CODEWITHSUNDEEP
haskellrandom
vondip
vondip

Reputation: 14029

Haskell - generate and use the same random list

In Haskell, I'd like to generate a random list of Ints and use it throughout my program. My current solution causes the array to be created randomly each time I access/use it.

How can I overcome this problem?

Upvotes: 1

Views: 178

Answers (2)

Random Dev
Random Dev

Reputation: 52270

Here is a simple example (@luqui mentioned) you should be able to generalize to your need:

module Main where

import Control.Monad (replicateM)
import System.Random (randomRIO)

main :: IO ()
main = do
  randomList <- randomInts 10 (1,6)
  print randomList
  let s = myFunUsingRandomList randomList
  print s

myFunUsingRandomList :: [Int] -> Int
myFunUsingRandomList list = sum list

randomInts :: Int -> (Int,Int) -> IO [Int]
randomInts len bounds = replicateM len $ randomRIO bounds

remarks

  • randomInts is an IO-action that will produce you your random list of integers, you see it's use in the very first line of mains do-block. Once it is used there the list will stay the same.
  • myFunUsingRandomList is a very simple example (just summing it) of using such a list in a pure fashion - indeed it just expects a list of integers and I just happen to call it with the random list from main

Upvotes: 2

Alec
Alec

Reputation: 32309

Follow this example. Then, pass rs from main to whatever function needs it.

You can't have a global variable for this without using something like unsafePerformIO, which, as the name indicates, should not be used. The reason is that picking random numbers is an inherently non-deterministic operation, dependent on the outside IO state.

Upvotes: 0

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