Feedback on use of State monad
Hello Everyone, In the HPFFP book, there's a homework problem where you're supposed to refactor a recursive function that rolls dice to return the number of rolls it takes to reach or exceed an input sum and the list of die that occurred. As practice, decided to take the solution and implement it using the State monad: -- Refactor rollsCountLoggged to use State -- limit sum count StdGen [Die] type RollsState = (Int, Int, Int, StdGen, [Die]) -- count [Die] type RollsValue = (Int, [Die]) runRolls :: State RollsState RollsValue runRolls = do (l, s, c, sg, xs) <- get if (s >= l) then return (c, reverse xs) else do let (d, ng) = randomR (1, 6) sg put (l, s + d, c + 1, ng, intToDie d : xs) runRolls rollsCountState :: Int -> IO () rollsCountState n = print $ evalState (runRolls) (n, 0, 0, mkStdGen 0, []) I'm wondering if that 'do' block under the else is a not so great stylistic choice. If the 'let' and 'put' is moved above the 'if', then that's just an unnecessary call at the end. Or is it just the nature of running inside a monad? Thank You, Andrea Sent with [ProtonMail](https://protonmail.com) Secure Email.
On 2017-07-13 23:29, Atrudyjane via Haskell-Cafe wrote:
runRolls :: State RollsState RollsValue runRolls = do (l, s, c, sg, xs) <- get if (s >= l) then return (c, reverse xs) else do let (d, ng) = randomR (1, 6) sg put (l, s + d, c + 1, ng, intToDie d : xs) runRolls
rollsCountState :: Int -> IO () rollsCountState n = print $ evalState (runRolls) (n, 0, 0, mkStdGen 0, [])
I'm wondering if that 'do' block under the else is a not so great stylistic choice. If the 'let' and 'put' is moved above the 'if', then that's just an unnecessary call at the end. Or is it just the nature of running inside a monad?
It is the nature of running inside a monad. But at the same time, runRolls contains the implementation of two things. Which, to a purist like me, would be one too many. Pulling one of them out into a second function already makes this one much nicer. But there are other, more pressing stylistic choices to make. Namely to use records and mnemonic names. Here is an adapted version taken to more beauty and then beyond into ridiculousness: {-# LANGUAGE RecordWildCards , MultiWayIf #-} […] rollsCountState :: Int -> IO () rollsCountState rollLimit = print $ evalState runRolls initialRollState where runRolls = get >>= \RollsState{..} -> if | rollSum >= rollLimit -> pure (rollCount, reverse rolledDice) | otherwise -> modify rollDice >> runRolls The “ideal” version is probably somewhere in between. Maybe with one of the many whileM or untilM implementations thrown in that everyone keeps reinventing. Cheers, MarLinn
Will need more experience before discerning between beauty and ridiculousness in Haskell code, but your version is helpful nonetheless. Can see that separating out the dice rolls at least makes a big difference. And since there's only one bind, it's readable enough without do syntax. Thank you for your suggestions! Regards, Andrea Sent with [ProtonMail](https://protonmail.com) Secure Email.
runRolls :: State RollsState RollsValue runRolls = do (l, s, c, sg, xs) <- get if (s >= l) then return (c, reverse xs) else do let (d, ng) = randomR (1, 6) sg put (l, s + d, c + 1, ng, intToDie d : xs) runRolls
rollsCountState :: Int -> IO () rollsCountState n = print $ evalState (runRolls) (n, 0, 0, mkStdGen 0, [])
I"m wondering if that "do" block under the else is a not so great stylistic choice. If the "let" and "put" is moved above the "if", then that"s just an unnecessary call at the end. Or is it just the nature of running inside a monad? It is the nature of running inside a monad. But at the same time, runRolls contains the implementation of two things. Which, to a purist
-------- Original Message -------- Subject: Re: [Haskell-cafe] Feedback on use of State monad Local Time: July 14, 2017 3:22 AM UTC Time: July 14, 2017 8:22 AM From: monkleyon@gmail.com To: haskell-cafe@haskell.org On 2017-07-13 23:29, Atrudyjane via Haskell-Cafe wrote: like me, would be one too many. Pulling one of them out into a second function already makes this one much nicer. But there are other, more pressing stylistic choices to make. Namely to use records and mnemonic names. Here is an adapted version taken to more beauty and then beyond into ridiculousness: {-# LANGUAGE RecordWildCards , MultiWayIf #-} […] rollsCountState :: Int -> IO () rollsCountState rollLimit = print $ evalState runRolls initialRollState where runRolls = get >>= \RollsState{..} -> if | rollSum >= rollLimit -> pure (rollCount, reverse rolledDice) | otherwise -> modify rollDice >> runRolls The “ideal” version is probably somewhere in between. Maybe with one of the many whileM or untilM implementations thrown in that everyone keeps reinventing. Cheers, MarLinn _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
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Atrudyjane -
MarLinn