Working with data types parameterized by a functor

Question

I recently defined a type whose fields I might fail to compute:

data Foo = Foo {x, y :: Int, others :: NonEmpty Int}

data Input

computeX, computeY :: Input -> Maybe Int
computeOthers :: Input -> Maybe (NonEmpty Int)

Now, one obvious thing I might do would be to just use liftA3:

foo :: Input -> Maybe Foo
foo i = liftA3 Foo (computeX i) (computeY i) (computeOthers i)

That works fine, but I thought it might be interesting to generalize Foo to hold Maybes as well, and then transform one type of Foo to another. In some similar cases, I could give the Foo type a type parameter and derive Traversable. Then after creating a Foo (Maybe Int), I could invert the whole thing at once with sequenceA :: Foo (Maybe Int) -> Maybe (Foo Int). But this doesn't work here, because my function doesn't give me a NonEmpty (Maybe Int), it gives me a Maybe (NonEmpty Int).

So I thought I'd try parameterizing by a functor instead:

data Foo f = Foo {x, y :: f Int, others :: f (NonEmpty Int)}

But then the question is, how do I turn a Foo Maybe into a Maybe (Foo Identity)? Obviously I can write that function by hand: it's isomorphic to the liftA3 stuff above. But is there some parallel of Traversable for this higher-order type, so that I can apply a more general function to this problem rather than re-doing it with a bespoke function?

Tabard

Answer 1

Such data types are called "Higher-Kinded Data" (HKD). Manipulating them is often done with Generics or Template Haskell.

There are libraries like higgledy which provide built-in functionality for HKD. I believe construct is the function you are looking for:

{-# LANGUAGE DeriveGeneric #-}

import Data.Generic.HKD
import GHC.Generics
import Data.Monoid

data Foo = Foo { x, y :: Int, z :: [Int] }
  deriving (Generic, Show)

emptyFoo :: HKD Foo Last
emptyFoo = mempty

sampleFoo :: HKD Foo Last
sampleFoo = deconstruct (Foo 1 2 [3])

emptyFoo' :: Last Foo
emptyFoo' = construct emptyFoo

sampleFoo' :: Last Foo
sampleFoo' = construct sampleFoo

main = do
  print emptyFoo'
  print sampleFoo'

This will print:

Last {getLast = Nothing}
Last {getLast = Just (Foo {x = 1, y = 2, z = [3])}

---

Edit: I just found out that a much more popular library is barbies (higgledy also depends on barbies). The function that you are looking for is also present in that library as an application of btraverse:

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE UndecidableInstances #-}

import Data.List.NonEmpty
import Barbies
import GHC.Generics
import Data.Functor.Identity

data Foo f = Foo {x, y :: f Int, others :: f (NonEmpty Int)}
  deriving (Generic, FunctorB, TraversableB, ConstraintsB)

deriving instance AllBF Show f Foo => Show (Foo f)

f :: Applicative f => Foo f -> f (Foo Identity)
f = btraverse (fmap Identity)

main :: IO ()
main = do
  print (f (Foo (Just 1) (Just 2) (Just (3 :| []))))

This prints:

Just (Foo {x = Identity 1, y = Identity 2, others = Identity (3 :| [])})

Obstruction