你可以建模很多结构。这里是一组:
class Group a where
mult :: a -> a -> a
identity :: a
inverse :: a -> a
instance Group Integer where
mult = (+)
identity = 0
inverse = negate
-- S_3 (group of all bijections of a 3-element set)
data S3 = ABC | ACB | BAC | BCA | CAB | CBA
instance Group S3 where
mult ABC x = x
... -- some boring code
identity = ABC
inverse ABC = ABC
... -- remaining cases
-- Operations on groups. Dual:
data Dual a = Dual { getDual :: a }
instance Group a => Group (Dual a) where
mult (Dual x) (Dual y) = Dual (mult y x)
identity = Dual identity
inverse (Dual x) = Dual (inverse x)
-- Product:
instance (Group a, Group b) => Group (a,b) where
mult (x,y) (z,t) = (x `mult` z, y `mult` t)
identity = (identity, identity)
inverse (x,y) = (inverse x, inverse y)
mult(Dual CAB,5)(Dual CBA,1)
并获得结果。这将是在S 3 * ⨯ Z群中的计算。您可以添加其他组,并以任何可能的方式组合它们并对它们进行计算。
Applicative
,Monoid
,Monad
,Arrow
等)进行详细解释。 - stakx - no longer contributing