Generic.Data.Internal.Traversable

Generic implementation of Foldable and Traversable.

There is already a naive implementation using the generic Rep's own instances of Foldable and Traversable. However, deriving then generates a lot of code that may not be simplified away by GHC, that results in unnecessary run-time overhead.

In contrast, this implementation guarantees that the generated code is identical to stock-derived instances of Foldable and Traversable, which have the following syntactic properties:

• constructors with zero fields use pure once;

• constructors with one field use fmap once;

• constructors with n >= 2 fields use liftA2 once and (<*>) n-2 times.

The heavy lifting is actually done by the ap-normalize library.