derive-topdown

This library will help you generate Haskell empty Generic instance and deriving type instances from the top automatically to the bottom.

An example to generate Out class for Person, Name and Address. Out class in genericpretty package has to provide a default implementation for the function it declears.

{-# LANGUAGE TemplateHaskell, DeriveGeneric,DeriveDataTypeable,TypeSynonymInstances #-}
{-# OPTIONS_GHC -ddump-splices #-}
module Data.Derive.TopDown.Test where 
import Data.Derive.TopDown.Generic
import Data.Derive.TopDown.Derive (derivings)
import Text.PrettyPrint.GenericPretty (Out)
import Data.DeriveTH
import GHC.Generics

data A a b = A a (B b) deriving (Show, Generic)
data B a = B a deriving (Show, Generic)

data Person = Person Names Address
            | Student Names Address deriving Generic
data Names  = Names String deriving Generic
data Address = Address Gate  deriving Generic
type Gate = (String,Int)

derivings ''Eq makeEq ''Person
instances ''Out ''A
instances ''Out ''Person

================================

Data\Derive\TopDown\Test.hs:1:1: Splicing declarations
    derivings ''Eq makeEq ''Person
  ======>
    Data\Derive\TopDown\Test.hs:23:1-30
    instance Eq Names where
      (==) (Names x1) (Names y1) = (x1 == y1)
    instance Eq Address where
      (==) (Address x1) (Address y1) = (x1 == y1)
    instance Eq Person where
      (==) (Person x1 x2) (Person y1 y2) = ((x1 == y1) && (x2 == y2))
      (==) (Student x1 x2) (Student y1 y2) = ((x1 == y1) && (x2 == y2))
      (==) _ _ = False
Data\Derive\TopDown\Test.hs:1:1: Splicing declarations
    instances ''Out ''A
  ======>
    Data\Derive\TopDown\Test.hs:24:1-19
    instance Out a_a1IiG => Out (B a_a1IiG)
    instance (Out a_a1IiH, Out b_a1IiI) => Out (A a_a1IiH b_a1IiI)
Data\Derive\TopDown\Test.hs:1:1: Splicing declarations
    instances ''Out ''Person
  ======>
    Data\Derive\TopDown\Test.hs:25:1-24
    instance Out Names
    instance Out Address
    instance Out Person

For generating 4 empty instances

instance Out Person
instnace Out Nmads
instance Out Address
instance Out Gate

you just write:

instances ''Out ''Person

For derive Eq typeclass you just write

derivings ''Eq makeEq ''Person

It will generate all instances that Person dependend on including Person.

Do not forget to use :set -ddump-splices, you will get

instances ''Out  ''Person
======>
    ~\Test.hs:13:1-18
    instance Out Names
    instance Out Gate
    instance Out Address
    instance Out Person
    Ok, modules loaded: CompositeDataInstancesGen, Main.

You can also use instnaceList to generate a list of class. Solution 1 is to use derive package

derivings ''Eq makeEq ''A 

If you enable -ddump-splices, you will get:

Data\Derive\TopDown\Test.hs:1:1: Splicing declarations
   derives ''Eq makeEq ''A
 ======>
   Data\Derive\TopDown\Test.hs:18:1-25
   instance Eq a_1627720873 => Eq (B a_1627720873) where
     (==) (B x1) (B y1) = (x1 == y1)
   instance (Eq a_1627720874, Eq b_1627720875) =>
            Eq (A a_1627720874 b_1627720875) where
     (==) (A x1 x2) (A y1 y2) = ((x1 == y1) && (x2 == y2))

We have to specify makeEq or other Derivation values here and I am not sure how to eleminate it.

Solution 2 is is to use standalone deriving. it will be supposrt in GHC 7.10 standalone deriving will be supported and you do not need to write. Currently unimplemented.

deriving (Data, Typeable, Generic, Ord,Eq,Show)

for each data type declarations ever again. In stead you will be able to write:

derivings instance [''Eq, ''Typeable, ''Generic] A