No instance for (Control.Monad.Fail.MonadFail App) on P66

[paulspencerwilliams]

When trying to compile the last source code changes for chapter 3, I received the following error:

Lib.hs:46:3: error:
    • No instance for (Control.Monad.Fail.MonadFail App)
        arising from a do statement
        with the failable pattern ‘Just registeredEmail’
    • In a stmt of a 'do' block: Just registeredEmail <- getUser uId
      In the expression:
        do let email = either undefined id $ mkEmail "ecky@test.com"
               passw = either undefined id $ mkPassword "1234ABCDefgh"
               ....
           register auth
           Just vCode <- M.getNotificationsForEmail email
           verifyEmail vCode
           ....
      In an equation for ‘action’:
          action
            = do let email = ...
                     ....
                 register auth
                 Just vCode <- M.getNotificationsForEmail email
                 ....
   |
46 |   Just  registeredEmail <- getUser uId

I found a resolution on Reddit, and I've copied it to ensure it survives.

Into Lib.hs I imported

import           Control.Monad                  ( MonadFail )

and added MonadFail to App's deriving list:

newtype App a = App
  { unApp :: ReaderT State IO a
  } deriving (Applicative, Functor, Monad, MonadReader State, MonadIO, MonadFail)

which allowed me to build, and run somefunc into the repl

ghci> someFunc 
("1Bk40ZG6XGJ1bsqWV",1,Email {emailRaw = "ecky@test.com"})