Adding maybeness to a Haskell data type (with record syntax)

Question

Looking at an answer to this question:

https://stackoverflow.com/a/34164251/1052117

I see that it defines a data type that is used to parse a JSON object.

data Address = Address
    { house  :: Integer
    , street :: String
    , city   :: String
    , state  :: Maybe String
    , zip    :: String -- here I change the original, zip codes are strings, they have leading zeros.
    } deriving (Show, Eq)

$(deriveJSON defaultOptions ''Address)

This is helpful, but I wonder: How could I change the Address data type to have all json fields be nullable? Specifically I see a Maybe before the state field, but I'm imagining a larger data structure where it would be tedious to modify all of the fields to Maybe fields. For example, while I /could/ re-write above as:

data Address = Address
    { house  :: Maybe Integer
    , street :: Maybe String
    , city   :: Maybe String
    , state  :: Maybe String
    , zip    :: Maybe String
    } deriving (Show, Eq)

What function could I apply to the Address data type /in code/ to achieve this same result without rewriting all the code and manually inserting the Maybes?

Answer 1

As is discussed in the comments, using a functor-functor would work for this with just very small changes to the original data type.

If you start out with

data Address = Address
    { house  :: Integer
    , street :: String
    , city   :: String
    , state  :: Maybe String
    , zip    :: String
    } deriving (Show, Eq)

then it is equivalent to

import Data.Functor.Identity

data AddressF f = Address
  { house  :: f Integer 
  , street :: f String
  , city   :: f String
  , state  :: Maybe String
  , zip    :: f String 
  } deriving (Show, Eq)

type Address = AddressF Identity

and then you can get the second one by writing

type Address' = AddressF Maybe

To get back to the original definition, you can write

toOriginal (AddressF (Identity house) (Identity street) (Identity city) mbState (Identity zip)) = Address house street city mbState zip