[Haskell-cafe] Fwd: manage effects in a DSL

[Corentin Dupont]

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
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