# Haskell Quiz/Probable Iterations/Solution Dolio

From HaskellWiki

This quiz was pretty simple. The list monad makes generation of the test cases simple, and the writer monad is handy for capturing potential output for each line. I used a DList for the writer accumulator to avoid repeated copying.

```
module Main where
import Data.DList
import Control.Monad.Writer.Lazy
import System.Environment
import System.Exit
import Control.Monad
import Text.Printf
die = [1..6]
check :: ([Int] -> Bool) -> (Int, [Int]) -> Writer (DList String) Bool
check p (line, roll) = do tell $ if b then singleton hit else singleton noHit ; return b
where
b = p roll
noHit = printf "%12d %s" line (show roll)
hit = noHit ++ " <=="
sample :: Int -> Int -> (Int, (Int, DList String))
sample i j = (length l, runWriter . liftM length . filterM (check p) $ zip [1..] l)
where
p l = length (filter (==5) l) >= j
l = replicateM i die
chop :: [a] -> [a]
chop [] = []
chop (x:xs) = x : chop (drop 49999 xs)
main = do (v,s,i,j) <- processArgs
let (total, (selected, out)) = sample i j
if v
then mapM_ putStrLn $ toList out
else when s . mapM_ putStrLn . chop $ toList out
putStrLn ""
putStr "Number of desirable outcomes is "
print selected
putStr "Number of possible outcomes is "
print total
putStrLn ""
putStr "Probability is "
print $ fromIntegral selected / fromIntegral total
processArgs = do l <- getArgs
case l of
[i,j] -> return (False, False, read i, read j)
["-v", i, j] -> return (True, False, read i, read j)
["-s", i, j] -> return (False, True, read i, read j)
_ -> do putStrLn "Unrecognized arguments."
exitFailure
```