'pattern matching' on Typeable types

Question

Suppose, for example, we have the following data structure:

data Foo = Bool Bool | Int Int | Double Double

Now, is there an easier way to do this:

foo :: Typeable a => a -> Foo
foo x = maybe (error "i dunno") id $
  liftM Bool   (cast x) `mplus`
  liftM Int    (cast x) `mplus`
  liftM Double (cast x)

Has someone thought of making a syntax for pattern matching on Typeable types?

Answer 1

Use typeOf and guards:

foo x
    | tx == typeOf "str" = "string"
    | tx == typeOf True  = "bool"
    | otherwise          = "i dunno"
  where tx = typeOf x