Question

I need to generate a random number from 1 to 5 which isn’t already in the passed list.

Here is my code:

questionNumberGenerator:: [Int]->Int
questionNumberGenerator ql = do
    g <- newStdGen
    let qn=(fst((randomR (1, 5) g)))::Int
    let element = (elem qn ql)::Bool
    if(element==True)
        then
           questionNumberGenerator ql
        else do
           return qn

What am I doing wrong?

Answer 1

The problem is your type signature; it should be:

questionNumberGenerator :: [Int] -> IO Int

questionNumberGenerator returns an IO action (which it must, to use the global random number generator), but your type signature says it’s pure.

If you want to use questionNumberGenerator in pure code, you’ll have to thread the random number generator state explicitly, like this:

questionNumberGenerator :: (RandomGen g) => [Int] -> g -> (Int, g)
questionNumberGenerator ql g =
    let (qn, g') = randomR (1, 5) g
        element = elem qn ql
    in if (element==True)
           then questionNumberGenerator ql g'
           else (qn, g')

By the way, (element==True) is redundant; element means the exact same thing.