Generic.Data.Internal.Generically

A datatype whose instances are defined generically, using the Generic representation. Generically1 is a higher-kinded version of Generically that uses Generic1.

Generic instances can be derived via Generically A using -XDerivingVia.

{-# LANGUAGE DeriveGeneric      #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE DerivingVia        #-}

import GHC.Generics (Generic)

data V4 a = V4 a a a a
  deriving stock Generic

  deriving (Semigroup, Monoid)
  via Generically (V4 a)

This corresponds to Semigroup and Monoid instances defined by pointwise lifting:

instance Semigroup a => Semigroup (V4 a) where
  (<>) :: V4 a -> V4 a -> V4 a
  V4 a1 b1 c1 d1 <> V4 a2 b2 c2 d2 =
    V4 (a1 <> a2) (b1 <> b2) (c1 <> c2) (d1 <> d2)

instance Monoid a => Monoid (V4 a) where
  mempty :: V4 a
  mempty = V4 mempty mempty mempty mempty

Historically this required modifying the type class to include generic method definitions (-XDefaultSignatures) and deriving it with the anyclass strategy (-XDeriveAnyClass). Having a /via type/ like Generically decouples the instance from the type class.

A type whose instances are defined generically, using the Generic1 representation. Generically1 is a higher-kinded version of Generically that uses Generic.

Generic instances can be derived for type constructors via Generically1 F using -XDerivingVia.

{-# LANGUAGE DeriveGeneric      #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE DerivingVia        #-}

import GHC.Generics (Generic)

data V4 a = V4 a a a a
  deriving stock (Functor, Generic1)

  deriving Applicative
  via Generically1 V4

This corresponds to Applicative instances defined by pointwise lifting:

instance Applicative V4 where
  pure :: a -> V4 a
  pure a = V4 a a a a

  liftA2 :: (a -> b -> c) -> (V4 a -> V4 b -> V4 c)
  liftA2 (·) (V4 a1 b1 c1 d1) (V4 a2 b2 c2 d2) =
    V4 (a1 · a2) (b1 · b2) (c1 · c2) (d1 · d2)

Historically this required modifying the type class to include generic method definitions (-XDefaultSignatures) and deriving it with the anyclass strategy (-XDeriveAnyClass). Having a /via type/ like Generically1 decouples the instance from the type class.

Type with Enum instance derived via Generic with FiniteEnum option. This allows deriving Enum for types whose constructors have fields.

Some caution is advised; see details in FiniteEnum.

Example

>>> :{data Booool = Booool Bool Bool  deriving Generic  deriving (Enum, Bounded) via (FiniteEnumeration Booool):}

Product type with generic instances of Semigroup and Monoid.

This is similar to Generic.Data.Generically in most cases, but GenericProduct also works for types T with deriving via GenericProduct U, where U is a generic product type coercible to, but distinct from T. In particular, U may not have an instance of Semigroup, which Generic.Data.Generically requires.

Example

>>> import Data.Monoid (Sum(..))>>> data Point a = Point a a deriving Generic>>> :{  newtype Vector a = Vector (Point a)    deriving (Semigroup, Monoid)      via GenericProduct (Point (Sum a)):}

If it were via Generic.Data.Generically (Point (Sum a)) instead, then Vector's mappend (the Monoid method) would be defined as Point's (<>) (the Semigroup method), which might not exist, or might not be equivalent to Vector's generic Semigroup instance, which would be unlawful.