Category transformers

olf:

Should we stop using monads?

Counter-counterpoint: why are monad cakes even viewed as a good foundation?

The example in this article could be naively written as

isEnvGreaterThanInput :: Handle -> Int -> IO (Maybe Bool)
isEnvGreaterThanInput handle env = do
  input <- hGetLine handle
  case readMaybe input of
    Nothing -> pure Nothing
    Just n  -> pure $ Just (env > n)

It’s shorter than the other two versions, it doesn’t do magic (can’t read from a Handle if it was never passed to you), and it trivially composes with any other plain IO function.