To clarify the discussion:
Let's add the following instances:
A: Functor, Foldable, Traversable to Sum, Product, Dual, First, and Last
B: Data to everything in Data.Monoid (except Endo): Sum, Product, Dual, First and Last, All, Any, Alt
C: Applicative, Monad, MonadFix to Sum, Product, Dual, First and Last
D: Data to Identity
All seem to agree on A and B, C is "not sure", and D is new item.
- Oleg
On 21 Feb 2015, at 17:34, 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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