[Haskell-cafe] MPTC inheritance - was operator error
derek at solidmath.com
Tue Feb 24 14:33:13 EST 2009
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
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 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
> 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
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