src/Data/Semigroup/Additive.hs · patch

on develop help sign in

54 lines 1.7 kB history raw

1{-# LANGUAGE CPP #-}

2{-# LANGUAGE TypeOperators #-}

3{-# LANGUAGE TypeFamilies #-}

4-- |

5-- Module:

6-- Data.Semigroup.Additive

7-- Description:

8-- This module defines a class for commutative semigroups, until it is moved

9-- to another library.

10module Data.Semigroup.Additive

11 ( Additive

12 ) where

14import Data.Functor.Const (Const (..))

15import Data.Functor.Identity

16import Data.Proxy

17#if !MIN_VERSION_base(4,12,0)

18-- for :*: and :.: semigroup instances

19import Data.Orphans ()

20#endif

21#if !MIN_VERSION_base(4,11,0)

22import Data.Semigroup (Semigroup (..))

23#endif

24import GHC.Generics

26-- | An 'Additive' 'Semigroup' is one where (<>) is commutative

27class Semigroup q => Additive q where

29-- | Trivial additive semigroup.

30instance Additive ()

32-- | Product additive semigroup.

33-- A Pair of additive semigroups gives rise to a additive semigroup

34instance (Additive a, Additive b) => Additive (a, b)

36-- See https://gitlab.haskell.org/ghc/ghc/issues/11135#note_111802 for the reason Compose is not also provided.

37-- Base does not define Monoid (Compose f g a) so this is the best we can

38-- really do for functor composition.

39instance Additive (f (g a)) => Additive ((f :.: g) a)

41-- | Product of additive semigroups, Functor style.

42instance (Additive (f a), Additive (g a)) => Additive ((f :*: g) a)

44-- | Trivial additive semigroup, Functor style

45instance Additive (Proxy x)

47-- | Const lifts additive semigroups into a functor.

48instance Additive a => Additive (Const a x)

50-- | Identity lifts additive semigroups pointwise (at only one point)

51instance Additive a => Additive (Identity a)

53-- | Functions lift additive semigroups pointwise.

54instance Additive b => Additive (a -> b)