[Haskell-cafe] representing Haskell objects in a uniform way

[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/haskelld/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?


The full code follows:

{-# LANGUAGE FlexibleInstances ,UndecidableInstances ,OverlappingInstances #-}
import Data.Data

import Data.Dynamic

class AFun f where
    afun :: Data a => f -> ([Dynamic] -> a)

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"

instance Data v => AFun v where
  afun f [] = f
  afun _ _  = error "Too many arguments"


Thanks in advance,

               titto