Probability newtype with smart constructor and unidirectional pattern example

Main.hs

module Main where

import Data.Probability

main :: IO ()
main = do
  s <- readLn
  case probability s of
    -- This pattern match wouldn't work w/o PatternSynonyms
    -- (you could still pattern match on the Maybe, but not on
    -- the Probability)
    Just (Probability p) -> putStrLn $ "Valid probability: " <> show p
    Nothing              -> putStrLn $ "Invalid probability: " <> show s

Probability.hs

{-# LANGUAGE PatternSynonyms #-}

module Data.Probability
( Probability  -- note we do not export the data constructor 'UnsafeProbability'
, unwrap
, pattern Probability
, probability
) where

newtype Probability = UnsafeProbability { unwrap :: Double } deriving Show
pattern Probability p <- UnsafeProbability p

-- | 'Probability' "smart constructor"
probability :: Double -> Maybe Probability
probability p | 0 <= p && p <= 1 = Just (UnsafeProbability p)
              | otherwise        = Nothing