[Haskell-cafe] Feedback on use of State monad

[MarLinn]

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