[Haskell-cafe] Feedback on use of State monad

[Atrudyjane]

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

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> -------- 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 at gmail.com
> To: haskell-cafe at haskell.org
> 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
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