typeOf is usually invoked with an undefined parameter; it should use types, never values.  Here you've defined it to deconstruct what it's passed, which means that anything that uses it in the usual way (`typeOf (undefined :: someType)') will immediately throw undefined.

You don't need a deconstructor there; you (correctly) throw away the value, and it doesn't provide any type information not already available from the instance declaration.  `typeOf t' should be good enough.