Announcing Evoke, a GHC plugin for deriving type class instances quickly

by Taylor Fausak on

I’m excited to announce Evoke, a GHC plugin that automatically derives type class instances without using generics or Template Haskell. What follows is essentially a copy-paste of the README. You can find the latest documentation on GitHub.


In Haskell there are many ways to provide type class instances for data types. As a motivating example, let’s say you have the following data type:

data Person = Person
  { name :: String
  , age :: Int

And let’s also say that you want to send and receive this data type over the wire as JSON. Typically in Haskell you would use the Aeson library and implement the FromJSON and ToJSON instances. The simplest and easiest way to do that is to write the instances by hand, like this:

instance FromJSON Person where
  parseJSON = withObject "Person" $ \ object -> do
    theirName <- object .: "name"
    theirAge <- object .: "age"
    pure Person
      { name = theirName
      , age = theirAge

instance ToJSON Person where
  toJSON person = object
    [ "name" .= name person
    , "age" .= age person

This is not difficult, but it’s tedious and error prone. It’s tedious because the instance isn’t doing anything interesting — it’s all boilerplate. (What’s worse: We haven’t even written all the boilerplate! We should have implemented toEncoding for better performance, but we didn’t.) And it’s error prone because we have to type all the key names correctly, get them matched up to the correct fields, and make sure that FromJSON and ToJSON agree with each other.

Thankfully GHC has a couple ways to make this easier. The first is Template Haskell (TH), and thankfully the Aeson library provides some TH functions for generating instances. Instead of writing the above instances, we could simply write this:

$( deriveJSON defaultOptions ''Person )

The generated instance would be similar to what we wrote by hand, but we don’t have to actually write it. And if we update the data type, the instance will be updated automatically to match.

Unfortunately this approach has one major problem: It forces the module to be recompiled more often. If any of this module’s transitive dependencies changes, this module will be recompiled. For a small project that’s not too bad, but for a large project that means TH forces you to recompile way more often.

Fortunately there’s another approach that supports generating the instances without causing unnecessary recompilations: generics. It’s also simple to use:

data Person = Person
  { name :: String
  , age :: Int
  } deriving (Generic, FromJSON, ToJSON)

Again the generated instances will behavior similarly to our hand-written instances, but we don’t have to worry about keeping everything in sync. This feels like the best of all worlds, so what’s the problem?

Generics has one major problem: It’s slow to compile. Unlike TH, it avoids recompiling more often, but when it does recompile it’s much slower. And just like TH, this is fine for small projects but for large projects it means you’ll spend a lot of time waiting for generic type class instances to compile.

So what can you do? Ideally we could fix the recompilation checking for TH and/or fix the compile time performance for generics. Doing either of those things feels daunting to me, so I looked for alternative approaches. I saw that it was technically possible for GHC plugins to generate instances, so I created Evoke to explore that option.


Compared to TH and generics, why might you prefer Evoke?

  • It’s fast to compile. Generating the instances is basically instantaneous. Compiling the instances takes just as long as if you wrote them manually.

  • It doesn’t affect recompilation checks. You can generate instances without forcing the module to be recompiled more frequently like you would with TH.

  • It’s easy to debug. The entire plugin operates as a source-to-source conversion process. You can output the generated source to remove the plugin entirely if you’d like. Or you can run it as a standalone executable.

  • It doesn’t require any language extensions. Obviously it requires a plugin instead.


Compared to TH and generics, why might you avoid Evoke?

  • It operates syntactically. Things that are equivalent to the type checker, like a and (a), typically confuse Evoke.

  • It’s tightly coupled to GHC. Since it operates on GHC’s view of a parsed module, it naturally depends on that representation.

  • It only supports a subset of possible data types. Currently it doesn’t support types with multiple constructors or GADTs.

  • Each type class must be supported explicitly. And this support must be provided by the plugin itself rather than by various other libraries like Aeson.

  • Some things are uncomfortably magical. For instance a field with type Maybe a will be optional, but if its type is Prelude.Maybe a then it won’t be optional.


There are several answers to the question: “How fast is Evoke?”

The first answer relates to the plugin itself. How long does it take the plugin to run? If the plugin is enabled for a module that doesn’t actually derive any instances, it’s basically instantaneous. In that case Evoke just has to walk over the parsed module and make sure that nothing needs to be done. If the module does derive some instances, it’s still very fast. Generating the code for the instances does not take an appreciable amount of time. I hesitate to call it instantaneous, but you probably won’t notice how long it takes.

The second answer relates to the code generated by the plugin. How long does it take GHC to compile the generated code? Since this plugin generates code and then hands it off to GHC, it’s just as fast as manually writing the code yourself. It turns out that TH has the same performance characteristics, except that using TH causes more frequent recompiles. And compared to generic deriving this plugin is much faster.

The last answer relates to the runtime performance of the code generated by the plugin. How fast is the generated code? The answer here is practically the same as the last one: Just as fast as if you wrote it manually. That’s the same as TH and quite a bit better than generics.

And now for some specific numbers. At ACI Learning we have a Haskell codebase with just over 400 data types that use Evoke for deriving at least one type class instance. Previously we used generic deriving for those instances. By switching from generics to Evoke, our compilation time improved by at least 20%! Here’s a table summarizing the build times:

Flags Generic Evoke Change Percent
-O1 -j1 922s 750s -172s 81%
-O0 -j1 372s 229s -143s 62%
-O0 -j8 139s 94s -45s 68%

Our runtime also improved slightly, along with lowered CPU and memory usage. Those numbers are harder to quantify, but we definitely saw an improvement.


Currently Evoke only supports GHC 8.10. It is possible to support a wider range of versions, but I haven’t put in the legwork necessary to make that possible. Please let me know if you’d like to use Evoke with a different version of GHC.

You can install Evoke by adding it to the build-depends section of your *.cabal file. For example:

name: example
version: 0
  build-depends: evoke, whatever-else
--               ^^^^^


Since Evoke is a GHC plugin, you must enable it before you can use it. To enable it on a per-file basis, add the following pragma to the top of a Haskell file:

{-# OPTIONS_GHC -fplugin=Evoke #-}
--              ^^^^^^^^^^^^^^

It can be tedious to add that pragma to each file that needs it. Instead you can enable Evoke for every file in a package by adding it to the ghc-options section of your *.cabal file. For example:

name: example
version: 0
  ghc-options: -fplugin=Evoke -whatever-else
--             ^^^^^^^^^^^^^^

The performance impact of enabling Evoke for an entire package is negligible. In practice I recommend enabling Evoke once in the *.cabal file rather than with a pragma in each source file.

Enabling Evoke will not change the behavior of any modules that previously compiled. In order to convince Evoke to derive an instance for you, you must write a deriving clause like this:

newtype Person = Person
  { name :: String
  } deriving SomeTypeClass via "Evoke"
--                         ^^^^^^^^^^^

Note that "Evoke" is a literal string. You do not need to have any language extensions, such as DerivingVia or DerivingStrategies, enabled in order to use Evoke.


The Evoke plugin itself accepts options, which you can pass using GHC’s -fplugin-opt option. For example, this will output the generated instances:

{-# OPTIONS_GHC -fplugin=Evoke -fplugin-opt=Evoke:--verbose #-}
--                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Each type class can accept its own options, which you can pass by adding them to the "Evoke" string used. For example:

newtype Person = Person
  { name :: String
  } deriving SomeTypeClass via "Evoke --some-option"
--                                    ^^^^^^^^^^^^^

Type Classes

These type classes accept the following options:

  • --kebab: Convert field names into kebab-case. For example firstName would become first-name.

  • --camel: Convert field names into camelCase. For example FirstName would become firstName. This is usually combined with --strip PREFIX.

  • --snake: Convert field names into snake_case. For example firstName would become first_name.

  • --strip PREFIX: Removes the given prefix from field names. If the field name doesn’t begin with the prefix, an error will be thrown. For example, with --strip person the field name personFirstName would become FirstName.

  • --title: Convert field names into TitleCase. For example firstName would become FirstName.

Options are processed in order. So for example --strip person --kebab would change personFirstName into first-name.


Here’s an example of how to derive Arbitrary using Evoke:

{-# OPTIONS_GHC -fplugin=Evoke #-}
data Person = Person
  { name :: String
  , age :: Int
  } deriving Arbitrary via "Evoke"

And here’s what the generated instance might look like:

instance Test.QuickCheck.Arbitrary Person where
  arbitrary = do
    name_1 <- Test.QuickCheck.arbitrary
    age_2 <- Test.QuickCheck.arbitrary
    Control.Applicative.pure Person
      { name = name_1
      , age = age_2


Here’s an example of how to derive FromJSON using Evoke:

{-# OPTIONS_GHC -fplugin=Evoke #-}
data Person = Person
  { name :: String
  , age :: Int
  } deriving FromJSON via "Evoke"

And here’s what the generated instance might look like:

instance Data.Aeson.FromJSON Person where
  parseJSON = Data.Aeson.withObject "Main.Person" (\ object_1 -> do
    name_2 <- object_1 Data.Aeson..: Data.Text.pack "name"
    age_3 <- object_1 Data.Aeson..: Data.Text.pack "age"
    Control.Applicative.pure Person
      { name = name_2
      , age = age_3


Here’s an example of how to derive ToJSON using Evoke:

{-# OPTIONS_GHC -fplugin=Evoke #-}
data Person = Person
  { name :: String
  , age :: Int
  } deriving ToJSON via "Evoke"

And here’s what the generated instance might look like:

instance Data.Aeson.ToJSON Person where
  toJSON var_1 = Data.Aeson.object
    [ Data.Text.pack "name" Data.Aeson..= name var_1
    , Data.Text.pack "age" Data.Aeson..= age var_1
  toEncoding var_2 = Data.Aeson.pairs (Data.Monoid.mconcat
    [ Data.Text.pack "name" Data.Aeson..= name var_2
    , Data.Text.pack "age" Data.Aeson..= age var_2


Here’s an example of how to derive ToSchema using Evoke:

{-# OPTIONS_GHC -fplugin=Evoke #-}
data Person = Person
  { name :: String
  , age :: Int
  } deriving ToSchema via "Evoke"

And here’s what the generated instance might look like:

instance Data.Swagger.ToSchema Person where
  declareNamedSchema _proxy_1 = do
    name_2 <- Data.Swagger.declareSchemaRef (Data.Proxy.Proxy :: Data.Proxy.Proxy String)
    age_3 <- Data.Swagger.declareSchemaRef (Data.Proxy.Proxy :: Data.Proxy.Proxy Int)
    Control.Applicative.pure (Data.Swagger.NamedSchema
      (Data.Maybe.Just (Data.Text.pack "Main.Person"))
          Control.Lens.& Data.Swagger.type_ Control.Lens.?~ Data.Swagger.SwaggerObject
          Control.Lens.& Control.Lens..~ Data.HashMap.Strict.InsOrd.fromList
            [ (Data.Text.pack "name", name_2)
            , (Data.Text.pack "age", age_3)
          Control.Lens.& Data.Swagger.required Control.Lens..~
            [ Data.Text.pack "name"
            , Data.Text.pack "age"

Note that this instance will require the ScopedTypeVariables extension if the type has any type variables that are used in any of the fields.