multi-instance

Alternative versions of common typeclasses, augmented with a phantom type parameter x. The purpose of this is to deal with the case where a type has more than one candidate instance for the original, unaugmented class.

Example: Integer sum and product

The canonical example of this predicament is selecting the monoid instance for a type which forms a ring (and thus has at least two strong candidates for selection as the monoid), such as Integer. This therefore gives rise to the Sum and Product newtype wrappers, corresponding to the additive and multiplicative monoids respectively.

The traditional fold-based summation of a list of integers looks like this:

>>> import Data.Foldable (fold)
>>> import Data.Monoid (Sum (..))
>>> getSum (fold [Sum 2, Sum 3, Sum 5]) :: Integer
10

By replacing fold with multi'fold, whose constraint is MultiMonoid rather than Data.Monoid.Monoid, we can write the same thing without the newtype wrapper:

>>> :set -XFlexibleContexts -XTypeApplications
>>> multi'fold @Addition [2, 3, 5] :: Integer
10