Hi Daniil, By "embedded" DSL, we usually mean identifying meta-language (Haskell) expressions with object language (DSL) expressions, rather than having an "Exp" data type. Then you just use meta-language variables as object-language variables. The new data types you introduce are then domain-oriented rather than language-oriented. Is there a reason that this kind of "embedded" approach doesn't work for you? Cheers, - Conal On 6/7/07, Daniil Elovkov wrote:

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?

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