[Haskell-cafe] Typeable and Dynamic

minh thu noteed at gmail.com
Mon Nov 3 17:58:30 EST 2008


Given a TypeRep and a Dynamic value with type corresponding to the
TypeRep, I'd like to be able to use fromDyn *without* specifying a type
(given as an additional 'default' argument) (since it is somewhat known
from the TypeRep).

That is, I'd like an undefinedOf that can be used in

extract :: Typeable a => TypeRep -> Dynamic -> a
extract typerep dyn = fromDyn (undefinedOf typerep) dyn

For instance, if TypeRep is Int,
extract Int dyn = fromDyn (undefined::Int) dyn

Is it possible to write "extract" ?


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