Hello café,

I have a little DSL in my program as follow.
Now I'd like to add a Map constructor in it. Thats where I would need help!


> data Obs a where

>     AllPlayers :: Obs [Int]
>     Plus       :: (Num a) => Obs a -> Obs a -> Obs a

>     And        :: Obs Bool -> Obs Bool -> Obs Bool
>     Vote       :: Obs String -> Obs Int -> Obs Bool

>     Map        :: (Obs a -> Obs b) -> Obs [a] -> Obs [b]
>     -- and others


Here is the evaluator for Obs:

> evalObs :: Obs a -> Evaluator a
> evalObs (Konst a)   = return $ pure a

> evalObs (Not a)     = liftE  not   (evalObs a)
> evalObs (Plus a b)  = liftE2 (+)   (evalObs a) (evalObs b)

> evalObs (Minus a b) = liftE2 (-)   (evalObs a) (evalObs b)
> evalObs (Time a b)  = liftE2 (*)   (evalObs a) (evalObs b)

> evalObs (And a b)   = liftE2 (&&)  (evalObs a) (evalObs b)
> evalObs (Or a b)    = liftE2 (||)  (evalObs a) (evalObs b)

> evalObs (Equ a b)   = liftE2 (==)  (evalObs a) (evalObs b)
> evalObs (If a b c)  = liftE3 (if3) (evalObs a) (evalObs b) (evalObs c)


How you can see it is quite neat...
But how can I write the evaluator for Map?

Actually I have some half baked solution, 15 lines long that I don't dare to show ;)


Actually compiling code excerpt is here: http://hpaste.org/40897/map_contstructor_in_a_dsl


Thanks for your help.
Corentin



Below is some helper code:

type Evaluator a = StateT Game Comm a (Either Actions a)



-- | Combined lifters for Evaluator
liftE  = liftM  . liftA
liftE2 = liftM2 . liftA2
liftE3 = liftM3 . liftA3



instance Applicative (Either Actions) where

        pure x = Right x
        (Right f) <*> (Right x) = Right $ f x


        (Right _) <*> (Left u) = Left u
        (Left u) <*> (Right _) = Left u


        (Left u) <*> (Left v) = Left $ u ++ v