Adding a Control.Arrow.Transformer.Error.ErrorArrow to an
arrow type, but not necessarily as the outer arrow transformer.
Typically a composite arrow type is built by applying a series
of arrow transformer to a base arrow (usually either a function
arrow or a Kleisli arrow. One can add a transformer to the
top of this stack using the Control.Arrow.Transformer.lift
method of the Control.Arrow.Transformer.ArrowTransformer class,
or remove a state transformer from the top of the stack using the
Control.Arrow.Transformer.Error.runError encapsulation operator.
The methods of this class add and remove state transformers anywhere
in the stack. In the instance
instance Arrow a => ArrowAddError ex (ArrowError ex a) a
they are equivalent to Control.Arrow.Transformer.lift and
Instances are lifted through other transformers with
instance ArrowAddError ex a a' =>
ArrowAddError ex (FooArrow a) (FooArrow a')
This could be combined with Control.Arrow.Transformer.Error.handle,
since the resulting arrow is always the arrow of the handler.
Separating them has the advantage of consistency with the other arrows,
and might give more helpful type error messages.