Haskell Quiz/Sampling/Solution Kuklewicz

This puzzle seems far too simple for the use of data structures or even monads. A single tail recursive helper function does the trick.

-- Linear solution by Chris Kuklewicz <haskell@list.mightyreason.com>

-- It is important to realize you are picking from the possible
-- combinations of the digits from 0 to (n-1).  The probability that
-- an element is chosen is (r/n).  This "rolls the dice" for each element
-- of the range in ascending order.
module Main where

import System.Random
import System(getArgs)

main = do [r,n] <- fmap (map read) getArgs
          g <- newStdGen
          mapM_ print (pick r n g)

pick :: Int -> Int -> StdGen -> [Int]
pick r n g  | 0<=r && r<=n  = pick' r n g 0
            | otherwise  = error "r must be between 0 and n"
  where pick' 0 _ _  _ = []
        pick' r n g1 i | r==n = [i..(i+n-1)]
                       | otherwise =
          let (x,g2) = randomR (1,n) g1
          in if x <= r
               then i : (pick' (pred r) (pred n) g2 $! (succ i))
               else      pick' r        (pred n) g2 $! (succ i)