MyLib.hs

{-# LANGUAGE OverloadedStrings #-}

module MyLib where

import Data.These
import Data.Text (Text)

f0 :: These [Text] Int
f0 = This ["f0 didn't work"]

-- | Produces result without error.
f1 :: These [Text] Int
f1 = That 33

f2 :: These [Text] Int
f2 = This ["f2 didn't work"]

f3 :: These [Text] Int
f3 = That 2

-- | Should return…
--
-- >>> sumAll 1 2
-- This ["a should not be less than 33"]
--
-- >>> sumAll 33 1
-- That 34
sumAll :: Int -> Int -> These [Text] Int
sumAll a b | a < 33 = This ["a should not be less than 33"]
           | otherwise = That $ a + b

-- | `caller1` does not short-circuit after encountering error.
--
-- >>> caller1
-- This ["f2 didn't work"]
caller1 :: These [Text] Int
caller1 = case (f0, f1, f2) of
            (This a, This b, This c) -> This $ a ++ b ++ c
            (This a, This b, That c) -> This $ a ++ b
            (This a, That b, This c) -> This $ a ++ c
            (That a, This b, This c) -> This $ b ++ c
            (That a, That b, This c) -> This c
            (That a, This b, That c) -> This b
            (This a, That b, That c) -> case (sumAll b c) of
                                            This d -> This $ a ++ d
                                            That d -> These a d
            (That a, That b, That c) -> sumAll b c

-- | `caller2` does not short-circuit after encountering error.
--
-- >>> caller2
-- This ["f0 didn't work", f2 didn't work", "a should not be less than 33"]
caller2 :: These [Text] Int
caller2 = do
  res1 <- f1
  res3 <- f3
  endResult <- sumAll res3 res1
  res2 <- f2
  res0 <- f0
  pure $ endResult