Question

For testing purposes of a RandomGen-reliant function I need a RandomGen instance which will return the numbers I tell it to on instantiation so that I could compare the results of a tested function to the correct ones. I.e. on each call to random I want this function to return the next number from a list [5, 8, 2, 6, 6] (it may throw an error if it reaches the end or it may cycle). I expect an instance of that generator to be created as simply as StaticGen [5, 8, 2, 6, 6].

I am wondering if there exists such a library or maybe if I should take some different approach at my testing.

On Updates: turns out I originally stated the question wrong by specifying the next function instead of random as the one I wanted to return the values from a list. My apologies.

Answer 1

Well, you could distribute such a library. But it would be very short.

newtype StaticGen = StaticGen [Int]
instance RandomGen StaticGen where
  next (StaticGen (x:xs)) = (x, StaticGen xs)
  next _ = error "No more random numbers!"
  split x = (x, x)           -- broken, but irrelevant
  genRange _ = (0, maxBound) -- choose your own

Well, as mentioned this won’t give you out the sequence you pass in. You could rewrite your functions, so instead of taking a random number generator like this:

myFunction :: RandomGen g => g -> a -> (b, g)

They work like this instead:

myFunction :: forall g. (g -> (Int, g)) -> g -> a -> (b, g)

That way you can use it with a random number generator:

myFunction random g a

Or you can use it with a list:

myFunction (\(x:xs) -> (x, xs)) [1,2,3,4] a