Generics.Deriving.Base

Get the precedence of a fixity value.

Products: encode multiple arguments to constructors

Sums: encode choice between constructors

Composition of functors

Datatype to represent the associativity of a constructor

Tag for M1: constructor

Type synonym for encoding meta-information for constructors

Class for datatypes that represent data constructors

Tag for M1: datatype

Type synonym for encoding meta-information for datatypes

Class for datatypes that represent datatypes

The strictness that GHC infers for a field during compilation. Whereas there are nine different combinations of SourceUnpackedness and SourceStrictness, the strictness that GHC decides will ultimately be one of lazy, strict, or unpacked. What GHC decides is affected both by what the user writes in the source code and by GHC flags. As an example, consider this data type:

data E = ExampleConstructor {-# UNPACK #-} !Int !Int Int

• If compiled without optimization or other language extensions, then the fields of ExampleConstructor will have DecidedStrict, DecidedStrict, and DecidedLazy, respectively.

• If compiled with -XStrictData enabled, then the fields will have DecidedStrict, DecidedStrict, and DecidedStrict, respectively.

• If compiled with -O2 enabled, then the fields will have DecidedUnpack, DecidedStrict, and DecidedLazy, respectively.

Datatype to represent the fixity of a constructor. An infix | declaration directly corresponds to an application of Infix.

This variant of Fixity appears at the type level.

Representable types of kind *. This class is derivable in GHC with the DeriveGeneric flag on.

A Generic instance must satisfy the following laws:

from . to ≡ Prelude.id
to . from ≡ Prelude.id

Representable types of kind * -> * (or kind k -> *, when PolyKinds is enabled). This class is derivable in GHC with the DeriveGeneric flag on.

A Generic1 instance must satisfy the following laws:

from1 . to1 ≡ Prelude.id
to1 . from1 ≡ Prelude.id

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.

Constants, additional parameters and recursion of kind *

Meta-information (constructor names, etc.)

Datatype to represent metadata associated with a datatype (MetaData), constructor (MetaCons), or field selector (MetaSel).

• In MetaData n m p nt, n is the datatype's name, m is the module in which the datatype is defined, p is the package in which the datatype is defined, and nt is 'True if the datatype is a newtype.

• In MetaCons n f s, n is the constructor's name, f is its fixity, and s is 'True if the constructor contains record selectors.

• In MetaSel mn su ss ds, if the field uses record syntax, then mn is Just the record name. Otherwise, mn is Nothing. su and ss are the field's unpackedness and strictness annotations, and ds is the strictness that GHC infers for the field.

Used for marking occurrences of the parameter

Tag for K1: recursion (of kind Type)

Type synonym for encoding recursion (of kind Type)

Recursive calls of kind * -> * (or kind k -> *, when PolyKinds is enabled)

Generic representation type