What's the practical value of all those newtype wrappers in `Data.Monoid`?

Question

When looking at Data.Monoid, I see there are various newtype wrappers, such as All, Sum, or Product, which e

Answer 1

Suppose you are working in the Writer monad and you want to store the sum of everything you tell. In that case you would need the newtype wrapper.

You would also need the newtype to use functions like foldMap that have a Monoid constraint.

The ala and alaf combinators from Control.Lens.Wrapped in the lens package can make working with these newtypes more pleasant. From the documentation:

>>> alaf Sum foldMap length ["hello","world"]
10

>>> ala Sum foldMap [1,2,3,4]
10