Excerpts from Edward Kmett's message of Fri Sep 18 16:04:47 +0200 2009:
I'ved chewed on the associativity and precedence issue for <> a little bit. Here are my thoughts.
infixl 6 <> would match the precedence of +, which is nice on paper, and how I happen to implement (+) in Data.Monoid.Sugar in the monoids library. I now believe that it is not quite right.
On paper infixr vs. infixl is basically irrelevant because the result is monoidal, but in practice infixr tends to work better for a lot of real world monoids we care about like list appending. Take a look at the behavior of ((... ++ b) ++ c) vs (a ++ (b ++ ...) for a compelling justification for why infixr is probably better in practice for the poster child of the monoid lineup.
Ross's infixr 5 >< in Data.Sequence is the same precedence and fixity as ++, which also seems like a good answer at first, but infixr 5 <> would change the behavior of programs that use Text.PrettyPrint.HughesPJ, which relies on the fixity of <> and <+> being higher than $$ and $+$ which are infixl 5.
The original proposed infixr/l 4 is low enough that you couldn't even mix the use of <> with the various comparators from Eq and Ord, and it exacerbates all of the above issues -- by inverting the precedence of $$ and <> -- so I think it should be off the table. For similar reasons I don't like lowering the fixity off $$ and $+$ in HughesPJ to 4 to permit infixr 5 <>.
So, in light of all of that, it would seem that the most compatible general change would be to set:
Good work.
infixr 6 <>
+1 -- Nicolas Pouillard http://nicolaspouillard.fr