A small state monad example demonstrating the creation of a number stream collection accumulated in a string

state_monad_sample.hs

type State = (String, Integer)
data ST x = S(State -> (x, State))

apply st (S f) = f st

instance Functor ST where
    fmap f (S f') = S(\s -> let (x, s') = f' s in (f x, s'))
     
instance Applicative ST where
    pure x = S(\s -> (x, s))
    (S f) <*> s = S(\s' -> let (S f') = fmap (\x -> (fst $ f s') x) s in f' s')
    
instance Monad ST where
    return x = S(\s -> (x, s))
    (S f) >>= f' = S(\s -> let (x, s') = f s in apply s' $ f' x)


double = S(\(acc, last) -> let n = last * 2 in (n, (acc ++ "\n" ++ show n, n)))
triple = S(\(acc, last) -> let n = last * 3 in (n, (acc ++ "\n" ++ show n, n)))
comment str = S(\(acc, last) -> (last, (acc ++ "\n" ++ str, last)))

streamOps :: ST Integer
streamOps = do
    first <- double
    double
    third <- triple
    comment $ "The first and third operation results are " ++ show first ++ " and " ++ show third
    triple
    end <- double
    comment $ "The stream ends with " ++ show end

intStreamString :: String
intStreamString = let (_, (str, _)) = apply ("--Start--", 1) streamOps in str

main = putStrLn intStreamString

-- Main results:
{-
    --Start--
    2
    4
    12
    The first and third operation results are 2 and 12
    36
    72
    The stream ends with 72
-}