[Haskell-cafe] Inferred type is not general enough

[Ivan Tihonov]

I start some ip networks related work in haskell and wrote two basic 
classes Location and Packet. Before writing IpLocation and IpPacket 
instances i have written simple TestLocation and TestPacket instances 
just to compile this and check for errors in class definitions. But 
looks like i misunderstand some haskell principles...

======================================================
class Location a where
         point :: a -> String

class Packet a where
         source, destination :: Location b => a -> b
         size :: Num b => a -> b

------------------------------------------------------

data TestLocation = TestSource | TestDestination
data TestPacket = TestPacket

------------------------------------------------------

instance Location TestLocation where
         point a = "location"

instance Packet TestPacket where
         source p = TestSource
         destination p = TestDestination
         size p = 99


$ hugs Test.hs

ERROR Test.hs:20 - Inferred type is not general enough
*** Expression    : source
*** Expected type : (Packet TestPacket, Location a) => TestPacket -> a
*** Inferred type : (Packet TestPacket, Location TestLocation) => 
TestPacket -> TestLocation

======================================================

But if i remove source and destination from class and instance 
definitions alone "size" compiles well.

How write this in a haskell way?