On Sun, Sep 09, 2007 at 12:58:25PM +0100, Conor McBride wrote:
My usual rule of thumb is that inherent natural monoidal structure should have a higher priority than just applicative lifting of monoidal structure from the value type. Here it's a bit harder to call because it's a choice of two lifts, but I'd suggest that (Cont r) has natural monoidal structure if r is a monoid, and hence this should take precedence over the applicative lifting (which is what liftM2 does, but with too restrictive a type).
I guess my fairly feeble reason for this rule of thumb is that it fits with what one would expect for []. It may also be to do with the way I read types: when I see a type application (f t), I think of it as an f-like thing with t-like details, hence the natural properties of f are somehow the more significant. Moreover, preferring the f-specific thing is no big deal if you have a cheap and uniform way to get the other thing (eg, liftA2 or idiom brackets). The f-specific thing usually requires special consideration, hence benefits most from overloading.
Okay, conversion successful. I want your instance now. :) Stefan