Is there a way to apply Maybe constructor to each field of record with generics?

Question

I have two data types and the second one is the copy of first, but with Maybe on each field.

data A = {a :: Int, b :: String}
data B = {c :: Maybe Int, d ::          


        

Answer 1

How about:

{-# LANGUAGE RankNTypes #-}

data R f = R { a :: f Int, b :: f String, c :: f Char }

newtype I a = I { unI :: a }

fromMaybeI :: I a -> Maybe a -> I a
fromMaybeI a Nothing = a
fromMaybeI _ (Just a) = I a

fromMaybeR :: R I -> R Maybe -> R I
fromMaybeR ri rm =
  R (go a) (go b) (go c)
  where
    go :: (forall f. R f -> f a)  -> I a
    go x = fromMaybeI (x ri) (x rm)

R Maybe is the record with Maybe values, R I is the record with concrete values.

Using RankNTypes reduces the amount of boilerplate code in fromMaybeR.

One downside is that you have use I and unI to construct and access the field values.