Thanks to the kind replies I got off-list, I figured out that this was actually operator error on my part. class (Node a, Edge b) => Graph a b was actually class (Node a, Edge b) => Graph g a b but this was in code I'd written quite some time ago and I've written a lot of day-job C++ in the intervening period. Main lesson learned: Read the ghc error message carefully, it contains all the information you need. Thankyou once again. - Derek Derek Gladding wrote:
Please forgive me if I'm still mentally contaminated by the OO way of seeing (and discussing) the universe, but I'm trying to figure out how to "inherit an interface" from a multi-parameter type class.
I have a Graph class that's parameterisable by Node and Edge type:
class (Node a, Edge b) => Graph a b where (lots of stuff that you can do with Graph a b)
Now, I'd like to build a FooGraph on top of this that adds additional capabilities:
class (Graph a b) => FooGraph a b where (lots of additional stuff)
but this isn't allowed (kind mismatch).
Of couse, I can do:
class (Node a, Edge b) => FooGraph a b
but this means that I have to manually replicate the Graph a b operations in the FooGraph a b class definition, which is (a) work that the machine should (?) be able to do for me, and (b) fragile.
Any pointers / wisdom would be very much appreciated.
- Derek
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe