{- |
Copyright    : 2007 Eric Kidd
License      : BSD3
Stability    : experimental
Portability  : non-portable (newtype deriving)

Support for probability values.
-}

module Data.Probability.Base (
    Probability,
    prob, fromProb,
    pnot, padd, pmul
  ) where

import Data.Monoid

-- | The probability of an event occuring.  We provide this as a type
-- class, allowing users of this library to choose among various
-- representations of probability.
class (Eq p, Monoid p) => Probability p where
  -- TODO: Should 'prob' and 'fromProb' work with Rational or another type?
  -- They exist mostly to interface with
  -- 'Control.Monad.Distribution.weighted'.

  -- | Create a probability from a rational number between 0 and 1, inclusive.
  prob :: Rational -> p
  -- | Convert a probability to a rational number.
  fromProb :: p -> Rational

  -- | Given the probability of an event occuring, calculate the
  -- probability of the event /not/ occuring.
  pnot :: p -> p
  -- | Given the probabilities of two disjoint events, calculate the
  -- probability of either event occuring.
  padd :: p -> p -> p
  -- | Given the probabilities of two indepedent events, calculate the
  -- probability of both events occuring.
  pmul :: p -> p -> p