Am Freitag, den 15.02.2019, 19:04 -0500 schrieb theindigamer:
Thoughts on adding a newtype with the following Alternative instance? (The name chosen is a placeholder, I don't have any particular preference).
newtype Mon f a = Mon { unMon :: f a }
instance (forall a. Monoid (f a)) => Alternative (Mon f) where
empty = Mon mempty
(<|>) (Mon m1) (Mon m2) = Mon (m1 <> m2)
If I remember correctly, then something that is an `Applicative` and a `Monoid` isn’t necessarily an `Alternative` and something that is a `Monad` and a `Monoid` isn’t necessarily a `MonadPlus`. There are certain axioms that are supposed to hold, which link the `Applicative` or `Monad` structure to the `Monoid` structure. At least this is how it ought to be. For details see the paper [“From Monoids to Near-Semirings: The Essence of `MonadPlus` and `Alternative`”][*]. All the best, Wolfgang [*]: https://usuarios.fceia.unr.edu.ar/~mauro/pubs/FromMonoidstoNearsemirings.pdf "From Monoids to Near-Semirings: The Essence of `MonadPlus` and `Alternative`"