Generics example: creating Monoid instances

October 2, 2012

GravatarMichael Snoyman

I recently was working on a project which included a very large datatype for holding configuration data. The configuration data was parsed from a file. One trick was that each config file could reference another "parent" config file. The desired semantics are that the settings in the "child" override those in the parent. To make things a bit more concrete, consider something like:

data Config = Config
    { userLanguage :: Text -- only one allowed
    , translationFolder :: [FilePath] -- can be many
    }

Obviously, I was dealing with many more fields. The trick for dealing with the parent folders was simple; my algorithm looked like:

parseConfig :: FilePath -> IO Config
parseConfig fp = do
    doc <- readFile fp -- it's stored as XML, but that's irrelevant
    let parentFPs = getParents doc
    parents <- mapM parseConfig parentFPs
    let config = getConfig doc
    return $ mconcat $ config : parents

In other words, we just use a Monoid instance to put together the different config files. To simplify the task of creating this Monoid instance, I made sure to add appropriate Monoid wrappers to each field as necessary. For the example above, I would add First to userLanguage, since we only wanted to get the first one. For translationFolder, since we want to grab the folders from the parents in addition to the child, we'd leave it as a list. Then, the Monoid instance is just some boilerplate:

instance Monoid Config where
    mempty = Config mempty mempty
    Config a b `mappend` Config x y = Config (a `mappend` x) (b `mappend` y)

Of course, writing such an instance by hand quickly becomes tedious. What I wanted was some way to generate that boilerplate automatically. And the solution I found was GHC 7.4's new Generics implementation. The code I wrote is heavily based on the GHC documentation, which happens to be a great coverage of the topic. The docs give the example of serialization, which uses a unary function. Implementing Monoid involves a nullary and a binary function, which makes it a good follow-up to the serialization example.

(Note: full code available as a Github gist.)

The first step is to create a generic version of our Monoid typeclass:

class GMonoid f where
    gmempty :: f a
    gmappend :: f a -> f a -> f a

This looks very similar to our standard Monoid typeclass. One tweak is the fact that the instance of the typeclass now takes an argument (a.k.a., it's of kind * -> * instead of *). The reason is that the Generics datatypes all have a phantom type variable. My understanding is that this type variable is currently unused.

Once we have this typeclass, we need to create instances for the different Generic datatypes. There are five datatypes available: U1, K1, M1, :+:, and :*: (please see the linked documentation for an explanation). Thankfully, most of these instances are incredibly straight-forward.

Our first instance is for U1, which represents a nullary constructor. In non-generic world, it's easy to deal with this case. mempty would just be the constructor, and mappending two identical nullary constructors should result in the same constructor. The generic version is just as simple:

instance GMonoid U1 where
    gmempty = U1
    gmappend U1 U1 = U1

Next, let's consider product types, e.g. data Foo = Foo Bar Baz. mempty would want to take advantage of the mempty provided for Bar and Baz. mappend would like to mappend the fields in the left and right Foo. We can express this almost identically in the generic version:

instance (GMonoid a, GMonoid b) => GMonoid (a :*: b) where
    gmempty = gmempty :*: gmempty
    gmappend (a :*: x) (b :*: y) = gmappend a b :*: gmappend x y

Sum types are a bit trickier. It's not immediately clear what the right thing to do is. Consider the datatype data Foo = Foo1 Bar | Foo2 Baz. Should mempty use the first or second constructor? As for mappend, if both input values use the same constructor, the solution is relatively simple. But what happens if we have something like mappend (Foo1 x) (Foo2 y)? There's no obvious solution.

So I decided to just leave off the sum type instance. What's wonderful about the generics implementation is that this means, at compile time, trying to use the generics code will fail on any sum type.

Nonetheless, for completeness sake, I did put together an instance. Its semantics are to arbitrarily choose the first constructor for mempty, and the first argument to mappend if there's a constructor conflict. This looks like:

instance (GMonoid a, GMonoid b) => GMonoid (a :+: b) where
    gmempty = L1 gmempty
    gmappend (L1 x) (L1 y) = L1 (gmappend x y)
    gmappend (R1 x) (R1 y) = R1 (gmappend x y)
    gmappend x _ = x

Ultimately, we'll end up hitting non-generic values (the actual values contained by our datatype). At that point, we want to switch over to standard Monoid functions. Again, the generics implementation will prevent us from using datatypes which are not instances of Monoid.

instance Monoid a => GMonoid (K1 i a) where
    gmempty = K1 mempty
    gmappend (K1 x) (K1 y) = K1 $ mappend x y

And finally, we need to deal with the M1 datatype, which is just a metadata container:

instance GMonoid a => GMonoid (M1 i c a) where
    gmempty = M1 gmempty
    gmappend (M1 x) (M1 y) = M1 $ gmappend x y

Now that we've implemented all of our instances, how do we use this? The Generic typeclass provides two methods: to and from, to convert a generic representation to a value and vice-versa. So we just take advantage of those, together with our generic gmempty and gmappend, to come up with default mempty and mappend functions:

def_mempty :: (Generic a, GMonoid (Rep a)) => a
def_mempty = to gmempty

def_mappend :: (Generic a, GMonoid (Rep a)) => a -> a -> a
def_mappend x y = to $ from x `gmappend` from y

If we had control of the Monoid typeclass ourselves, we could also use the DefaultSignatures extension right now to bake this directly into the Monoid typeclass. Then, any time we wrote instance Monoid Foo, it would use def_mempty and def_mappend. However, in our case, we have to do it manually:

instance Monoid Config where
    mempty = def_mempty
    mappend = def_mappend

Still much cleaner than having to write it all out manually.

If you're looking for a more sophisticated example of generics usage, check out the ToJSON and FromJSON typeclasses in the aeson package.

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