Unit testing IO actions with Hspec

I have found other questions on similar lines but nothing that answers my question in this particular scenario. Furthermore, there seem to be few resources which succinctly cover the subject of unit testing IO actions in Haskell.

Let's say I have this typeclass for my database communication:

data Something = Something String deriving Show

class MonadIO m => MonadDB m where
  getSomething :: String -> m Something
  getSomething s = do
    ... -- assume a DB call is made and an otherwise valid function

instance MonadDB IO

and this function which uses it:

getIt :: MonadDB m => m (Int, Something)
getIt = do
  s@(Something str) <- getSomething "hi"
  return (length str, s) -- excuse the contrived example

I wish to test this getIt function with hspec but without it talking to the database, which presumably means replacing which MonadDB it uses, but how do I achieve that?

Solution

Would this work for you?

#!/usr/bin/env stack
-- stack exec --package transformers --package hspec -- ghci
import Control.Monad.IO.Class
import Control.Monad.Trans.Identity

import Data.Char
import Test.Hspec

data Something = Something String deriving (Eq, Show)

class MonadIO m => MonadDB m where
  getSomething :: String -> m Something
  getSomething s = return $ Something (map toUpper s)

instance MonadDB IO

instance MonadIO m => MonadDB (IdentityT m)

getIt :: MonadDB m => m (Int, Something)
getIt = do
  s@(Something str) <- getSomething "hi"
  return (length str, s)

main :: IO ()
main = hspec $ do
  describe "Some tests" $ do
    it "test getIt" $ do
      runIdentityT getIt `shouldReturn` (2, Something "HI")

    it "test getIt should fail" $ do
      runIdentityT getIt `shouldReturn` (1, Something "HI")

You might also be able to use ReaderT or StateT to "supply" data or a transformation for getSomething to use upon test querying.

Edit: Example use from within hspec.