On Thu, Mar 24, 2011 at 11:36 AM, Simon Peyton-Jones <simo...@microsoft.com> wrote: > | class Monoid (GeneratorOf a) => Generable a where > | type GeneratorOf a :: * -> * > | construct :: GeneratorOf a -> a > | > | Now, it seems I need FlexibleInstances to do this when I'm using an > | associated type synonym, but I don't need the flexibility when using a > | multiparameter type class. > > > Suppose you have these wierd instances: > type instance GeneratorOf (Tree a) = Tree (Tree a) > instance Generable a => Monoid (Tree a) > instance Generable (Tree a) > > Now, in the last of these we need to cough up an instance of Generable (Tree > a)'s superclasses. Ah, that's Monoid (GeneratorOf (Tree a)) > Ah, that's Monoid (Tree (Tree a)) > We have an instance of Monoid, but it needs, well Generable (Tree a), which > is where we started. > > If I'd nested things a bit more deeply you can see I'd get into an infinite > regress. So you have to take responsibility that instance solving will > terminate, hence FlexibleInstances. > > As you say, the same thing can happen with fundeps. The fact that the thing > is allowed is probably a bug in the Fundep stuff.
Thanks, it's good to know that I was, in fact, being naughty in both instances (and not merely being constrained from doing Good Things by the typing rules for associated types). Back to the drawing board. -Jan _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
