Hi all,
> > Concerning "2. combination with overlapping instances", you say "The > > solution has been described already in the HList paper: provide > > something the typeOf at the type level. That is, assume a type > > function "TypeOf :: * -> TYPEREP". > > > > ... > > > > Incidentally Pedro's new "deriving Generic" stuff does derive a kind of > type-level type representation for types, I think. It's more or less as > described in their paper. > > http://www.dreixel.net/research/pdf/gdmh_nocolor.pdf > > I'm very excited about this new Representable class and have just > started playing with it, so it is too early for me to say for sure. > However, my initial impression is that this is going to make me want > OverlappingInstances even more because of all the cool things you can > do with recursive algorithms over the Rep types... > Yes, most likely. I have a package defining a few generic functions and showing some example uses ( http://hackage.haskell.org/package/generic-deriving). There I use OverlappingInstances (and even UndecidableInstances). > > An initial issue I'm running into (and again, this is preliminary, > maybe I'll figure out a way without OverlappingInstances) is in > implementing a function to serialize Generic data types to JSON. If > the Haskell data type has selectors (i.e., is a record), then I would > like to serialize/unserialize the type as a JSON object, where the > names of the selectors are the JSON field names. If the Haskell data > type does not have selectors, then I would like to serialize it as a > JSON array. > > Generic deriving gives me the conIsRecord function, but this is at the > value level, and I want to do something different at the type level. > For a record, I need a list of (String, Value) pairs, while for a > non-record, I need a list of Values. This leads me to write code such > as the following: > > class JSON1 a r | a -> r > toJSON1 :: a -> r > > instance (JSON a) => JSON1 (S1 NoSelector (K1 c a)) [Value] where > toJSON1 (M1 (K1 a)) = [toJSON a] > > instance (Selector x, JSON a) => JSON1 (S1 x (K1 c a)) [(String, Value)] > where > toJSON1 s@(M1 (K1 a)) = [(nameOf s undefined, toJSON a)] > where nameOf :: S1 c f p -> c -> String > nameOf _ c -> selName c > > The key piece of magic I need here (and in so many other places) is to > be able to do one thing at the type level if x is a particular type > (e.g., NoSelector) or sometimes one of a small number of types, and to > do something else if x is any other type. > Right. I think this is often essential. Cheers, Pedro
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