[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?
_______________________________________________
Haskell-Cafe mailing list
[EMAIL PROTECTED]
http://www.haskell.org/mailman/listinfo/haskell-cafe