Cont monad defined in Frege

[zhou...@163.com]

No big deal, only recording my progress striving to understand monad and monad transformer with this tutorial https://en.wikibooks.org/wiki/Haskell/Continuation_passing_style and this https://wiki.haskell.org/All_about_monads#The_Continuation_monad

data Cont r a = Cont { runCont :: ((a -> r) -> r)}

instance Monad (Cont r) where

  pure x = cont ($ x)

  s >>= f = cont $ \c -> runCont s $ \x -> runCont (f x) c

cont :: ((a -> r) -> r) -> Cont r a

cont f = Cont { runCont = f}

runCont :: Cont r a -> (a -> r) -> r

runCont c = Cont.runCont c

callCC f = cont $ \h -> runCont (f (\a -> cont $ \_ -> h a)) h

fun :: Int -> String

fun n = (`runCont` id) $ do

   str <- callCC $ \exit1 -> do

     when (n < 10) (exit1 (show n))

     let ns = map digitToInt (unpacked (show (n `div` 2)))

     n' <- callCC $ \exit2 -> do

       when ((length ns) < 3) (exit2 (length ns))

       when ((length ns) < 5) (exit2 n)

       when ((length ns) < 7) $ do

         let ns` = map intToDigit (reverse ns)

         exit1 $ packed $ (dropWhile (== '0') ns')

       return $ sum ns

     return $ "(ns = " ++ (show ns) ++ ") " ++ (show n')

   return $ "Answer: " ++ str

[zhou...@163.com]

The code is working on my 3.24.100 Frege Repl, and I am trying to reinvent ContT monad transformer in Frege now, hope I can learn more about Haskell

在 2017年11月23日星期四 UTC+8上午10:07:07，zhou...@163.com写道：

[Dierk König]

Cool! Bring it on!

Dierk

sent from:mobile 

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