Cookbooks
Haskell
Chapter 3. Numbers
Generating random numbers

Haskell:
Generating random numbers

How to:

To generate random numbers in Haskell, one typically uses the random package, which is part of the Haskell Platform. Here’s a step-by-step guide:

First, ensure you have the random package installed. If not, you can get it via Cabal or Stack.

Generating a Random Number

To generate a simple random number, you can use the randomRIO function, which produces a random value within a specified range.

import System.Random (randomRIO)

main :: IO ()
main = do
  randomNumber <- randomRIO (1, 10) :: IO Int
  putStrLn $ "Random number: " ++ show randomNumber

Generating a List of Random Numbers

Generating a list of random numbers is slightly more involved but still straightforward:

import System.Random (randomRIO)

randomList :: Int -> IO [Int]
randomList 0 = return []
randomList n = do
  r <- randomRIO (1, 100)
  rs <- randomList (n-1)
  return (r:rs)

main :: IO ()
main = do
  numbers <- randomList 5
  print numbers

This code snippet creates a function randomList that generates a list of random integers. Replace (1, 100) with your desired range.

Deep Dive

The Haskell random package provides a pseudo-random number generator (PRNG), which means the numbers generated are not truly random but can appear to be random for many applications. The core of Haskell’s random generation capability lies in the RandomGen type class, which abstracts different methods of generating random numbers, and the Random type class, which includes types that can be generated randomly.

Historically, Haskell’s approach to random number generation has emphasized purity and reproducibility. This is why operations involving randomness are explicitly handled in the IO monad or require manually passing and updating generator states — to maintain referential transparency.

In certain applications, such as cryptography, the pseudo-random numbers generated by the default PRNG may not be secure enough. For these use cases, Haskell programmers often turn to more specialized libraries like crypto-random, which are designed to meet the stringent requirements of cryptographic applications.

Moreover, alternative libraries like mwc-random offer better performance and quality of random numbers for simulations and other applications, by implementing modern algorithms such as the Mersenne Twister.

When choosing a random number generation approach in Haskell, it’s essential to consider the application’s needs regarding randomness quality, performance, and security to select the most appropriate tool or library.

Rounding numbers