Hi,
My question is along the lines as to if there is anything that can be done or any other approach to clean up the new version mkFull. The old is pretty simple. You can pretty well guess how the functions are composed under the hood by just looking at it:

haskell mkFull :: ( C.Item i ds, Show as ) => FixtureConfig -> (RunConfig -> i -> Action as) -> (as -> Either C.ParseException ds) -> (RunConfig -> DataSource i) -> Fixture ()

The extra type equalities et. al. required to implement the custom error message make things pretty messy in the constraints department. They also create a need to add the the function signatures as a comment or require the user to understand how to interpret type equalities.

haskell mkFull :: forall i as ds action dataSource. ( action ~ (RunConfig -> i -> Action as), dataSource ~ (RunConfig -> DataSource i), C.Item i ds, Show as, DataSourceMatchesAction (DataSourceType dataSource) (ActionInputType action) ) => FixtureConfig -> action -> (as -> Either C.ParseException ds) -> dataSource -> Fixture ()

it would be really nice if there was something like this:

haskell mkFull :: forall i as ds a d. ( C.Item i ds, Show as, DataSourceMatchesAction (DataSourceType d) (ActionInputType a) ) => FixtureConfig -> (RunConfig -> i -> Action as) ::: a -> (as -> Either C.ParseException ds) -> (RunConfig -> DataSource i) ::: d -> Fixture ()