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
