Yesterday morning I decided to stop procrastinating and start learning me some Haskell.

So far I've made this, which is a simple cli 'dice rolling' utility that can be used like:

$ ./dice 3d20
You rolled: 17

The code looks like this:

import Data.List.Split
import Data.Char
import Control.Monad
import Control.Monad.Random
import System.Environment

type Dice = (Int, Int)

diceCode :: String -> Dice
diceCode die = (parts!!0, parts!!1)
    parts = [read x :: Int | x <- take 2 (splitOn "D" (map toUpper die))]

rollDie :: (RandomGen g) => Int -> Rand g Int
rollDie sides = getRandomR (1, sides)

rollDice :: (RandomGen g) => Dice -> Rand g Int
rollDice dice = liftM sum (sequence(replicate rolls (rollDie sides)))
    rolls = fst dice
    sides = snd dice

main = do
  args <- getArgs
  roll <- evalRandIO (rollDice (diceCode (args!!0)))
  putStrLn ("You rolled: " ++ show roll)

I was able to get the diceCode 'parsing' function by myself without too much trouble.

The rollDie function is almost straight from an example in the Control.Monad.Random docs, which helped a lot.

I struggled for quite a while to find a recipe for summing the rolls in rollDice... it seemed to me I ought to use msum but I couldn't find a way to make it work. liftM sum seems to do exactly what I wanted though.

I also found the use of tuples quite cumbersome. In Python I could just do:
rolls, sides = dice
but I seem to have to use the horribly-named fst and snd functions to access the members in Haskell (?)

I guess the next part of my adventure is to try and incorporate this into a larger program, eg a simple game. It seems like the monadically-wrapped random int values are going to force the rest of the code to be 'monad-aware' (i.e. lots of use of liftM) and I wonder if there is a way to avoid this?


1 Answer 1


The list comprehension you use in diceCode is a bit overkill: after all, you only want to split the list of characters into two. You can use break instead.

Here I have to use fmap tail to act on the second list because break retains the value it breaks the list at (the 'd' character here). This would be a good place to handle the sort of errors that would arise if there is no 'd' in the string.

diceCode :: String -> Dice
diceCode die = (read rolls, read sides) where
  (rolls, sides) = fmap tail $ break ('D' ==) $ fmap toUpper die

Your rollDice is a bit complex. Here is how I would rewrite it:

  • (rolls, sides) on the left hand side exposes the two components of the tuple
  • [1..rolls] generates a list of length rolls
  • mapM (const $ rollDie sides) replaces all the numbers in that list with an invocation of rollDie sides and performs the same job as sequence thus returning an Random g [Int]
  • sum <$> (or fmap sum $) goes under the Random g part and sums the elements in the [Int] value.

Putting all this together we get:

rollDice :: (RandomGen g) => Dice -> Rand g Int
rollDice (rolls, sides) = sum <$> mapM (const $ rollDie sides) [1..rolls]

The rest of the code is quite idiomatic. One thing I was a bit concerned about is the fact that a Dice is represented as a pair of Ints: to understand which is which, you need to read the code manipulating them and if you make a mistake the compiler won't warn you: they have the same type! It's quite annoying. One thing you could do is use a record type instead in order to name the two fields:

data Dice' = Dice' { rolls :: Int
                   , sides :: Int }

This way you can access them by their names, build the Dice' using the named syntax, etc.

  • \$\begingroup\$ thanks, I will digest this a bit. first question - I'm just googling what is fmap and why not map... it seems like fmap is more 'general' and I should always just use it instead of map? stackoverflow.com/a/6824333/202168 \$\endgroup\$
    – Anentropic
    Commented Dec 22, 2015 at 11:24
  • \$\begingroup\$ I don't understand whyfmap tail only strips the first char from the second half and not from the first half too. I tried in ghci fmap tail ([1,2,3],[4,1,2,3,4]) returns ([1,2,3],[1,2,3,4]) ...I don't understand why this result \$\endgroup\$
    – Anentropic
    Commented Dec 22, 2015 at 11:35
  • \$\begingroup\$ following your hint in 4th bullet point I wrote this line rollDice (rolls, sides) = fmap sum $ sequence $ replicate rolls (rollDie sides) which seems to work fine... however I don't understand why fmap sum is appropriate here (or even works). liftM sum made sense to me but now I am confused. \$\endgroup\$
    – Anentropic
    Commented Dec 22, 2015 at 12:01
  • \$\begingroup\$ on the other hand, I just found this works rollDice (rolls, sides) = sum <$> (sequence $ replicate rolls (rollDie sides)) ...this makes sense to me if I understand <$> as a sort of 'monadic $' or ...an infix liftM? I had to add parentheses around the right-hand-side to get it to compile \$\endgroup\$
    – Anentropic
    Commented Dec 22, 2015 at 12:06
  • 1
    \$\begingroup\$ I'm going to have to read more about fmap, it's hurting my brain. If I understood the docs you linked it does different things depending on the type of the second arg. In ghci it does what I'd expect if passed a list, so this special behaviour for a pair seems a major WTF. I'm sure there's good reason for this but I find it hard to conceptualise currently \$\endgroup\$
    – Anentropic
    Commented Dec 23, 2015 at 1:02

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