Overriding Type Class Instances (Part 2)

March 25, 2020

As we discovered in part 1, we can build some infrastructure to override instances for a type class we’ve created. But what if we want to do this for an existing type class? In most cases, we likely won’t be able to tamper with the source code to inject our override machinery. Furthermore, we’d like to be able to achieve this with as little boilerplate as possible.

Spoiler Alert

The goal I set out to achieve was to end up with the following syntax. Note that this allows us to override type class instances by type or field name.

The idea is that we can piggyback off of the existing generic machinery used by ToJSON. We will use DerivingVia to derive an instance of ToJSON MyRec via Override MyRec with a type-level list specifying the type substitutions to make.

In our example, we are saying that when deriving a ToJSON MyRec instance, for its fields, instead of ToJSON String use ToJSON CharArray and, for the baz field, instead of ToJSON Text use ToJSON Uptext. These alternate instances will be provided via newtype wrappers, something we’re likely all familiar with at this point.

Now, via the derived ToJSON MyRec instance, we should be able to serialize this Haskell value -

into this JSON -

Ok, now on to making this dream come true.

The Approach

How can we possibly make this work for any type class? Well let us consider how generic derivation works in the first place.

The ToJSON type class delegates to genericToJSON for its default implementation -

It’s not too important to know the implementation of genericToJSON for our uses, so we’ll just worry about its type signature.

So it has its own generic version of ToJSON called GToJSON which operates on Rep values. Rep is an associated type family introduced by the Generic type class which tells us the generic representation of its type argument.

To give a concrete example, we can observe the generic representation type for our MyRec type from earlier using GHC.Generics.from -

A bit noisy, but you can see that the structure of our MyRec type is represented here.

Here’s the important part!

What we can do is create our own type class that will produce a different representation when we are passing things through our Override type. We can replace the leaves of the generic representation with another type that will do the override magic for us. We’ll call this type Overridden.

As you can see, the leaf nodes of our generic representation have now been tagged as Overridden and include the field name, underlying type, and type-level list of overrides.

Making it really work for real

First, we’ll need to define the types needed for this syntax.

The entry point for our generic override machinery will be the Override newtype.

This takes two type parameters -

  • a - The type which contains fields for which we will be overriding instances during generic derivation
  • xs - A type-level list of overrides

Next, we need to define the As type which can be conveniently used infix via TypeOperators. This is similar to how it was defined in part 1 except this time it uses PolyKinds so we can target either a concrete type or a field name to override.

This takes two type parameters -

  • o - The target to be overridden. Should be either the concrete type to or field name to override.
  • n - The type to replace o with.

Now we’ll create the Overridden type used at the leaf nodes of the generic representation of an Override type.

This has the same type parameters as Override except for ms. The ms holds an optional type-level string which, when available, will contain the field name for a leaf node we can override.

We’ll also need to introduce a type family called Using which will be used to “pick” which instance to use from a list of overrides.

The use of PolyKinds with our As type allows us to override by Symbol or concrete type. The ms and x supplied to Using will be the same Maybe Symbol and a as from Overridden. This allows us to conditionally match on the field name or type.


In order for us to build our own Rep for our Override type (instead of relying on the one provided by GHC) we’ll need to introduce a new type class.

The OverrideRep is analogous to the Rep from GHC.Generics, except it takes a Rep and produces a new one which has Overridden injected at its leaves. The same goes for overrideFrom and overrideTo, which are analogous to from and to from GHC.Generics but perform runtime injection of Overridden at the leaves of the generic representation.

We can then define the instance for Generic Override as follows, delegating to our GOverride type class.

We can then define instances for GOverride which operate on the generic representation nodes -

That last instance is probably the most important. We extract both the M1 S and K1 so we can supply the optional field name ms and field type c to Overridden.

Aeson Glue

Now we’ll talk about writing glue code which binds our generic override machinery to the aeson library, specifically, the ToJSON type class.

As previously mentioned, ToJSON uses genericToJSON for generic derivation -

It has another function, toEncoding, which is generically derived with, you guessed it, genericToEncoding -

So we can now write an instance for ToJSON Override which leverages the default implementations for ToJSON -

In this case, we don’t have to implement the toJSON and toEncoding methods; the default implementations are exactly what we would have written.

Next we’ll need to implement the instance for the Overridden type which was injected at the leaves of our generic representation.

And that’s it! We now have generic override support for ToJSON!

Exploring Overrides

Let’s revisit our original MyRec declaration.

Now let’s print out its JSON representation.

Nice, it works!

Let’s explore the possibilities for a moment using a function for easily creating overrides on the fly.

This allows us to specify ad hoc overrides as a proxy and summon the appropriate instances as such. Let’s try it out!

No overrides will give us the default generically derived instance.

We can specify a single override as before.

We can also specify an override for a single field and a default for all other fields matching a type.

We can even tell a field to use its original instance, overriding the override!

What if we get the types wrong? Is it safe? Yes!

<interactive>:121:1: error:
    • Couldn't match representation of type ‘Text’
                               with that of ‘[Char]’
        arising from a use of ‘enc’
    • In the expression:
        enc $ override r $ Proxy @'[String `As` Uptext]
      In an equation for ‘it’:
          it = enc $ override r $ Proxy @'[String `As` Uptext]

This also works for fields just as well.

<interactive>:122:1: error:
    • Couldn't match representation of type ‘Text’
                               with that of ‘[Char]’
        arising from a use of ‘enc’
    • In the expression: enc $ override r $ Proxy @'["bar" `As` Uptext]
      In an equation for ‘it’:
          it = enc $ override r $ Proxy @'["bar" `As` Uptext]

There is one gotcha, however, and that is that you will want to supply field overrides first and type overrides last.

For those in which “just get it right” is not an acceptable solution, we could devise some sort of ValidateOverride type class that, upon attempting to do any sort of overriding, checks for situations like this and reports them as compiler errors. Or we could specialize the Using type family to prefer fields over types in overrides.

However, I think it’s always a good idea to test your codecs and instances anyway, especially when doing any sort of specialization, so until such machinery were implemented this is probably an acceptable gotcha to be aware of.

That’s it for now

There’s still likely more exploration and work to be done in this area, but so far I’ve packaged this up into a workable library with tests and more examples.