Pure Monad

Maybe.hs

import Data.Map.Strict

-- Monadic
f :: Map String Int -> String -> Maybe Int
f m x = do 
    y <- lookup m x
    let y' = g y
    y'' <- if y' > 8 
        then Just $ y' * 10
        else Nothing
    return $ y'' - 6
  
-- Not monadic
f' :: Map String Int -> String -> Maybe Int
f' m x =
    let 
        y = lookup m x 
        y' = case y of
            Just z  -> Just $ g z
            Nothing -> Nothing
        y'' = case y' of
            Just z | z > 8 -> Just $ y * 10
            _              -> Nothing
    in
        case y'' of
            Just z'' -> Just $ z'' - 6
            Nothing -> Nothing 

-- Some pure function
g :: Int -> Int
g x = x * 46