foldMap for generic data types.

foldMap can be considered a two-step process:

• map every element a of a t a (where Foldable t) to some Monoid m
• combine elements using (<>)

Applying this to generic data types:

• map every field of a constructor to some Monoid m
• combine elements using (<>)

Field mappings are handled using a per-monoid type class. You need a monoid m with an associated type class which has a function a -> m. Write a GenericFoldMap instance for your monoid which points to your type class. If a field type doesn't have a matching instance, the generic instance emits a type error.

Sum types (with multiple constructors) are handled by (<>)-ing the constructor with its contents (in that order). You must provide a String -> m function for mapping constructor names. If you need custom sum type handling, you may write your own and still leverage the individual constructor generics.

This function can provide generic support for simple fold-y operations like serialization.