haskell - How to pick a random list element in a pure function?

Question

I want to make a Haskell function that can pick out a random number from a given list. My type signature is:

randomPick :: [a] -> a 

What should I do?

Answer 1

Part of the definition of a "pure" function in Haskell is that it is referentially transparent, that is, interchangeable with the result of evaluating it. This implies that the result of evaluating it must be the same every time. Ergo, the function you want isn't possible, I'm afraid. To generate random numbers in Haskell, a function needs to do one of two things:

Take and return a pseudorandom number generator, e.g.:

randomPick :: RNG -> [a] -> (a, RNG)

Or use IO to access randomness from "the outside world":

randomPick :: [a] -> IO a

Both styles are provided by the module System.Random. Also, in the former case, passing the PRNG around can be abstracted out using the State monad, or perhaps a special-purpose Random monad.