2008/11/4, [EMAIL PROTECTED] <[EMAIL PROTECTED]>: > > 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.
Indeed, that's what I was asking. > 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: I'll look at Template Haskell then. It's a bit cumbersome to add type annotations everywhere when using Dynamic. > > 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 Thank you, Thu _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
