liftM2 (/) sum length. Anyway, the closest you can get in Haskell is something like this, using the infix expressions of Ken Shan and Dylan Thurstonhttp://www.haskell.org/pipermail/haskell-cafe/2002-July/003215.html
Hmm.. that might be decent if you added rules to pretty-print them in lhs2tex. The src code would be slightly messy but the formatted code would be very clean. And it opens the doors for other binop decorators. Interesting idea.
I finally get my lhs2tex set up and put together a better example of what I was talking about: http://www.thenewsh.com/%7Enewsham/Hats.pdf Though I don't know if you can coax lhs2tex into accepting something like %format -:liftM2 (+):- = "\widehat{+}" Tim Newsham http://www.thenewsh.com/~newsham/