But here we need to get the grammar from users. How can I do that? Or how can input the grammar in my code?? Sent from my iPhone On Jul 22, 2011, at 5:04 AM, Yves Parès wrote:

You can do that using the Typeable class and possibly the Dynamic type. (Data.Typeable and Data.Dynamic in base)

When you have:

foo :: (Typeable a, Num a) => a -> a foo x = case (cast x :: Maybe Int) of Just x' -> x + 12 _ -> x

foo will accept every instance of Num but will add 12 to it only if it is an Int.

Dynamic wraps an instance of Typeable.

2011/7/22 Patrick Browne Is it possible to access type information and perform either of the following in a Haskell program?

1) Find the type/class of a variable e.g. a type predicate: is y of-type A, or is y of-Class A

2) Assert the type of a variable e.g. if y.length > 100 then y is of type big.

Regards, Pat

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On Fri, Jul 22, 2011 at 10:31 AM, Stephen Tetley wrote:

As Haskell is statically typed, if y.length < 100, y is still of type Big...

You could achieve this kind of thing with smart constructors and GADTs. I don't know how useful it is, though =). data Sized t a where Small :: a -> Sized Small a Big :: a -> Sized Big a data Small -- empty data Big -- empty retrieve :: Sized t a -> a retrieve (Small a) = a retrieve (Big a) = a toSmall :: [a] -> Maybe (Sized Small [a]) toSmall xs = if length xs < 100 then Just (Small xs) else Nothing toBig :: ... -- almost the same as toSmall toUnknown :: [a] -> (forall t. Sized t [a] -> r) -> r toUnknown xs f = if length xs < 100 then f (Small xs) else f (Big xs) -- Felipe.