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".
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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