Functions to mechanically derive ToJSON and FromJSON instances. Note that you need to enable the TemplateHaskell language extension in order to use this module.

An example shows how instances are generated for arbitrary data types. First we define a data type:

data D a = Nullary
         | Unary Int
         | Product String Char a
         | Record { testOne   :: Double
                  , testTwo   :: Bool
                  , testThree :: D a
                  } deriving Eq

Next we derive the necessary instances. Note that we make use of the feature to change record field names. In this case we drop the first 4 characters of every field name. We also modify constructor names by lower-casing them:

$(deriveJSON defaultOptions{fieldLabelModifier = drop 4, constructorTagModifier = map toLower} ''D)

Now we can use the newly created instances.

d :: D Int
d = Record { testOne = 3.14159
           , testTwo = True
           , testThree = Product "test" 'A' 123
           }

>>> fromJSON (toJSON d) == Success d
> True

This also works for data family instances, but instead of passing in the data family name (with double quotes), we pass in a data family instance constructor (with a single quote):

data family DF a
data instance DF Int = DF1 Int
                     | DF2 Int Int
                     deriving Eq

$(deriveJSON defaultOptions 'DF1)
-- Alternatively, one could pass 'DF2 instead

Please note that you can derive instances for tuples using the following syntax:

-- FromJSON and ToJSON instances for 4-tuples.
$(deriveJSON defaultOptions ''(,,,))