Generics.Deriving.Monoid

This module provides two main features:

1. GMonoid, a generic version of the Monoid type class, including instances of the types from Data.Monoid
2. Default generic definitions for the Monoid methods mempty and mappend

The generic defaults only work for types without alternatives (i.e. they have only one constructor). We cannot in general know how to deal with different constructors.

GMonoid type class

class GMonoid a where

Minimal complete definition

Nothing

Methods

gmempty :: a

Generic mempty

gmappend :: a -> a -> a

Generic mappend

gmconcat :: [a] -> a

Generic mconcat

Instances

GMonoid Ordering

GMonoid ()

GMonoid All

GMonoid Any

GMonoid [a]

GMonoid a => GMonoid (Identity a)

GMonoid a => GMonoid (Dual a)

GMonoid (Endo a)

Num a => GMonoid (Sum a)

Num a => GMonoid (Product a)

GMonoid (First a)

GMonoid (Last a)

GMonoid a => GMonoid (Maybe a)

GMonoid b => GMonoid (a -> b)

(GMonoid a, GMonoid b) => GMonoid (a, b)

GMonoid a => GMonoid (Const a b)

GMonoid (Proxy * s)

(GMonoid a, GMonoid b, GMonoid c) => GMonoid (a, b, c)

Alternative f => GMonoid (Alt * f a)

(GMonoid a, GMonoid b, GMonoid c, GMonoid d) => GMonoid (a, b, c, d)

(GMonoid a, GMonoid b, GMonoid c, GMonoid d, GMonoid e) => GMonoid (a, b, c, d, e)

(GMonoid a, GMonoid b, GMonoid c, GMonoid d, GMonoid e, GMonoid f) => GMonoid (a, b, c, d, e, f)

(GMonoid a, GMonoid b, GMonoid c, GMonoid d, GMonoid e, GMonoid f, GMonoid g) => GMonoid (a, b, c, d, e, f, g)

(GMonoid a, GMonoid b, GMonoid c, GMonoid d, GMonoid e, GMonoid f, GMonoid g, GMonoid h) => GMonoid (a, b, c, d, e, f, g, h)

Default definitions

GMonoid

gmemptydefault :: (Generic a, GMonoid' (Rep a)) => a

gmappenddefault :: (Generic a, GMonoid' (Rep a)) => a -> a -> a

Monoid

These functions can be used in a Monoid instance. For example:

-- LANGUAGE DeriveGeneric

import Generics.Deriving.Base (Generic)
import Generics.Deriving.Monoid

data T a = C a (Maybe a) deriving Generic

instance Monoid a => Monoid (T a) where
  mempty  = memptydefault
  mappend = mappenddefault

memptydefault :: (Generic a, Monoid' (Rep a)) => a

mappenddefault :: (Generic a, Monoid' (Rep a)) => a -> a -> a

The Monoid module

This is exported for convenient access to the various wrapper types.

module Data.Monoid