Problems with products, part 2

Higher-kinded products

Definitions carried over from part one:

newtype Decode k v a = Decode{ decode :: Map k v -> a }
newtype Encode k v a = Encode{ encode :: a -> Map k v }
data Codec k v a = Codec{ co :: Encode k v a, dec :: Decode k v a }

I first saw the following technique used with optparse-applicative, though I cannot remember where. Here is a typical-looking type that m…