear when to choose an "applicative" or a "monadic" DSL? Betsides, what's the rational behind the name "let_" (never seen it before)? Linsey, Jacques: Thanks for the pointer! I learned about data kinds. I tried to apply your suggestions to add a phantom type parameter to Exp. I came up to (I dropped the Free monad idea, which seems unrelated to the problem):
data Eff = Effect | NoEffect
-- first type parameter is used to track effects data Exp :: Eff -> * -> * where ReadAccount :: Exp r Int --ReadAccount can be used in whatever monad (with or without effect) WriteAccount :: Exp NoEffect Int -> Exp Effect () --WriteAccount takes an effect-less expression, and returns an effectfull expression SetVictory :: Exp NoEffect Bool -> Exp Effect () -- same for SetVictory OnTimer :: Exp Effect () -> Exp Effect () --OnTime can program whatever expression to be triggered every minutes, in particular effectful ones Return :: a -> Exp r a Bind :: Exp r a -> (a -> Exp r b) -> Exp r b
This is the (simplified) game state:
data Game = Game { bankAccount :: Int, victory :: Exp NoEffect Bool, timerEvent :: Exp Effect ()}
-- victory when account > 100 victoryRule' :: Exp Effect ()
victoryRule' = SetVictory $ do m <- readAccount --WriteAccount (return $ m + 1) --won't compile (good)
return (m > 100)
--increase my bank account by 1 every minute myTimer :: Exp Effect () myTimer = OnTimer $ do m <- readAccount
writeAccount (return $ m + 1)
Do you have a better name suggestion for Eff? Other pointers where this idiom is realised?? Thanks!! Corentin