Safe Haskell	Safe-Inferred
Language	Haskell2010

Control.Foldl.Incremental

Contents

Increment
common incremental folds
move to Foldl
reexports

Description

This module provides incremental statistics folds based upon the foldl library

>>> import Control.Foldl.Incremental
>>> import qualified Control.Foldl as L

The folds represent incremental statistics such as `moving averages`.

Statistics are based on exponential-weighting schemes which enable statistics to be calculated in a streaming one-pass manner. The stream of moving averages with a rate of 0.9 is:

>>> scan (incMa 0.9) [1..10]

or if you just want the moving average at the end.

>>> fold (incMa 0.9) [1..10]

Synopsis

Increment

data Increment Source

An Increment is the incremental state within an exponential moving average fold.

Constructors

Increment
Fields _adder :: !Double _counter :: !Double _rate :: !Double

Instances

Show Increment

incrementalize :: (a -> Double) -> Double -> Fold a Double Source

Incrementalize takes a function and turns it into a Fold where the step incremental is an Increment with a step function iso to a step in an exponential moving average calculation.

>>> incrementalize id

is a moving average of a foldable

>>> incrementalize (*2)

is a moving average of the square of a foldable

This lets you build an exponential standard deviation computation (using Foldl) as

>>> std r = (\s ss -> sqrt (ss - s**2)) <$> incrementalize id r <*> incrementalize (*2) r

An exponential moving average approach (where average id abstracted to function) represents an efficient single-pass computation that attempts to keep track of a running average of some Foldable.

The rate is the parameter regulating the discount of current state and the introduction of the current value.