On 22 Feb 2015, at 09:56, Edward Kmett <ekmett@gmail.com> wrote:

Many instances for data types in Data.Monoid have already been added in 7.10.

Are these still missing?

-Edward

On Sat, Feb 21, 2015 at 10:34 AM, Oleg Grenrus <oleg.grenrus@iki.fi> wrote:

I propose to add Functor, Applicative, Monad, Foldable, and Traversable and maybe even MonadFix instances to wrapper newtypes in the Data.Monoid module.
The same way as in the semigroups package, e.g. https://hackage.haskell.org/package/semigroups-0.16.1/docs/Data-Semigroup.html#t:Min <https://hackage.haskell.org/package/semigroups-0.16.1/docs/Data-Semigroup.html#t:Min>

Basically:

instance Functor Sum where
  fmap f (Sum x) = Sum (f x)

instance Foldable Sum where
  foldMap f (Sum a) = f a

instance Traversable Sum where
  traverse f (Sum a) = Sum <$> f a

instance Applicative Sum where
  pure = Sum
  a <* _ = a
  _ *> a = a
  Sum f <*> Sum x = Sum (f x)

instance Monad Sum where
  return = Sum
  _ >> a = a
  Sum a >>= f = f a

instance MonadFix Sum where
  mfix f = fix (f . getSum)

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