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 <patrick.browne@dit.ie>

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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