Building data constructors with GHC Generics

Let’s imagine for a moment that we are in the presence of the following data type:

data Coffee = MkCoffee { coffeeBeans :: String
                       , coffeeOriginCountry :: Country
                       , coffeeBrewMethod :: BrewMethod
  deriving (Generic)

However, we have only been given the type itself - and not a constructor for this value. That is, we don’t have access to this function:

MkCoffee :: String -> Country -> BrewMethod -> Coffee

Is it still possible to make Coffee? As we’ll see, by making use of the Generics type class it is indeed possible to make Coffee, and with a little bit of functor tricky, we can derive an implementation of the MkCoffee data value constructor ourselves.

GHC Generics

Before we get going, it’s worth discussing briefly what is meant be the idea of a generic representation, specifically GHC generics. The term “generic programming” is fairly overloaded, but for this post we’re interested in the idea of shape generics. This (and the other types of generic programming) are discussed in José Pedro Magalhães’ thesis “Less is More”, but loosely speaking we’re interested in abstracting over the “shape” of data types. This means we find another value that isomorphic to our, but is made out of a smaller (and importantly, closed) set of primitives.

GHC Generics is one such approach to shape genericity - Rep type is the type of isomorphic data, and we have to and from to move between both sides of the isomorphism. Let’s see what this all means for Coffee:

> :kind! (Rep Coffee)
Rep Coffee :: * -> *
= M1
       (M1 S Main.S1_0_0Coffee (K1 R [Char])
        :*: (M1 S Main.S1_0_1Coffee (K1 R Country)
             :*: M1 S Main.S1_0_2Coffee (K1 R BrewMethod))))

This is neither pretty nor succinct, but thankfully a lot of this is noise (for our purposes). Here’s another look at just the essential structure of Rep Coffee:

K1 [Char] :*: (K1 R Country :*: K1 R BrewMethod)

Now we can see that Coffee is isomorphic to the product of a string, a country and a BrewMethod. These correspond to the three fields in the above Coffee data type.

Using these generic representations, we can construct new Coffees:

> to (M1 (M1 (M1 (K1 "Single Origin") :*: (M1 (K1 (Country "Rwanda")) :*: M1 (K1 V60))))) :: Coffee
MkCoffee { coffeeBeans = "Single Origin"
         , coffeeOriginCountry = Country "Rwanda"
         , coffeeBrewMethod = V60

That’s pretty cool, no? It’s this idea that is at the heart of generic programming. We could imagine doing a similar thing by building these values by reading a binary serialisation from a file, or walking a JSON AST.

However, for our purposes we’ve not yet reached our goal. To recap, we really want to be able to build a function like MkCoffee, keeping the generic representation behind the scenes. To do this, we’ll need to interpret the generic representation into a function.

Interpreting Generic Representations

The standard way to work with a generic representation is to walk the tree using instances of a type class. We’ll do the same, and walk the generic representation to reach a functor that will contain Rep Coffee. Later, we’ll be able to use fmap to, turning Rep Coffee into real Coffee.

Our workhorse is the following type class.

class Functor f => Mk rep f | rep -> f where
  mk :: f (rep a)

As you can see, it’s a multi-parameter type class, taking the generic Rep type, and also indicating which functor can “build” this Rep. Each Rep uniquely determines the constructing functor, which we indicate with a functional dependency. This is essential for having sane type inference.

Starting “at the bottom”, we can begin by constructing a single field. In our Coffee example, we need to construct Strings, Countrys, and BrewMethods. In GHC Generics, each of these is represented with K1. To actually construct K1 we need a value, so our constructing functor will be a one-argument function:

-- Remember that ((->) c) means (c ->)
instance Mk (K1 i c) ((->) c) where
  mk = \x -> K1 x

A Coffee is more than just a single field though, and we need a way to combine individual fields together. This is done by the use of the :*: constructor, which we can think of as having “fields on the left” and “fields on the right”. To construct :*: we need to compose a builder on the left with the builder on the right, so we use Compose to join the left and right functors into one. The definition of mk itself is a little cumbersome, but does the job:

instance (Mk l fl, Mk r fr) => Mk (l :*: r) (Compose fl fr) where
  mk = Compose (fmap (\l -> fmap (\r -> l :*: r) mk) mk)

Finally, we just need to construct the M1 layers, which just hold meta-data. These don’t have any effect on what we’re trying to do, so their type class instance simply proxy through to other instances:

instance (Mk f f') => Mk (M1 i c f) f' where
  mk = M1 <$> mk

Believe it or not, we’re now a good step closer to getting what we want. Let’s have a look and see what we get if we try and mk some Coffee:

fmap (to :: Rep Coffee a -> Coffee) mk
  :: Compose
       ((->) String) (Compose ((->) Country) ((->) BrewMethod)) Coffee

Hey, that’s actually pretty close! If we squint, this is some sort of function that takes a String, a Country and a BrewMethod and yields some Coffee. All that we have to do is somehow get rid of all the Compose noise.

Unwrapping Compose

At this point, I was originally stuck, but then I remembered smart people have already come before me to work on very similar problems. Specifically, Ralf Hinze published a Functional Pearl a while ago called “Formatting: A Class Act”, which uses an identical construction (which I can honestly say was a happy accident!). Hinze then goes a step further with this magic little type class:

class Functor f => Apply f a b | f a -> b where
  apply :: f a -> b

instance Apply ((->) a) b (a -> b) where
  apply = id

instance (Apply g a b, Apply f b c) => Apply (Compose f g) a c where
  apply (Compose x) = apply (fmap apply x)

This time we have a type class of three types (!), where the first two types determine the third. However, it’s not so bad - f is the functor we need to expand, a is the type of data under the functor and b is the final type after expansion. If you look at the specific instances, it should be clear how this all plays out.

Now we can use apply with mk to generate a function!

> apply (fmap to mk) :: String -> Country -> BrewMethod -> Coffee
    No instance for (Apply
                       f0 a0 (String -> Country -> BrewMethod -> Coffee))
      arising from a use of ‘apply’
    The type variables ‘f0’, ‘a0’ are ambiguous
    Note: there are several potential instances:
      instance (Apply g a b, Apply f b c) => Apply (Compose f g) a c
        -- Defined at 2014-04-26-coffee.hs:39:10
      instance Apply ((->) a) b (a -> b)
        -- Defined at 2014-04-26-coffee.hs:36:10

Damn! What went wrong?

Unfortunately, if we use apply with mk we lose parametricity on f itself. There could be many different ways to reach the type we desire (especially as type classes are open), and it turns out that the crucial ingredient is having rep available.

However, from our perspective it should be possible to infer all of this from the return type of the function we are building. Of course, GHC can’t guess, so we will need to encode this information somehow. With GHC 7.8 we can easily express this with a closed type family:

type family Returns (f :: *) :: * where
  Returns (a -> b) = Returns b
  Returns r = r

Returns lets us figure out what the final type of a function is, and now we can complete the loop and build our final make function:

make :: forall b f z. (Generic (Returns b), Apply f (Returns b) b, Mk (Rep (Returns b)) f) => b
make = apply (fmap (to :: Rep (Returns b) z -> (Returns b)) (mk :: f (Rep (Returns b) z)))

We need to use ScopedTypeVariables to carry a bit of extra type information around, but luckily this is all behind the scenes.

Finally, we can now write mkCoffee:

mkCoffee :: String -> Country -> BrewMethod -> Coffee
mkCoffee = make

Does it work?

> mkCoffee "Single Origin" (Country "Rwanda") V60
Coffee { coffeeBeans = "Single Origin"
       , coffeeOriginCountry = Country "Rwanda"
       , coffeeBrewMethod = V60


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