Hey, I've solved the problem. To unify static and dynamic types I
didn't have to introduce FType m a. Rather I had to do the 'matches'
function a member of Typed class.
class Typed a where
typ :: m a -> Type m
matches :: m a -> Type m -> Maybe (m a)
with a trivial implementation
instance Typed Int where
...
macthes _ (TInt x) = Just x;
matches _ _ = Nothing;
and then
compute :: (Typed t) => Scope -> Exp t -> Val t
compute scope (Var t name) =
let opq = lookup name scope
val = case opq of
Nothing -> error "not in scope"
Just opq -> extract opq
expType = Val t
in case expType `matches` val of
Nothing -> error "type error"
Just v -> v
I like it.
btw, matches can be used for dynamic coercions like
instance Typed Double where
matches _ (TDouble x) = Just x
matches _ (TInt x) = Just (castIntToDouble x)
matches _ _ = Nothing
2007/6/7, Daniil Elovkov
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.
.....
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
there was a typo here, which sneaked in from previous variants of the code, sorry 'typ' shoud be instead of 'aux'
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