Hello, I am new not only to Haskell, but to programming in general. I hope that I am posting in the correct forum. I am working through "Practical Web Development With Haskell" by Ecky Putrady (2018), New York: Apress, and I am having some difficulty understanding the STM portion of Chapter three. More specifically, I cannot get the following code snippet to return the expected result in the REPL: -- snippet from Adapter.InMemory.Auth module as composed on pages 53-64 in the book import ClassyPrelude import qualified Domain.Auth as D -- as composed on pages 35-52 in the book import Conrol.Monad.Except import Text.StringRandom import Data.Has -- ... data State = State { stateAuths :: [(D.UserId, D.Auth)] -- ... initialState :: State initialState = State { stateAuths = [] -- ... type InMemory r m = (Has (TVar State) r, MonadReader r m, MonadIO m) addAuth :: InMemory r m => D.Auth -> m (Either D.RegistrationError D.VerificationCode) addAuth auth = do tvar <- asks getter -- ... rest of function These are the instructions for "Verification in REPL" provided on page 64 of the book: > :l Adapter.InMemory.Auth > let email = D.mkEmail "ecky@test.com" > let passw = D.mkPassword "1234ABCDefgh" > let auth = either undefined id $ D.Auth <$> email <*> passw > s <- newTVarIO initialState > addAuth s auth Calling the addAuth function is supposed to print the following: Right "aBNhtG653Bga9kas" -- (or whatever random vCode stringRandomIO generates) However, GHCI instead tells me: <interactive>:6.9: error: * Couldn't match expected type 'D.Auth' with actual type 'TVar State' * In the first argument of 'addAuth', namely 's' In the expression: addAuth s auth -- ... The source code for the book at https://github.com/Apress/practical-webdev-haskell was created with stack resolver lts-9.11, while I am using resolver lts-14.11. I am doing this intentionally, because debugging the compilation errors helps guide me to become more familiar with the documentation in the packages being implemented, as well as helping me to understand general Haskell concepts little by little. I have tried looking through the changelogs for the stm and classy-prelude packages on https://hackage.haskell.org, for any clues which would be intelligible to me (an extremely restrictive constraint, admittedly), that would help me understand why the addAuth function thinks the TVar State we pass to it should be a D.Auth, and why this presumably would not have been the case had I built the project with the lts-9.11 resolver, but I have not been able to understand it thus far.