Scala accessing variable outside function

Question

I was reading an article at https://medium.com/@aarshkshah1992/scala-functional-memoization-and-lazy-loading-caches-de116f24828

and this part is really interesting

def memoizedIsPrime: Int => Boolean = {
  def checkIfPrime(i: Int): Boolean = {
    2 to (i - 1) forall (x => i % x != 0)
  }

  var cache = Map.empty[Int, Boolean]
  i => {
    if (!cache.contains(i)) {
      print(s"Calling isPrime since input ${i} has not been seen before and caching the output")
      cache = cache updated(i, checkIfPrime(i))
    }
    else print(s"Input ${i} has been seen before , returning cached output")
    cache(i)
  }
}

val isPrime = memoizedIsPrime

Can anyone explain how i is being accessed and the how the code part after => is really working?

Thanks in advance :)

jwvh · Accepted Answer · 2018-04-21 04:56:28Z

2

The result of memoizedIsPrime is Int => Boolean, in other words a function that takes an Int and returns a Boolean.

So in the body of the code, after we get some preliminary def and val definitions out of the way, we need a way to reference the incoming Int:

i => ...

OK, so it is decided, we'll refer to the received Int as i. Now we need to produce a Boolean value. And that, of course, is the { } delimited block of code that comes after the => which gets executed every time a new Int arrives.

edited Apr 21, 2018 at 4:56

answered Apr 21, 2018 at 4:53

jwvh

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3 Comments

pramesh Over a year ago

What if memoizedIsPrime took multiple parameters? would it be (i, j)?

jwvh Over a year ago

A memoized function of arity 2 would create a 2-tuple with the received parameter values and use that tuple as the key to the cache lookup.

pramesh Over a year ago

Tried. Thanks :)

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