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
React
Typescript
Django
Laravel
Facebook
Microsoft
Google
Alibaba
D3
Tencent