Re: Drastic Prelude changes imminent

Actually, I don't consider this instance a hack. One of the things that convinced me of the elegance of this `Writer`-like instance for tuples is that you can use it to implement another instance: instance (Monoid a, Monoid b) => Monoid (a, b) where mempty = pure mempty mappend = liftA2 mappend This is a special case of the more general pattern of lifting `Monoid`s over `Applicative`s: instance (Applicative f, Monoid b) => Monoid (f b) where mempty = pure mempty mappend = liftA2 mappend On 2/11/15 1:01 PM, Henning Thielemann wrote:

Am 01.02.2015 um 23:02 schrieb Edward Kmett:

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On Sun, Feb 1, 2015 at 4:39 PM, Iavor Diatchki mailto:iavor.diatchki@gmail.com> wrote:

Really? This is not obvious at all. Is it because of the odd `Applicative` instance for pairs? I am saying 'odd' because there are no similar instances for tuples of higher arity; there are also no instance for `Monad` for the same type, which makes as much sense, I guess. Obviously this is just an opinion, but I don't think these instances fit pairs naturally, and make for confusing looking code.

instance Monoid m => Monad ((,) m)

is the "unnamed" writer monad that corresponds to the

That looks like a hack which requires another hack (namely moving Monoid to Prelude). Would it have been too difficult to use the Writer monad from transformers or mtl? Btw. I think if I use this instance this is certainly by accident. I prefer to be explicit about my programming intentions and would use the Writer monad.

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