There's nothing wrong with the definition of FooGraph. That's how you do it. -- Lennart On Tue, Feb 24, 2009 at 6:09 PM, 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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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. [...] but this isn't allowed (kind mismatch).

Kinds are a meta-type system for types. Because Haskell has such a rich type system, the types themselves need a type-like system. These are "kinds". You never declare kinds (apart from certain language extensions not in use here): the compiler infers them. This suggests that your problem is in the types lower down. Probably you are using "a" or "b" in a way that implies they take a type argument in one place (kind * -> *) and not in another place (kind *).