TestRandom

TestRandom allows for deterministically testing effects involving randomness.

TestRandom operates in two modes. In the first mode, TestRandom is a purely functional pseudo-random number generator. It will generate pseudo-random values just like scala.util.Random except that no internal state is mutated. Instead, methods like nextInt describe state transitions from one random state to another that are automatically composed together through methods like flatMap. The random seed can be set using setSeed and TestRandom is guaranteed to return the same sequence of values for any given seed. This is useful for deterministically generating a sequence of pseudo-random values and powers the property based testing functionality in ZIO Test.

In the second mode, TestRandom maintains an internal buffer of values that can be "fed" with methods such as feedInts and then when random values of that type are generated they will first be taken from the buffer. This is useful for verifying that functions produce the expected output for a given sequence of "random" inputs.

import zio.Random
import zio.test.environment.TestRandom

for {
  _ <- TestRandom.feedInts(4, 5, 2)
  x <- Random.nextIntBounded(6)
  y <- Random.nextIntBounded(6)
  z <- Random.nextIntBounded(6)
} yield x + y + z == 11

TestRandom will automatically take values from the buffer if a value of the appropriate type is available and otherwise generate a pseudo-random value, so there is nothing you need to do to switch between the two modes. Just generate random values as you normally would to get pseudo-random values, or feed in values of your own to get those values back. You can also use methods like clearInts to clear the buffer of values of a given type so you can fill the buffer with new values or go back to pseudo-random number generation.