{-# LANGUAGE GeneralizedNewtypeDeriving #-}
-- |
-- Module : Data.Semiring
-- Copyright : Thomas Wilke, Frank Huch, Sebastian Fischer
-- License : BSD3
-- Maintainer : Sebastian Fischer <mailto:mail@sebfisch.de>
-- Stability : experimental
--
-- This library provides a type class for semirings and instances for
-- standard data types.
--
module Data.Semiring (
Semiring(..), fromBool,
Numeric(..)
) where
infixr 6 .+.
infixr 7 .*.
-- |
-- A semiring is an additive commutative monoid with identity 'zero':
--
-- > a .+. b == b .+. a
-- > zero .+. a == a
-- > (a .+. b) .+. c == a .+. (b .+. c)
--
-- A semiring is a multiplicative monoid with identity 'one':
--
-- > one .*. a == a
-- > a .*. one == a
-- > (a .*. b) .*. c == a .*. (b .*. c)
--
-- Multiplication distributes over addition:
--
-- > a .*. (b .+. c) == (a .*. b) .+. (a .*. c)
-- > (a .+. b) .*. c == (a .*. c) .+. (b .*. c)
--
-- 'zero' annihilates a semiring with respect to multiplication:
--
-- > zero .*. a == zero
-- > a .*. zero == zero
--
-- All laws should hold with respect to the required `Eq` instance.
--
-- For example, the Booleans form a semiring.
--
-- * @False@ is an identity of disjunction which is commutative and
-- associative,
--
-- * @True@ is an identity of conjunction which is associative,
--
-- * conjunction distributes over disjunction, and
--
-- * @False@ annihilates the Booleans with respect to conjunction.
--
class Eq s => Semiring s where
zero, one :: s
(.+.), (.*.) :: s -> s -> s
-- | Auxiliary function to convert Booleans to an arbitrary semiring.
--
fromBool :: Semiring s => Bool -> s
fromBool False = zero
fromBool True = one
instance Semiring Bool where
zero = False; one = True; (.+.) = (||); (.*.) = (&&)
-- |
-- Wrapper for numeric types.
--
-- Every numeric type that satisfies the semiring laws (as all
-- predefined numeric types do) is a semiring.
--
newtype Numeric a = Numeric { getNumeric :: a }
deriving (Eq,Num)
instance Show a => Show (Numeric a) where
show = show . getNumeric
instance Num a => Semiring (Numeric a) where
zero = 0; one = 1; (.+.) = (+); (.*.) = (*)