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--- Library for Probabilistic Functional Logic Programming
--- @author Sandra Dylus, Jan Christiansen, Finn Teegen
--- @version October 2017

module PFLP
  ( Probability
  , Dist
  , enum
  , uniform
  , certainly
  , (>>>=)
  , joinWith
  , (??)
  , RT
  , pick
  , replicateDist
  ) where

import Float (i2f, (+.), (*.), (/.))
import Findall (allValues)

infixl 1 >>>=
infixr 1 ??

--- Probabilities.
--- Floating point numbers are used to model probabilities.
type Probability = Float

--- Probability distributions.
--- Distributions are abstract and can only be created using the functions
--- provided by this library, e.g., 'enum' and 'uniform'. Internally, Curry's
--- built-in non-determinism is used to model distributions with more than one
--- event-probability pair.
data Dist a = Dist { event :: a, prob :: Probability }

member :: [a] -> a
member = foldr (?) failed

--- Creates a distribution based on a given list of events and another list
--- providing the corresponding probabilities. This function also ensures that
--- the relevant probabilities add up to `1.0` and are strictly positive.
enum :: [a] -> [Probability] -> Dist a
enum xs ps
  | foldl (+.) 0.0 ps' == 1.0 && all (> 0.0) ps'
  = member (zipWith Dist xs ps')
  | otherwise
  = error ("PFLP.enum: probabilities do not add up to 1.0 " ++
             "or are not strictly positive")
 where ps' = take (length xs) ps

--- Creates a uniform distribution based on a given list of events. The list
--- of events must be non-empty.
uniform :: [a] -> Dist a
uniform [] = error "PFLP.uniform: list of events must be non-empty"
uniform xs@(_:_) = enum xs (repeat (1.0 /. i2f (length xs)))

--- Creates a single-event-distribution with probability `1.0`.
certainly :: a -> Dist a
certainly x = Dist x 1.0

--- Combines two (dependent) distributions.
(>>>=) :: Dist a -> (a -> Dist b) -> Dist b
d >>>= f = let Dist x p = d
               Dist y q = f x
           in Dist y (p *. q)

--- Combines two (independent) distributions with respect to a given function.
joinWith :: (a -> b -> c) -> Dist a -> Dist b -> Dist c
joinWith f d1 d2 = d1 >>>= \ x ->
                   d2 >>>= \ y ->
                   certainly (f x y)

filterDist :: (a -> Bool) -> Dist a -> Dist a
filterDist p d | p (event d) = d

--- Queries a distribution for the probabilitiy of events that satisfy a given
--- predicate.
(??) :: (a -> Bool) -> Dist a -> Probability
(??) p = foldr (+.) 0.0 . allValues . prob . filterDist p

--- Run-time choice values. Currently, the only way to construct a run-time
--- choice value is to explicitly use a lambda abstraction. The evaluation of
--- a run-time choice can be triggered by the function 'pick'.
type RT a = () -> a

--- Triggers the evaluation of a run-time choice value (see type synonym 'RT').
--- Everytime a run-time choice value is evaluated, a new choice is made.
pick :: RT a -> a
pick rt = rt ()

--- Independently replicates a distribution a given number of times. In order
--- to behave properly, the given distribution is required to be a run-time
--- choice value (see type synonym 'RT').
replicateDist :: Int -> RT (Dist a) -> Dist [a]
replicateDist n rt
  | n == 0    = certainly []
  | otherwise = joinWith (:) (pick rt) (replicateDist (n - 1) rt)