Data.Derive.TopDown

derive-topdown will make it easier to derive class instance for complex composited data types by using Template Haskell. For example:

data Gender = Male | Female
type Age = Int
data Person a = P {name :: String , age :: Int, gender :: Gender}
data Department a = D {dname :: String , head :: Person a, staff :: [Person a]}
data Company a = C {cname :: String, departments :: [Department a]}

derivings [''Eq, ''Ord, ''Generic] ''Company

You will get:

    derivings [''Eq, ''Ord, ''Generic] ''Company
  ======>
    deriving instance Eq Gender
    deriving instance Eq (Person a_adHv)
    deriving instance (Eq a_adHu, Eq a_adHu) => Eq (Department a_adHu)
    deriving instance Eq a_adHt => Eq (Company a_adHt)
    deriving instance Ord Gender
    deriving instance Ord (Person a_adHv)
    deriving instance (Ord a_adHu, Ord a_adHu) =>
                      Ord (Department a_adHu)
    deriving instance Ord a_adHt => Ord (Company a_adHt)
    deriving instance Generic Gender
    deriving instance Generic (Person a_adHv)
    deriving instance Generic (Department a_adHu)
    deriving instance Generic (Company a_adHt)

This will make sense if you have a deep composited data types, nomally an AST of a language.

instance_ and instances functions will generate empty instances. It is not quite useful with GHC newer 7.10 since they ahve DeriveAnyClass extension. However, with older GHC, it may help. The useage is the same with derive and derives.

For other classes whose instance can only be generated by using a function Name -> Q [Dec] like Arbitrary in QuickCheck, the derive package provides derive makeArbitrary function. For doing the top-down derive in these cases, deriving_th and deriving_ths are defined. Other example can the deriveXXXX functions in Data.Aeson.TH.

deriving_th (''FromJSON, deriveFromJSON defaultOptions) ''Company
deriving_th (''ToJSON, deriveToJSON defaultOptions)     ''Company

However, the poblem could be that the instance context is generated by the template haskell derive function instead mine, so it could be wrong in some circumtances. For example, type with high order type constructor:

data T1 k a b = T11 (k a) b | T12 (k (k b)) a b Int

It cannot be derived FromJSON and ToJSON with deriveFromJSON and deriveToJSON since it does not generate (k a) and (k (k b)) for in the instance context.

Also, there are some data types which are impossible to be derived as instances of a certain type class. For example, Word cannot be derived as Functor or Generic. Using derive-topdown is the same with hand-written code, so it is your responsiblity to make that right.

• NOTE! derive-topdown will NOT work with Typeable type class. See here.

Documentation

deriving_

Arguments

:: Name

class name

-> Name

type name

-> Q [Dec]

derivings

Arguments

:: [Name]

class names

-> Name

type name

-> Q [Dec]

derivingss

Arguments

:: [Name]

class names

-> [Name]

type names

-> Q [Dec]

instance_

Arguments

:: Name

class name

-> Name

type name

-> Q [Dec]

instances

Arguments

:: [Name]

class names

-> Name

type name

-> Q [Dec]

instancess

Arguments

:: [Name]

class names

-> [Name]

type names

-> Q [Dec]

deriving_th

Arguments

:: (Name, Name -> Q [Dec])

class name and corresponding isntance generation function

-> Name

type name

-> Q [Dec]

deriving_ths

Arguments

:: [(Name, Name -> Q [Dec])]

class names and corresponding instance generation functions

-> Name

type name

-> Q [Dec]

deriving_thss

Arguments

:: [(Name, Name -> Q [Dec])]

class names and corresponding instance generation functions

-> [Name]

type names

-> Q [Dec]