RowLabels type class and LabelList

[LiamGoodacre]

A type class RowLabels to compute the list of labels in a closed row. Defined as:

class RowLabels (row :: # Type) (labels :: LabelList) | row -> labels

Where LabelList is defined by:

foreign import kind LabelList
foreign import data Nil :: LabelList
foreign import data Cons :: Symbol -> LabelList -> LabelList
infixr 6 type Cons as :

The labels computed by RowLabels should be sorted and preserve duplicates, e.g:

RowLabels (b :: String, a :: Int, b :: Boolean) ("a" : "b" : "b" : Nil)

With this it is possible to compute a lot more based on rows. We can check if a row has a label, filter labels, rebuild a row with the associated types transformed in some way, perhaps reflect the list as an array of strings.

Any thoughts, ideas, criticism?

[LiamGoodacre]

With this I think we could remove support of closed records from generics-rep deriving 👍 - so that generics-rep only describes data constructors and their positional arguments.

[LiamGoodacre]

This would also let us define Show for records with closed rows.