Re: [Haskell-cafe] Introspection

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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