Scala: How to avoid var here

Question

I have a code snippet like this:

def until(terminationCond: List[A]=>Boolean, combiner: List[A]=>List[A] )(obj: List[A]): A = { var tempObj = obj while(!terminationCond(tempObj)) { tempObj = combiner(obj) } tempObj.head }

I am looking out a way to write this code from functional programming style, avoiding any mutable types.

This snippet combiner(obj) is, by definition, not FP because it is a function that repeatedly takes the same argument but returns different results. — jwvh
– jwvh, Commented Sep 22, 2016 at 16:40

vvg · Accepted Answer · 2016-09-22 16:21:28Z

4

Using recursion:

@tailrec def doUntilTerm(obj: List[A]) = if (terminationCond(obj)) obj.head else doUntilTerm(obj)

edited Sep 22, 2016 at 16:21

answered Sep 22, 2016 at 15:38

vvg

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1 Comment

Iadams Over a year ago

if (terminationCond(obj)) obj.head else doUntilTerm(combine(obj)) Otherwise right.

Travis Brown · Accepted Answer · 2016-09-22 16:49:09Z

I find the following a little more declarative than the explicitly recursive version:

def until[A]( terminationCond: List[A] => Boolean, combiner: List[A] => List[A] )(obj: List[A]): A = Stream.iterate(obj)(combiner).dropWhile(!terminationCond(_)).head.head

I.e. we create a stream of results by iteratively applying combiner, drop as long as the termination condition doesn't hold, and then return the head of the first for which it does.

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