Using ExceptT to catch errors in Haskell.

main.hs

{-# LANGUAGE NoImplicitPrelude #-}

module Main where

import Protolude
import qualified Data.Text as T

calculateThing :: ExceptT Text IO Text
calculateThing = do
  print $ T.pack "log"
  return $ T.pack "uccess"
  --throwError $ T.pack "fail"

main :: IO ()
main = do
  {-
    need to modify the wrapped value?
    fmap a lambda over ExceptT! (fmap (b -> c) (m a b) returns (m a c))
  -}
  result <- runExceptT $ fmap (\x -> T.cons 's' x) calculateThing
  case result of
    Right success -> putStrLn success
    Left  error   -> putStrLn error