{-# LANGUAGE CPP #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE FlexibleContexts #-}
#if __GLASGOW_HASKELL__ >= 704
{-# LANGUAGE Safe #-}
#else
{-# LANGUAGE Trustworthy #-}
#endif
-----------------------------------------------------------------------------
-- |
-- Module : Data.Semigroup.Generic
-- Copyright : (C) 2014-2015 Edward Kmett, Eric Mertens
-- License : BSD-style (see the file LICENSE)
--
-- Maintainer : Edward Kmett <ekmett@gmail.com>
-- Stability : provisional
-- Portability : portable
--
-- This module provides generic deriving tools for monoids and semigroups for
-- product-like structures.
--
----------------------------------------------------------------------------
module Data.Semigroup.Generic
( GSemigroup
, gmappend
, GMonoid
, gmempty
) where
import Data.Semigroup
import GHC.Generics
-- | Generically generate a 'Semigroup' ('<>') operation for any type
-- implementing 'Generic'. This operation will append two values
-- by point-wise appending their component fields. It is only defined
-- for product types.
--
-- @
-- 'gmappend' a ('gmappend' b c) = 'gmappend' ('gmappend' a b) c
-- @
gmappend :: (Generic a, GSemigroup (Rep a)) => a -> a -> a
gmappend x y = to (gmappend' (from x) (from y))
class GSemigroup f where
gmappend' :: f p -> f p -> f p
instance GSemigroup U1 where
gmappend' _ _ = U1
instance GSemigroup V1 where
gmappend' x y = x `seq` y `seq` error "GSemigroup.V1: gmappend'"
instance Semigroup a => GSemigroup (K1 i a) where
gmappend' (K1 x) (K1 y) = K1 (x <> y)
instance GSemigroup f => GSemigroup (M1 i c f) where
gmappend' (M1 x) (M1 y) = M1 (gmappend' x y)
instance (GSemigroup f, GSemigroup g) => GSemigroup (f :*: g) where
gmappend' (x1 :*: x2) (y1 :*: y2) = gmappend' x1 y1 :*: gmappend' x2 y2
-- | Generically generate a 'Monoid' 'mempty' for any product-like type
-- implementing 'Generic'.
--
-- It is only defined for product types.
--
-- @
-- 'gmappend' 'gmempty' a = a = 'gmappend' a 'gmempty'
-- @
gmempty :: (Generic a, GMonoid (Rep a)) => a
gmempty = to gmempty'
class GSemigroup f => GMonoid f where
gmempty' :: f p
instance GMonoid U1 where
gmempty' = U1
instance (Semigroup a, Monoid a) => GMonoid (K1 i a) where
gmempty' = K1 mempty
instance GMonoid f => GMonoid (M1 i c f) where
gmempty' = M1 gmempty'
instance (GMonoid f, GMonoid g) => GMonoid (f :*: g) where
gmempty' = gmempty' :*: gmempty'