Generic.Data.Microsurgery

Apply a microsurgery s to a type a for DerivingVia.

For the Monoid class, see ProductSurgery.

Example

{-# LANGUAGE DerivingVia #-}

-- The constructors must be visible.
import Generic.Data.Microsurgery
  (Surgery, Surgery'(..), Generically(..), Derecordify)

data T = T { unT :: Int }
  deriving Show via (Surgery Derecordify T)

-- T won't be shown as a record:
--   show (T {unT = 3}) == "T 3"

Apply a microsurgery s to a type a for DerivingVia for the Monoid class.

Plural of Surgery. Apply a list of microsurgeries.

Plural of ProductSurgery. Apply a list of microsurgeries.

See Surgery.

Apply a microsurgery represented by a symbol s (declared as a dummy data type) to a generic representation f.

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.

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.

Synthetic data type.

A wrapper to view a generic Rep as the datatype it's supposed to represent, without needing a declaration.

Conversion between a generic type and the synthetic type made using its representation. Inverse of fromData.

Inverse of toData.

onData :: _ => (Data r x -> Data s y) -> (Data r x -> Data s y)  -- possible specialization

Can be used with generic-lens for type-changing field updates with field_ (and possibly other generic optics).

A specialization of the identity function to be used to fix types of functions on Data, unifying the "spines" of input and output generic representations (the "spine" is everything except field types, which may thus change).