On 13-09-18 08:54 AM, Roman Cheplyaka wrote:
* Jacques Carette <care...@mcmaster.ca> [2013-09-18 08:21:51-0400]
Could someone please explain what the difference (if any!), in
semantics is between
class Foo f => Bar f g where
method1 :: f a -> g a
and
class Bar' g where
method2 :: Foo f => f a -> g a
Bar is more flexible than Bar'. If you have n types, you can write n^2
Bar instances (potentially having very different semantics) to convert
between them.
You can only write n Bar' instances, on the other hand. In these
instances you can dispatch based on 'g' but not on 'f'. 'f' is abstract,
and you only can access it through the Foo interface.
So they are quite different.
Right, that makes sense, thanks.
Turns out that, in my case, I frequently want the Bar' case, as I want
things to be fully polymorphic in Foo.
Jacques
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