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
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