ExceptT minimal usage example

ExceptT_example.hs

import           Control.Monad.Except
import           Control.Monad.Identity

data MyError   = InvalidNumber deriving Show
data MySuccess = CorrectNumber | NotACorrectNumber deriving Show

runMyMonad = runIdentity . runExceptT

type MyMonad = ExceptT MyError Identity MySuccess

m :: Int -> MyMonad
m x = do
  when (x /= 42) (throwError InvalidNumber)
  return CorrectNumber

m2 :: Int -> MyMonad
m2 x = do
  (m x) `catchError` (const $ return NotACorrectNumber)

run1 = runMyMonad (m  0 )
run2 = runMyMonad (m2 0 )
run3 = runMyMonad (m  42)
run4 = runMyMonad (m2 42)