Random Number Set Haskell

Question

i need to take one random element from a set of int. I am trying to use the function elemAt :: Int->Set a->a but i need to randomly generate the Int which will be used in the first argument of elemAt , but i am having a problem because randomRIO is a monadic function. How can i use randomRIO in a funcion that returns an int?Eg:

function :: Int->Int
function i = x <- randomRIO(0,i-1)

What i am trying to do is something like this:

element set = elemAt (function (size set)) set

Answer 1

In Haskell we want to know if our functions are pure - in your case you want to implement a function which is obviously not pure at all - it's random.

So you should be honest about this function and indicate this in the type. Now of course you can use randomRIO and in this case you indicate this by returning IO -computations.

Assuming you want to take random elements form Data.Set s it could look like this:

randomElement :: Set a -> IO a
randomElement set = do
   i <- randomRIO (0, size set - 1)
   return $ set `elemAt` i

for Map s, etc. it's similar