module Statistics.Distribution.Poisson
(
PoissonDistribution
, poisson
, poissonLambda
) where
import Data.Typeable (Typeable)
import qualified Statistics.Distribution as D
import qualified Statistics.Distribution.Poisson.Internal as I
import Statistics.Math (incompleteGamma)
newtype PoissonDistribution = PD {
poissonLambda :: Double
} deriving (Eq, Read, Show, Typeable)
instance D.Distribution PoissonDistribution where
cumulative (PD lambda) x
| x < 0 = 0
| otherwise = 1 incompleteGamma (fromIntegral (floor x + 1 :: Int)) lambda
instance D.DiscreteDistr PoissonDistribution where
probability (PD lambda) x = I.probability lambda (fromIntegral x)
instance D.Variance PoissonDistribution where
variance = poissonLambda
instance D.Mean PoissonDistribution where
mean = poissonLambda
instance D.MaybeMean PoissonDistribution where
maybeMean = Just . D.mean
instance D.MaybeVariance PoissonDistribution where
maybeStdDev = Just . D.stdDev
poisson :: Double -> PoissonDistribution
poisson l
| l <= 0 = error $ "Statistics.Distribution.Poisson.poisson:\
\ lambda must be positive. Got " ++ show l
| otherwise = PD l