Functional programming in Scala: how to unit test function composition

Question

I'm reading the book "Functional Programming in Scala" and I'm writing unit tests to check the implementation of the exercises.

One thing I'm not sure I understood is how to unit test exercises using function composition and passage of state, explained in the chapter 6 of the book.

For example one exercise (ex. 6.8 of the book) is related to the implementation of the function flatMap using passage of state, below the solution:

def flatMap[A, B](f: Rand[A])(g: A => Rand[B]): Rand[B] = {
  rng => {
    val (a, rng2) = f(rng)
    g(a)(rng2)
  }
}

Now, I do understand the implementation of the solution, but I'm not able to figure out how to write a unit test for it. In detail, I understand how to pass Rand[A] as parameter for the function f, but I don't know how to build the function g, that should return a Rand[B].

Has anyone came up with a solution for it?

Thanks in advance.

is you question about testing the solution or using the solution? — pedrofurla

metaphori metaphori · Accepted Answer · 2017-06-22T06:41:07

If you want to implement a function A => Rand[B] that takes an A and returns a Rand[B], you just need

A way to create a Rand[B] (where this is defined as an alias for RNG => (B,RNG)). You can do it explicitly (by creating an RNG and defining the function) or you can use other functions that return a Rand[B] (such as unit, map, ...).

A logic to produce a B from an A

val g: RNG = // ...
val ra: Rand[Int] = // ... you know how to build it, you say
val rb: Rand[String] = flatMap(ra)(n => unit("hello")) // E.g., you ignore 'n'
rb(g)._1 shouldEqual "hello"

Functional programming in Scala: how to unit test function composition

1 Answers