Hello folks Haskell is considered good for embedded DSLs. I'm trying to implement some simple EDSL in a typeful manner and having a problem with looking up variable values. I've got an Expression GADT, which admits variables. The problem is with writing compute function which will lookup variable values in a type-safe manner. The exp. data type is like this data Exp a where Const a :: Val a -> Exp a Var :: a -> String -> Exp a -- where the first comp. isn't used,only for type info. .... So, obviously, I have to perform lookups in some 'scope' to compute the expression. By scope I mean the list of (name,value) pairs. How do I represent the scope? Well, I haven't gone that far as to encode statically the information about the type of every variable in scope. Instead, I used existentials to hide their types and put'em all in a list. For that purpose I introduced pack/unpack. -- value with dynamic type annotation -- m here and below can be Val, Exp, etc. -- to represent Val Int, Exp Int, etc. data Type m = TInt (m Int) | TString (m String) | TDouble (m Double) class Typed a where typ :: m a -> Type m instance Typed Int where typ x = TInt x instance Typed String where typ x = TString x instance Typed Double where typ x = TDouble x data Opaque m = forall a. (Typed a) => Opaque (m a) -- extract to an annotated representation extract :: Opaque m -> Type m extract (Opaque a) = typ a How would you suggest, I write compute function? My try was compute :: Scope -> Exp t -> Val t compute scope (Const x) = x -- trivial compute scope (Var t name) = -- intereseting part let opq = lookup name scope val = case opq of Nothing -> error "not in scope" Just opq -> extract opq expType = aux t in case val `matches` expType of -- I'd like to make some 'matches' func. Nothing -> error "type error" -- which would either produce an error Just v -> v -- or return the value, based on run-time tags matches :: Typed m -> Typed m -> Maybe (m a) BUT of course this type is bad, there's no 'a' in the left side matches (TInt x) (TInt _) = Just x matches (TString x) (TString _) = Just x matches (TDouble x) (TDouble _) = Just x matches _ _ = Nothing So, clearly the problem is in that Type m has no evidence of a, which was its very purpose. Ok, so I made data FType m a where FInt :: m Int -> FType m Int FString :: m String -> FType m String FDouble :: m Double -> Aux m a class Typed a where typ :: m a -> Type m -- as before ftyp :: m a -> FType m a -- new one and again obvious instance instance Typed Int where ftyp x = FInt x ... And of course, I'd like to get that information somehow extract2 (Opaque a) = ftyp a I rewrote 'matches' accordingly but the problem is already in the type of extract2 its type Opaque m -> (forall a. (Typed a) => m a) is not good to ghc, less polymorphic than expected So, in principle it must be doable, since opaque data retains its dictionary, and by that I can get a dynamic tag, say FInt x, where x is proved to be Int. What would you suggest? Thank you