monoid-statistics-0.3: Monoids for calculation of statistics of sample

MaintainerAlexey Khudyakov <>







class Monoid m => StatMonoid m a whereSource

Monoid which corresponds to some stattics. In order to do so it must be commutative. In many cases it's not practical to construct monoids for each element so papennd was added. First parameter of type class is monoidal accumulator. Second is type of element over which statistic is calculated.

Statistic could be calculated with fold over sample. Since accumulator is Monoid such fold could be easily parralelized.

Instance must satisfy following law:

 pappend x (pappend y mempty) == pappend x mempty `mappend` pappend y mempty
 mappend x y == mappend y x

It is very similar to Reducer type class from monoids package but require commutative monoids


pappend :: a -> m -> mSource

Add one element to monoid accumulator. P stands for point in analogy for Pointed.

evalStatistic :: (Foldable d, StatMonoid m a) => d a -> mSource

Calculate statistic over Foldable. It's implemented in terms of foldl'.

Statistic monoids

data TwoStats a b Source

Monoid which allows to calculate two statistics in parralel




calcStat1 :: !a
calcStat2 :: !b


Typeable2 TwoStats 
(Eq a, Eq b) => Eq (TwoStats a b) 
(Show a, Show b) => Show (TwoStats a b) 
(Monoid a, Monoid b) => Monoid (TwoStats a b) 
(StatMonoid a x, StatMonoid b x) => StatMonoid (TwoStats a b) x 

Additional information

Statistic is function of a sample which does not depend on order of elements in a sample. For each statistics corresponding monoid could be constructed:

 f :: [A] -> B

 data F = F [A]

 evalF (F xs) = f xs

 instance Monoid F here
   mempty = F []
   (F a) `mappend` (F b) = F (a ++ b)

This indeed proves that monoid could be constructed. Monoid above is completely impractical. It runs in O(n) space. However for some statistics monoids which runs in O(1) space could be implemented. For example mean.

On the other hand some statistics could not be implemented in such way. For example calculation of median require O(n) space. Variance could be implemented in O(1) but such implementation won't be numerically stable.