Jerzy Karczmarczuk <jerzy.karczmarc...@unicaen.fr> wrote: > Le 26/03/2012 01:51, Chris Smith a écrit : > >> instance (Num a) => Num [a] where >> xs + ys = zipWith (+) xs ys >> >> You can do this in the sense that it's legal Haskell... but it is a bad idea >> [...]
> It MIGHT be a ring or not. The "real problem" is that one should not confuse > structural and algebraic (in the "classical" sense) properties of your > objects. Of course there are rings for which it's possible to represent the elements as lists. Nevertheless, there is definitely not one that defines (+) = zipWith (+), as did the one I was responding to. By the time you get a ring structure back by some *other* set of rules, particularly for multiplication, the result will so clearly not be anything like a general Num instance for lists that it's silly to even be having this discussion. -- Chris Smith _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
