从Haskell的某些列表中创建随机数

Question

I know that Haskell provides a function for producing a random number between a certain range.

Is there a way of producing random number from a set of numbers ?

Example: Suppose list of [1, 6, 9] exists and I want the random number to be out of this list ie 1 or 6 or 9.

Answer 1

Well there are certainly a lot of ways to accomplish this task. Perhaps the most straight forward is to 1) get a random number between 0 and length list - 1 2) Use that number to index the list ( list !! rand ).

For example, if you are OK with living in the IO monad for your random numbers then you could use randomRIO :

import System.Random

oneOf :: [a] -> IO (Maybe a)
oneOf [] = return Nothing
oneOf list = do
   rand <- randomRIO (0,length list - 1)
   return (Just $ list !! rand)