minh thu asked a tricky question, about writing
extract :: Typeable a => TypeRep -> Dynamic -> a
The question here is what determines the type 'a'. One answer is that 'a' is determined from the context, e.g., (extract tr dyn) + 1.0 fixes 'a' to be an Int. In that case, extract is equivalent to `read' (indeed, String is a primitive sort of Dynamic). We do not need any argument TypeRep though as we obtain 'a' from the context. fromDynamic is sufficient there. One can also construe the question to be asking about writing an `inverse' of typeOf. Whereas, typeOf :: Typeable a => a -> TypeRep can give the TypeRep for any (Typeable) type, undefinedOf :: Typeable a => TypeRep -> a could give (an undefined) value for a given TypeRep. Now, the _type_ 'a' is determined by the _value_ of TypeRep: seemingly we need dependent types. To write function of exactly that signature, we need Template Haskell. Oftentimes, we can get away with a weaker function:
data Dyn = forall a. Typeable a => Dyn a reflect :: TypeRep -> Dyn
to be used to implement dynApply, dynFst, dynSnd, dynHead and other operations on Dynamics. Perhaps the following message may help then http://www.haskell.org/pipermail/haskell-cafe/2007-December/036820.html