The book 'Functional Programming in Scala' demonstrates an example of pure functional random number generator as below
trait RNG {
def nextInt: (Int, RNG)
}
object RNG {
def simple(seed: Long): RNG = new RNG {
def nextInt = {
val seed2 = (seed*0x5DEECE66DL + 0xBL) &
((1L << 48) - 1)
((seed2 >>> 16).asInstanceOf[Int],
simple(seed2))
}
}
}
The usage will look like
val (randomNumber,nextState) = rng.nextInt
I do get the part that it's a pure function as it returns the next state and leaves it on the API client to use it to call nextInt
the next time it would need a random number but what I did not understand is 'how will the first random number be generated as we must provide seed
at least once.
Should there be another function to lift seed
to get a RNG
? And if so then how do we expect the client of this API to know about it (because in the non-functional implementation user just calls nextInt
and the state is maintained by API)
Can someone give a full example of pure functional random number generator in Scala and perhaps relate it to state Monad in general.
No, it isn't a pure function because its output doesn't depend only on the input provided (Math. random() can output any value), while pure functions should always output the same value for same inputs.
That random generator RNG
is pure functional, for the same inputs you get always the same outputs. The non-pure-functional part is left for the user of that API (you).
To use the RNG
in a pure-functional way you have to initialize it always with the same initial value, but then you will always get the same sequence of numbers, which is not so useful.
Otherwise, you will have to rely the initialization of RNG
to an external system (usually the wall-clock time) and so introducing side effects (bye pure functional).
val state0 = RNG.simple(System.currentTimeMillis)
val (rnd1, state1) = state0.nextInt
val (rnd2, state2) = state1.nextInt
val (rnd3, state3) = state2.nextInt
println(rnd1, rnd2, rnd3)
[EDIT]
Inspired by the answer of @Aivean, I created my version of randoms Stream
:
def randoms: Stream[Int] = Stream.from(0)
.scanLeft((0, RNG.simple(System.currentTimeMillis)))((st, _) => st._2.nextInt)
.tail
.map(_._1)
println(randoms.take(5).toList)
println(randoms.filter(_ > 0).take(3).toList)
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