It's possible, but it's not nice. You need to be able to "get out of the monad" to make the types match, i.e. lam f = I (return $ \x -> let y = I (return x) in unsafePerformIO $ unI (f y)) The use of IO 'forces' lam to transform its effectful input into an even more effectful result. Actually, same goes for any monad used inside a 'repr'. let_ and fix follow a similar pattern (although you can hide that somewhat by re-using lam if you wish). Jacques Günther Schmidt wrote:
Hi all,
I'm playing around with finally tagless.
Here is the class for my Syntax:
class HOAS repr where lam :: (repr a -> repr b) -> repr (a -> b) app :: repr (a -> b) -> repr a -> repr b fix :: (repr a -> repr a) -> repr a let_ :: repr a -> (repr a -> repr b) -> repr b
int :: Int -> repr Int add :: repr Int -> repr Int -> repr Int sub :: repr Int -> repr Int -> repr Int mul :: repr Int -> repr Int -> repr Int
and here is one instance of that class for semantics.
newtype I a = I { unI :: IO a }
instance HOAS I where app e1 e2 = I (do e1' <- unI e1 e2' <- unI e2 return $ e1' e2') int i = I (putStrLn ("setting an integer: " ++ show i) >> return i) add e1 e2 = I (do e1' <- unI e1 e2' <- unI e2 putStrLn (printf "adding %d with %d" e1' e2') return $ e1' + e2') sub e1 e2 = I (do e1' <- unI e1 e2' <- unI e2 putStrLn (printf "subtracting %d from %d" e1' e2') return $ e1' - e2') mul e1 e2 = I (do e1' <- unI e1 e2' <- unI e2 putStrLn (printf "multiplying %d with %d" e1' e2') return $ e1' * e2')
I'm stuck with the "lam" method, for 2 days now I've been trying to get it right, but failed.
Is there a possibility that it isn't even possible?
Günther
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