CODEWITHSUNDEEP
scalamonads
Kevin Meredith
Kevin Meredith

Reputation: 41939

Understanding Gen.unit(x)

Working through Functional Programming in Scala, the book shows the Gen Monad definition. Gen, as I understand, is a ScalaCheck trait.

val genMonad = new Monad[Gen] {
    def unit[A](a => A): Gen[A] = Gen.unit(a)
    def flatMap[A, B](ma: Gen[A])(f: A => Gen[B]) = 
        ma.flatMap(f)
}

I believe that OptionMonad.unit is defined as Some(a), but I don't understand Gen.unit(a).

How is Gen.unit(a) defined?

Upvotes: 1

Views: 126

Answers (1)

Dave Griffith
Dave Griffith

Reputation: 20515

A Gen[A] is just an object which can be repeatedly called to give instances of type A. These "generators" are used to drive the ScalaCheck automated testing framework, which lets programmers specify properties of objects of a given type, and then repeatedly generates instances of that type and checks those properties. Gen forms a monad, which is to say it supports the operations of "unit" and "bind", about which approximately a zillion tutorials can be found on the internet. Scala's idioms for monads are a bit inconsistent, as monad types have a standard method of bind, called flatMap, but none for unit. This is because Scala is object oriented, and unit doesn't take an object of it's monad, but instead returns one, so it doesn't make any sense to make unit a method of the underlying class. Instead, most Scala monads leave the unit method implicit, often as a single-element constructor of the monad type.

So with that background out of the way, what's unit of Gen[A]. Well it needs to be something which takes an object of type A as an argument, and then allows repeated generation of objects of type A. Since A could literally be anything, there's really only one thing we can come up with which fits this bill. unit(a) must be a boring generator which repeatedly returns a . Simple once you think it through.

Upvotes: 2

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