I would suggest that the coerce-based instances be stolen from
Data.Identity.
On Feb 21, 2015 10:34 AM, "Oleg Grenrus"
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.ht... < https://hackage.haskell.org/package/semigroups-0.16.1/docs/Data-Semigroup.ht...
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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