Note: This case is valid also for the “old” #method method usage. The reason why I mention that in the “dot-colon” context, is the fact that due to the syntax sugar addition, this style of coding will surely be used more intensely.

Note: This feature has been reverted. See details here: bugs.ruby-lang.org/issues/16275.

Note: Benchmarks and the optimization approach still applies to the #method method usage.

One of the most interesting for me features of the upcoming Ruby 2.7 is the syntax sugar for the method reference. I like using the #method method together with the #then ( #yield_self ) operator in order to compose several functions in a “pipeline” like fashion. It’s particularly useful when you process data streams or build ETL pipelines.

Here’s an example of how you could use it:

class Parse def self.call(string) string.to_f end end class Normalize def self.call(number) number.round end end class Transform def self.call(int) int * 2 end end # Simulate a long-running data producing source with batches # Builds a lot of stringified floats (each unique) stream = Array.new(10_000) do |i| Array.new(100) { |i| "#{i}.#{i}" } end stream.each do |batch| batch .map(&Parse.:call) .map(&Normalize.:call) .map(&Transform.:call) end

It’s nice, it’s clear, it’s short. So what is wrong with it?

Well, what is wrong is Ruby itself. Each time you reference a method using the #method method, Ruby gives you a new instance of a #Method class. Even when you’re fetching the method of the same instance of an object. That’s not all! Since we’re using the & operator, each of the fetched method references is later on converted into a Proc object using the #to_proc method.

nil.:nil?.object_id #=> 47141222633640 nil.:nil?.object_id #=> 47141222626280 nil.:nil?.object_id #=> 47141222541360 # In general nil.:nil?.object_id == nil.:nil?.object_id #=> false nil.:nil?.to_proc == nil.:nil?.to_proc #=> false

It means that when you process a lot of data samples, you may spin up a lot of objects and pay a huge performance penalty. Especially when you operate on a per entity basis:

stream.each do |batch| batch.each do |message| message .then(&Parse.:call) .then(&Normalize.:call) .then(&Transform.:call) end end

If you run the same code as above, but in a way like this:

stream.each do |batch| batch.each do |message| Transform.call( Normalize.call( Parse.call(message) ) ) end end

you end up having 12 million fewer objects and you will be able to run your code almost 10 times faster!

See for yourself:

require 'benchmark/ips' GC.disable class Parse def self.call(string) string.to_f end end class Normalize def self.call(number) number.round end end class Transform def self.call(int) int * 2 end end # Builds a lot of stringified floats (each unique) stream = Array.new(10_000) do |i| Array.new(100) { |i| "#{i}.#{i}" } end Benchmark.ips do |x| x.config(time: 5, warmup: 1) x.report('std') do stream.each do |batch| batch.each do |message| Transform.call( Normalize.call( Parse.call(message) ) ) end end end # This case was pointed out by Vladimir Dementyev # See the comments for more details x.report('std-then') do stream.each do |batch| batch.each do |message| message.then do |message| Parse.call(message) end.then do |message| Normalize.call(message) end.then do |message| Transform.call(message) end end end end x.report('dot-colon') do stream.each do |batch| batch.each do |message| message .then(&Parse.:call) .then(&Normalize.:call) .then(&Transform.:call) end end end x.compare! end

Results:

Warming up -------------------------------------- std 1.000 i/100ms std-then 1.000 i/100ms dot-colon 1.000 i/100ms Calculating ------------------------------------- std 6.719 (± 0.0%) i/s - 34.000 in 5.060580s std-then 3.085 (± 0.0%) i/s - 16.000 in 5.187639s dot-colon 0.692 (± 0.0%) i/s - 4.000 in 5.824453s Comparison: std: 6.7 i/s std-then: 3.1 i/s - 2.18x slower dot-colon: 0.7 i/s - 9.70x slower

Same for the allocation of the objects:

tao1 = GC.stat[:total_allocated_objects] stream.each do |batch| batch.each do |message| Transform.call( Normalize.call( Parse.call(message) ) ) end end tao2 = GC.stat[:total_allocated_objects] stream.each do |batch| batch.each do |message| message.then do |message| Parse.call(message) end.then do |message| Normalize.call(message) end.then do |message| Transform.call(message) end end end tao3 = GC.stat[:total_allocated_objects] stream.each do |batch| batch.each do |message| message .then(&Parse.:call) .then(&Normalize.:call) .then(&Transform.:call) end end tao4 = GC.stat[:total_allocated_objects] p "Std allocated: #{tao2 - tao1}" p "Std-then allocated: #{tao3 - tao2}" p "Dot-colon allocated: #{tao4 - tao3}"

Std allocated: 1 Std-then allocated: 2 Dot-colon allocated: 12000002

So, shouldn’t we use the new feature (and method reference in general) at all? Not exactly. There are two things you need to do if you want to use it and not slow down your application that much.

Memoize your method references

Instead of fetching the method reference for each of the objects (or batches), fetch it once and re-use:

parse = Parse.:call normalize = Normalize.:call transform = Transform.:call stream.each do |batch| batch.each do |message| message .then(&parse) .then(&normalize) .then(&transform) end end

This will save you from creating 3 milions objects and will make your code 7 times slower than the base one.

Convert the memoized methods into procs

Since Ruby will do that for you anyhow (in a loop), why not be smarter and do it for him:

parse = Parse.:call.to_proc normalize = Normalize.:call.to_proc transform = Transform.:call.to_proc stream.each do |batch| batch.each do |message| message .then(&parse) .then(&normalize) .then(&transform) end end

This will make the code above only 2.5 times slower than the base one (usually it’s fine), and at the same time, it will save you almost all out of the 12 milion additional objects!

Dot-colon and the method reference further development

Some of you might know that I’ve been involved a bit in this feature. I proposed and submitted a patch, that will make the .: Method object frozen. It may seem like not much, but freezing keeps a window of opportunity for introducing method reference caching in case it would be needed because the method object is immutable.

This proposal was an aftermath of my discussion with Ko1 and Matz this summer in Bristol. When using the #method method (not the syntax-sugar), due to the backwards compatibility (that I hope will be broken in this case), the Method instance is not frozen. However, the .: will be. It’s a rare corner case (why would you even want to mutate an object like that?), but it does create a funny “glitch”:

nil.:nil? == nil.method(:nil?) #=> true nil.:nil?.frozen? #=> true nil.method(:nil?).frozen? #=> false

Note: I’m planning to work on adding the last-method cache after the 2.7 is released and after I’m able to statistically justify that the majority of cases are as those presented above.

Cover photo by Rahel Samanyi on Attribution 2.0 Generic (CC BY 2.0) license.