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partial.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE LambdaCase #-}
{-# OPTIONS_GHC -fno-warn-incomplete-patterns #-}
module Partial where

import Control.Exception (PatternMatchFail, catch, evaluate)
import Data.Maybe (mapMaybe)
import System.IO.Unsafe (unsafePerformIO)


class Partial a where
  partial :: a -> a

instance Partial (a -> Bool) where
  partial = partialWith False

instance Partial (a -> Maybe b) where
  partial = partialWith Nothing


partialWith :: b -> (a -> b) -> a -> b
partialWith z f x = unsafePerformIO (evaluate (f x) `catchPatternMatchFail` \_ -> return z)

catchPatternMatchFail :: IO a -> (PatternMatchFail -> IO a) -> IO a
catchPatternMatchFail = catch


xs :: [Int]
xs = filter (partial $ \case 3 -> True; 7 -> True) [1..10]

ys :: [Int]
ys = mapMaybe (partial $ \case 3 -> Just 8; 7 -> Just 1) [1..10]