On Sun, Apr 18, 2010 at 5:01 AM, Limestraël
There must be some kind of a private joke I don't get...
BTW, all you've said is pretty scaring...
And somewhat exaggerated, of course. Reasonable uses exist for all three extensions, but they're firmly in the category of "avoid unless you know what you're doing". Well, at least two of them are, I'm not sure when IncoherentInstances is a good idea (if ever). It's worth experimenting with them in some toy code for a while before trying to use them "for real". In any case, if you do use those extensions, they can usually be isolated to some extent. A library can use them internally without requiring client code to enable them, and in an application use can be restricted to just a few modules, enabling the extensions on a per-module basis. My rule of thumb is the "sausage principle"--outside code should be able to act as if GHC somewhere picked up a more expressive means of specifying instance heads and/or a smarter termination checker and carry on blissfully ignorant of by what providence the instances were obtained. That is, if one eats sausage, it is best to not dwell on how it is made, so to speak.
It's strange I can't declare a generic instance for Binary types... I thought I was trying to do something quite common in Haskell. Apparently I'm still a young padawan with many things to learn. Anyway, it's not the first time I get worked up with multi-param typeclasses and functionnal dependencies....
What you're trying to do is perfectly reasonable, unfortunately it doesn't mesh well with how type classes/instances work. A lot of the reason why the distressing extensions under discussion exist at all is working around those limitations. Type families are a start on cleaning up one aspect of the type class system--namely, the awkwardness of functional dependencies. Unfortunately, type families don't really help on the "how to write generic but not completely general instances" right now, and in fact are incompatible with overlapping instances, making some things impossible! I think there have been some discussions of proposals toward fixing this as well, but I'm not sure what the status of those are. - C.