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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