wta: Implement Probability in terms of Data.Scientific

                     , text

                     , random >= 1.1 && <1.2

                     , mtl >= 2.2 && <2.3

                     , megaparsec >= 7

                     , megaparsec >= 7 && <8

                     , scientific >= 0.3 && <0.4

{-# LANGUAGE DerivingStrategies #-}

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module Probability (Probability(..), readProbability, decide) where

import           System.Random

import           Text.Printf

import           Data.Scientific

data Probability = Probability Integer Int

newtype Probability = Probability Scientific

  deriving newtype (Show)

readProbability :: String -> Either String Probability

readProbability input = case input of

  "0" -> Right (Probability 0 0)

readProbability input = Probability <$> case input of

  "0" -> Right (scientific 0 0)

  ('0':'.':rest) -> case reads rest of

    [(digits, "")] -> Right (Probability digits (length rest))

    [(digits, "")] -> Right (scientific digits (negate (length rest)))

    _ -> failure

  "1" -> Right (Probability 1 0)

  "1" -> Right (scientific 1 0)

  ('1':'.':rest)

    | all (=='0') rest -> Right (Probability 1 0)

    | all (=='0') rest -> Right (scientific 1 0)

    | otherwise -> failure

  _ -> failure

  where failure = Left "Could not parse probability"

instance Show Probability where

  show (Probability digits 0) = show digits

  show (Probability digits exp) = "0." ++ printf "%0*d" exp digits

decide :: Probability -> IO Bool

decide (Probability digits exp) = do

decide (Probability science) = do

  let digits = coefficient science

  let exp  = negate (base10Exponent science)

  randomNumber <- randomRIO (0, (10^exp)-1)

  return $ randomNumber < digits