Write a function to extract a given number of randomly selected elements from a list

I'm going through the 99 Haskell problems and I have a few question about the solution I implemented for Problem 23.

It asks:


Extract a given number of randomly selected elements from a list.

This is the code I came up with:

-- Extract a given number of randomly selected elements from a list.
import System.Random

randomFromList :: StdGen -> Int -> [a] -> ([a], StdGen)

randomFromList generator 0 _ = ([], generator)
randomFromList generator toDo xs =
  let (nextInt, generator') = next generator
      index = mod nextInt (length xs)
      (otherNumbers, generator'') = randomFromList generator' (toDo -1) xs
  in  ((xs !! index) : otherNumbers, generator'')

It works, but I'm not sure this is idiomatic Haskell nor that it is the best way to address this problem.

None of the solutions in the Haskell wiki uses the let approach so I was wondering if it is discouraged for some reason.

Your solution looks good to me, no reason to discourage using let as far as I know.

Tip: if you make the type randomFromList :: Int -> [a] -> StdGen -> ([a], StdGen), then you can call it like this: main = getStdGen >>= print . randomFromList 7 ['A'..'Z']

Also, if the requirement is interpreted as selecting randomly without replacement, your version only needs slight modification (this also uses randomR rather than mod):

dropAt :: Int -> [a] -> [a]
dropAt k xs = take k xs ++ drop (k+1) xs

randomFromList :: Int -> [a] -> StdGen -> ([a], StdGen)
randomFromList 0 _ generator = ([], generator)
randomFromList _ [] generator = ([], generator)
randomFromList toDo xs generator =
  let (index, generator') = randomR (0, length xs - 1) generator
      (otherNumbers, generator'') = randomFromList  (toDo -1) (dropAt index xs) generator'
  in  ((xs !! index) : otherNumbers, generator'')

See also this shuffle implementation for some attempt at optimisation. (You could potentially solve the random selection by something like take n . shuffle.)

StackExchange Code Review Q#62910, answer score: 2