Lifty.hs

module Main where
  
import Control.Monad.Writer
import Control.Monad.Except
import Data.Functor.Identity

f :: Monad m
  => (String -> String -> m String)
  -> String
  -> String
  -> m (Either String (), [String])
f g a b = runWriterT . runExceptT $ do
  v <- lift . lift $ g a b
  when (length v <= 6) $ throwError "too small!"
  tell ["we got this"]
  return ()

s :: Monad m
  => String
  -> String
  -> m String
s a b = return $ a ++ b

main :: IO ()
main = do
  let v1 = runIdentity $ f s "holy" "crap"
  print v1
  let v2 = runIdentity $ f s "hol" "cra"
  print v2