Am Mittwoch 04 November 2009 22:03:09 schrieb Pasqualino "Titto" Assini:
Hi,
I am writing a little IPC system to make Haskell values and functions remotely invokable.
To do so, I need (or so I believe) to make my objects accessible via a generic interface as in:
class AFun f where afun :: Data a => f -> ([Dynamic] -> a)
So my generic object is something that takes an array of parameters, that being Dynamic can be anything, and returns a Data, that I can easily serialise and send back on the wire.
I start by defining an instance for functions:
instance (Typeable a,AFun b) => AFun (a->b) where afun f (p:ps) = let Just v = fromDynamic p in afun (f v) ps afun _ _ = error "Too few arguments"
So far so good, but when I try to define an instance for values:
instance Data v => AFun v where afun f [] = f afun _ _ = error "Too many arguments"
I get:
Couldn't match expected type `a' against inferred type `v' `a' is a rigid type variable bound by the type signature for `afun' at /home/titto/.quid2/state/ubuntu.local.8080/wikidata/haskell/package/haskell d/src/HaskellD/Test.hs:7:17 `v' is a rigid type variable bound by the instance declaration ....
Why is that? a and v are both declared to be a Data, why should they not match?
afun's type sgnature says "whatever type you (the caller) want, I can give it to you, as long as it's a member of Data" But v is one particular type belonging to Data, and it's the only one, the implementation of afun can provide. Maybe you want class (Data a) => AFun f a | f -> a where afun :: f -> ([Dynamic] -> a) or, with type families: class AFun f where type RType f :: * afun :: (Data (RType f)) => f -> ([Dynamic] -> RType f) I'm not quite sure whether that compiles, let alone does what you want, but it might be worth a try.