In Scala one might define methods inside other methods. This limits their scope of use to inside of definition block. I use them to improve readability of code that uses several higher-order functions. In contrast to anonymous function literals, this allows me to give them meaningful names before passing them on.
For example:
class AggregatedPerson extends HashSet[PersonRecord] { def mostFrequentName: String = { type NameCount = (String, Int) def moreFirst(a: NameCount, b: NameCount) = a._2 > b._2 def countOccurrences(nameGroup: (String, List[PersonRecord])) = (nameGroup._1, nameGroup._2.size) iterator.toList.groupBy(_.fullName). map(countOccurrences).iterator.toList. sortWith(moreFirst).head._1 } }
Is there any runtime cost because of the nested method definition I should be aware of?
Does the answer differ for closures?
What is a nested method. If a method is defined inside another method, the inner method is said to be nested inside the outer method or it is called Nested method. All languages do not support nesting a method inside another method but python allows you to do so.
Java does not support “directly” nested methods. Many functional programming languages support method within method. But you can achieve nested method functionality in Java 7 or older version by define local classes, class within method so this does compile.
No. It is invalid syntax.
C# Local Functions are a great new feature in C# 7. Local functions are nested functions. They are methods declared in another method and run in the context of that method. They are methods that are used only by one other method and help to keep each method small and focused.
During compilaton, the nested functions are moveFirst
and countOccurences
are moved out to the same level as mostFrequentName
. They get compiler synthesized names: moveFirst$1
and countOccurences$1
.
In addition, when you refer to one of these methods without an argument list, it is lifted into a function. So map(countOccurences)
is the same as writing map((a: (String, List[PersonRecord])) => countOccurences(a))
. This anonymous function is compiled to a separate class AggregatedPerson$$anonfun$mostFrequentName$2
, which does nothing more than forward to countOccurences$
.
As a side note, the process of lifting the method to a function is called Eta Expansion. It is triggered if you omit the argument list in a context where a function type is expected (as in your example), or if you use _
in place of the entire argument list, or in place of each argument (val f1 = countOccurences _ ; val f2 = countOccurences(_)
.
If the code was directly in the closure, you would have one fewer method call in your stack, and one fewer synthetic method generated. The performance impact of this is likely to be zero in most cases.
I find it to be a fantastically useful tool to structure code, and consider your example very idiomatic Scala.
Another useful tool is using small blocks to initialize a val:
val a = { val temp1, temp2 = ... f(temp1, temp2) }
You can use scalac -print
to see exactly how Scala code is translated into a form ready for the JVM. Heres the output from your program:
[[syntax trees at end of cleanup]]// Scala source: nested-method.scala package <empty> { class AggregatedPerson extends scala.collection.mutable.HashSet with ScalaObject { def mostFrequentName(): java.lang.String = AggregatedPerson.this.iterator().toList().groupBy({ (new AggregatedPerson$$anonfun$mostFrequentName$1(AggregatedPerson.this): Function1) }).map({ { (new AggregatedPerson$$anonfun$mostFrequentName$2(AggregatedPerson.this): Function1) } }, collection.this.Map.canBuildFrom()).$asInstanceOf[scala.collection.MapLike]().iterator().toList().sortWith({ { (new AggregatedPerson$$anonfun$mostFrequentName$3(AggregatedPerson.this): Function2) } }).$asInstanceOf[scala.collection.IterableLike]().head().$asInstanceOf[Tuple2]()._1().$asInstanceOf[java.lang.String](); final def moreFirst$1(a: Tuple2, b: Tuple2): Boolean = scala.Int.unbox(a._2()).>(scala.Int.unbox(b._2())); final def countOccurrences$1(nameGroup: Tuple2): Tuple2 = new Tuple2(nameGroup._1(), scala.Int.box(nameGroup._2().$asInstanceOf[scala.collection.SeqLike]().size())); def this(): AggregatedPerson = { AggregatedPerson.super.this(); () } }; @SerialVersionUID(0) @serializable final <synthetic> class AggregatedPerson$$anonfun$mostFrequentName$1 extends scala.runtime.AbstractFunction1 { final def apply(x$1: PersonRecord): java.lang.String = x$1.fullName(); final <bridge> def apply(v1: java.lang.Object): java.lang.Object = AggregatedPerson$$anonfun$mostFrequentName$1.this.apply(v1.$asInstanceOf[PersonRecord]()); def this($outer: AggregatedPerson): AggregatedPerson$$anonfun$mostFrequentName$1 = { AggregatedPerson$$anonfun$mostFrequentName$1.super.this(); () } }; @SerialVersionUID(0) @serializable final <synthetic> class AggregatedPerson$$anonfun$mostFrequentName$2 extends scala.runtime.AbstractFunction1 { final def apply(nameGroup: Tuple2): Tuple2 = AggregatedPerson$$anonfun$mostFrequentName$2.this.$outer.countOccurrences$1(nameGroup); <synthetic> <paramaccessor> private[this] val $outer: AggregatedPerson = _; final <bridge> def apply(v1: java.lang.Object): java.lang.Object = AggregatedPerson$$anonfun$mostFrequentName$2.this.apply(v1.$asInstanceOf[Tuple2]()); def this($outer: AggregatedPerson): AggregatedPerson$$anonfun$mostFrequentName$2 = { if ($outer.eq(null)) throw new java.lang.NullPointerException() else AggregatedPerson$$anonfun$mostFrequentName$2.this.$outer = $outer; AggregatedPerson$$anonfun$mostFrequentName$2.super.this(); () } }; @SerialVersionUID(0) @serializable final <synthetic> class AggregatedPerson$$anonfun$mostFrequentName$3 extends scala.runtime.AbstractFunction2 { final def apply(a: Tuple2, b: Tuple2): Boolean = AggregatedPerson$$anonfun$mostFrequentName$3.this.$outer.moreFirst$1(a, b); <synthetic> <paramaccessor> private[this] val $outer: AggregatedPerson = _; final <bridge> def apply(v1: java.lang.Object, v2: java.lang.Object): java.lang.Object = scala.Boolean.box(AggregatedPerson$$anonfun$mostFrequentName$3.this.apply(v1.$asInstanceOf[Tuple2](), v2.$asInstanceOf[Tuple2]())); def this($outer: AggregatedPerson): AggregatedPerson$$anonfun$mostFrequentName$3 = { if ($outer.eq(null)) throw new java.lang.NullPointerException() else AggregatedPerson$$anonfun$mostFrequentName$3.this.$outer = $outer; AggregatedPerson$$anonfun$mostFrequentName$3.super.this(); () } } }
There is a small runtime cost. You can observe it here (apologies for the long code):
object NestBench { def countRaw() = { var sum = 0 var i = 0 while (i<1000) { sum += i i += 1 var j = 0 while (j<1000) { sum += j j += 1 var k = 0 while (k<1000) { sum += k k += 1 sum += 1 } } } sum } def countClosure() = { var sum = 0 var i = 0 def sumI { sum += i i += 1 var j = 0 def sumJ { sum += j j += 1 var k = 0 def sumK { def sumL { sum += 1 } sum += k k += 1 sumL } while (k<1000) sumK } while (j<1000) sumJ } while (i<1000) sumI sum } def countInner() = { var sum = 0 def whileI = { def whileJ = { def whileK = { def whileL() = 1 var ksum = 0 var k = 0 while (k<1000) { ksum += k; k += 1; ksum += whileL } ksum } var jsum = 0 var j = 0 while (j<1000) { jsum += j; j += 1 jsum += whileK } jsum } var isum = 0 var i = 0 while (i<1000) { isum += i; i += 1 isum += whileJ } isum } whileI } def countFunc() = { def summer(f: => Int)() = { var sum = 0 var i = 0 while (i<1000) { sum += i; i += 1 sum += f } sum } summer( summer( summer(1) ) )() } def nsPerIteration(f:() => Int): (Int,Double) = { val t0 = System.nanoTime val result = f() val t1 = System.nanoTime (result , (t1-t0)*1e-9) } def main(args: Array[String]) { for (i <- 1 to 5) { val fns = List(countRaw _, countClosure _, countInner _, countFunc _) val labels = List("raw","closure","inner","func") val results = (fns zip labels) foreach (fl => { val x = nsPerIteration( fl._1 ) printf("Method %8s produced %d; time/it = %.3f ns\n",fl._2,x._1,x._2) }) } } }
There are four different methods for summing integers:
And we see the results on my machine in terms of nanoseconds taken in the inner loop:
scala> NestBench.main(Array[String]()) Method raw produced -1511174132; time/it = 0.422 ns Method closure produced -1511174132; time/it = 2.376 ns Method inner produced -1511174132; time/it = 0.402 ns Method func produced -1511174132; time/it = 0.836 ns Method raw produced -1511174132; time/it = 0.418 ns Method closure produced -1511174132; time/it = 2.410 ns Method inner produced -1511174132; time/it = 0.399 ns Method func produced -1511174132; time/it = 0.813 ns Method raw produced -1511174132; time/it = 0.411 ns Method closure produced -1511174132; time/it = 2.372 ns Method inner produced -1511174132; time/it = 0.399 ns Method func produced -1511174132; time/it = 0.813 ns Method raw produced -1511174132; time/it = 0.411 ns Method closure produced -1511174132; time/it = 2.370 ns Method inner produced -1511174132; time/it = 0.399 ns Method func produced -1511174132; time/it = 0.815 ns Method raw produced -1511174132; time/it = 0.412 ns Method closure produced -1511174132; time/it = 2.357 ns Method inner produced -1511174132; time/it = 0.400 ns Method func produced -1511174132; time/it = 0.817 ns
So, bottom line is: nesting functions really doesn't hurt you at all in simple cases--the JVM will figure out that the call can be inlined (thus raw
and inner
give the same times). If you take a more functional approach, the function call can't be completely neglected, but the time taken is vanishingly small (approximately 0.4 ns extra per call). If you use a lot of closures, then closing them gives an overhead of something like 1 ns per call at least in this case where a single mutable variable is written to.
You can modify the code above to find answers to other questions, but the bottom line is that it is all very fast, ranging between "no penalty whatsoever" through to "only worry about in the very tightest inner loops that otherwise have minimal work to do".
(P.S. For comparison, the creation of a single small object takes ~4 ns on my machine.)
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