By what mechanism does Generic interact with Aeson's ToJSON class?

Question

Looking at part of the servant example, I see:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}

module Main where

import Prelude ()
import Prelude.Compat

import Control.Monad.Except
import Control.Monad.Reader
import Data.Aeson.Types
import Data.Attoparsec.ByteString
import Data.ByteString (ByteString)
import Data.List
import Data.Maybe
import Data.String.Conversions
import Data.Time.Calendar
import GHC.Generics
import Lucid
import Network.HTTP.Media ((//), (/:))
import Network.Wai
import Network.Wai.Handler.Warp
import Servant
import System.Directory
import Text.Blaze
import Text.Blaze.Html.Renderer.Utf8
import qualified Data.Aeson.Parser
import qualified Text.Blaze.Html

type UserAPI1 = "users" :> Get '[JSON] [User]

data User = User
  { name :: String
  , age :: Int
  , email :: String
  , registration_date :: Day
  } deriving (Eq, Show, Generic)

instance ToJSON User

When I removed the deriving of Generic, I got the following error:

• No instance for (Generic User)
    arising from a use of ‘aeson-1.1.2.0:Data.Aeson.Types.ToJSON.$dmtoJSON’

So, it appears that the Generic typeclass instance for User enables instance ToJSON User to, I'm assuming, create a JSON Encoder for User.

What's the machinery of instance ToJSON User, i.e. type signature, if that's the right word?

I'm trying to look at its type from the stack ghci, i.e. REPL, but failing:

λ: >:t instance
<interactive>:1:1: error: parse error on input ‘instance’
λ: >:i instance
<interactive>:1:1: error: parse error on input ‘instance’

Answer 1

Let's look at the source for ToJSON:

class ToJSON a where
    -- | Convert a Haskell value to a JSON-friendly intermediate type.
    toJSON     :: a -> Value

    default toJSON :: (Generic a, GToJSON Zero (Rep a)) => a -> Value
    toJSON = genericToJSON defaultOptions

The ToJSON class has a default toJSON implementation with additional type constraints (including Generic, as you've noticed). This requires the DefaultSignatures extension; notice at the top of that module you can see

{-# LANGUAGE DefaultSignatures #-}

The other constraint, GToJSON Zero (Rep a), imposes some further restrictions on the structure of a, and so not every type with a Generic instance will satisfy this signature.

Regarding your question about GHCi: instance is a Haskell keyword. Inspecting toJSON may be what you want instead. This will show you the same information we saw in the source:

λ> :i toJSON
class ToJSON a where
  toJSON :: a -> Value
  default toJSON :: (GHC.Generics.Generic a,
                     GToJSON Zero (GHC.Generics.Rep a)) =>
                    a -> Value
  ...
        -- Defined in ‘aeson-1.1.2.0:Data.Aeson.Types.ToJSON’