Functional Programming in Ruby with Lambdas RubyConf Houston, Texas December 1, 2022 Keith R. Bennett (@)keithrbennett on: GMail, LinkedIn, Speakerdeck, Twitter, Github 1

Ruby not Rails 2 I am a developer.

Disclaimer 3 I am not a functional programming expert.

Lambda Topics 4 What is a lambda? What does it look like? What is it good for? How is it used?

What is a lambda? 5 A lambda is a free fl oating function.

6 Remember when you encountered code blocks for the fi rst time? Remember how they were confusing at fi rst? But we persevered and mastered them. Was it worth the e ff ort? Of course it was.

7 Lambdas are the next step in that progression.

Bene fi ts of Lambdas 8 • Simplicity • Extensibility • Productivity • Fun

Ways to Call a Lambda 9 # First, let’s define a lambda: is_even = ->(n) { n % 2 == 0 } is_even.call(2) is_even.(2) is_even[2] # Don't do this! is_even === 2 # Don't do this either! case 2 when is_even # (case equality, ===) puts 'is even' end

“Callables” — Objects Responding to `call` 10 • All Proc instances (lambda and non lambda) • Modules or Classes with a `call` class method • Instances of classes having a `call` instance method • Any other object having a `call` method added to it directly.

“Callables” — Objects Responding to `call` 11 def callable?(object) object.respond_to?(:call) end class C; def self.call; end end class I; def call; end end p callable?(-> {}) # -> true p callable?(Proc.new {}) # -> true p callable?(C) # -> true p callable?(I.new) # -> true The .() abbreviation can be used to call any and all of these!

12 You can use a lambda where a code block is expected by preceding it with &. logger = MyLogger.new # The usual case is to pass a code block like this: logger.info { 'I am inside a block' } # But we can instead adapt a lambda: proclaimer = -> { 'I am inside a lambda' } MyLogger.new.info(&proclaimer) Using a Lambda Where a Code Block is Expected

13 Using a Method Where a Lambda is Expected You can use a method where a lambda is expected using this notation: lambda(&method(:foo)) def foo puts 'Hello, world.' end fn = lambda(&method(:foo)) fn.() # Hello, World

14 Lambda Syntax In Ruby versions <= 1.8: In Ruby versions >= 1.9, the "stabby lambda" syntax is added: lambda {} lambda do end -> {} -> do end

15 In Ruby versions <= 1.8, parameters are speci fi ed the same way as in a code block: lambda { |param1, param2| } lambda do |param1, param2| end Lambda Syntax In the "stabby lambda" alternate syntax for Ruby versions >= 1.9, the parameter syntax is identical to method syntax: ->(param1, param2) {} ->(param1, param2) do end

16 Self Invoking Anonymous Functions -> do source_dir_root = File.join(File.dirname(__FILE__), 'my_gem') all_files_spec = File.join(source_dir_root, '**', '*.rb') Dir[all_files_spec].each { |file| require file } end.()

17 .() Abbreviated Notation: Without Parameters, Parentheses are Mandatory

18 Lambda and non-Lambda Procs

19 Comparing Lambda and Nonlambda Return Behavior A lambda's return returns only from the lambda, and not from the enclosing method or lambda. def foo -> { return }.() puts "still in foo" end # > foo # still in foo

20 In contrast, a non-lambda Proc (proc) return returns from its enclosing scope: def foo proc { return }.() puts "still in foo" end # > foo # > (no output) Comparing Lambda and Nonlambda Return Behavior

21 Comparing Lambdas and Procs Arity Checking Lambdas have strict arity checking; blocks and non-lambda procs do not. # ------------------------------------ Lambdas two_arg_lambda = -> (a, b) {} two_arg_lambda.(1) # ArgumentError: wrong number of arguments (1 for 2) # ------------------------------------ Procs two_arg_proc = proc { |a, b| } two_arg_proc.(1) # (No complaint.)

22 Comparing Lambdas and Procs Arity Checking Lambdas have strict arity checking; blocks and non-lambda procs do not.

23 Lambdas and Procs are Selfless # Executed in irb, in which the top level object's to_s returns 'main'. # # Lambdas: -> { puts self }.() # main # Procs: (proc { puts self }).() # main

24 Lambdas are Safer Unless the looser behavior of code blocks and non-lambda procs is needed, I believe that lambdas are safer and therefore preferable. Arity Checking Return Behavior

25 Unnecessary complexity is our enemy.

26 As software developers, we strive to maximize the ratio: Functionality Complexity

Why Do We Use Local Variables? 27 • To limit their scope. Does it improve: • reliability? • simplicity? • readability?

28 One Measure of Complexity: The Number of Possible Paths of Interaction Having n instance methods means n * (n-1) possible call paths from one instance method to another

29 If only we could de fi ne method-like things that were nested inside a method, and local to it...

30 We Can De fi ne Methods Within Other Methods class C def outer puts 'defining inner' def inner puts 'inner' end end end c = C.new c.outer # => defining inner c.outer.inner # => undefined method `inner' for :inner:Symbol (NoMethodError) c.inner # => inner C.instance_methods.grep /er/ # => [:outer, :inner]

Lambdas as Inner Methods! 31 Encapsulation ‘Lite’ Would you notice anything interesting about the structure? def compute_something(input) compute_part = ->(part) do # ... end [compute_part.(input.left), compute_part.(input.right)] end left_part, right_part = compute_something(input) Imagine there were several lambdas there, and not only one. We can use lambdas as local nested functions:

32 A Method with Nested Lambdas Can Easily Be Converted to a Class class ComputerOfSomething def compute_part(part) # ... end # other lambdas would also be converted to methods def compute(input) [compute_part(input.left), compute_part(input.right)] end end left_part, right_part = ComputerOfSomething.new.compute(input)

33 …Or a Module module ComputerOfSomething class << self def compute_part(part) # ... end # other lambdas would also be converted to methods def compute(input) [compute_part(input.left), compute_part(input.right)] end end end left_part, right_part = ComputerOfSomething.compute(input)

34 Lambdas as Nested Functions: Formatters def report_times(times) format_line = ->(caption, value) { format("%-16.16s: %s", caption, value) } duration_s = ((times[:finish] - times[:start]).round(2)).to_s <<~RETURN_VALUE #{format_line.("Start Time", times[:start])} #{format_line.("End Time", times[:finish])} #{format_line.("Duration (secs)", duration_s)} RETURN_VALUE end puts report_times({ start: Time.now, finish: Time.now + 10 }) • DRY (Don't Repeat Yourself) • Separates high from low level code # Returns a multiline string like this: # # Start Time : 2022-11-04 18:44:23 -0400 # End Time : 2022-11-04 18:44:33 -0400 # Duration (secs) : 10.0

35 Lambdas in Behavior Lookups Example: Formatters def formatters @formatters ||= { amazing_print: ->(obj) { obj.ai << "\n" }, inspect: ->(obj) { obj.inspect + "\n" }, json: ->(obj) { obj.to_json }, marshal: ->(obj) { Marshal.dump(obj) }, none: ->(_obj) { nil }, pretty_json: ->(obj) { JSON.pretty_generate(obj) }, pretty_print: ->(obj) { obj.pretty_inspect }, puts: ->(obj) { sio = StringIO.new; sio.puts(obj); sio.string }, to_s: ->(obj) { obj.to_s + "\n" }, yaml: ->(obj) { obj.to_yaml }, } end

36 Lambdas in Behavior Lookups Example: Parsers def input_parsers @input_parsers ||= { json: ->(string) { JSON.parse(string) }, marshal: ->(string) { Marshal.load(string) }, none: ->(string) { string }, yaml: ->(string) { YAML.load(string) }, } end private def init_parser_and_formatters @input_parser = lookups.input_parsers[options.input_format] @output_formatter = lookups.formatters[options.output_format] @log_formatter = lookups.formatters[options.log_format] end

Lambdas and Threads 37 fetch_type_1_data = -> { `uptime` } # complex and long running task #1 fetch_type_2_data = -> { `whoami` } # complex and long running task #2 type_1_data, type_2_data = [ Thread.new(&fetch_type_1_data), Thread.new(&fetch_type_2_data), ].map(&:value) # now do something useful with type_1_data and type_2_data. puts "\nType 1 data:"; ap type_1_data puts "\nType 2 data:"; ap type_2_data

38 Lambdas are Closures n = 15 # Create the lambda and call it immediately: -> { puts n }.() # 15

39 n = 15 -> { n = "I just overwrote n." }.() puts n # I just overwrote n. Lambdas are Closures

40 Lambda Locals n = 15 ->(;n) { n = 'I did NOT overwrite n.' }.() puts n # still 15

41 This illustrates the closure behavior using the `binding`. Lambdas and Bindings name = 'Joe' f = -> { puts "Name is #{name}" } f.() # 'Name is Joe' f.binding.eval("name = 'Anil'") f.() # 'Name is Anil' name # 'Name is Anil'

42 Private Methods Aren’t Really Private They can be accessed using the send method: class ClassWithPrivateMethod private def my_private_method puts "Hey! You're invading my privacy!" end end ClassWithPrivateMethod.new.send(:my_private_method) # --> Hey! You're invading my privacy.

43 Lambdas Local to a Method Really Are Private class ClassWithLocalLambda def foo my_local_lambda = -> do puts "You can't find me." end # ... end end Can’t access this lambda from outside this method. So you can't get to it with a unit test either.

44 Lambdas Are Great Lightweight Event Handlers event_handler = -> (event) do puts "This event occurred: #{ event}" end something.add_event_handler(event_handler) # Or, even more concisely, by eliminating the intermediate variable (this notation is very similar to # block notation): something.add_event_handler( -> (event) do puts "This event occurred: #{ event}" end)

45 Customizable Behavior in Ruby Using Classes class EvenFilter def call(n) n.even? end end class OddFilter def call(n) n.odd? end end def filter_one_to_ten(filter) (1 .. 10).select { |n| filter.(n) } end puts filter_one_to_ten(EvenFilter.new).to_s # [2, 4, 6, 8, 10]

46 Customizable Behavior in Ruby Using Lambdas If we use lambdas instead, we can dispense with the ceremony and verbosity of classes. See how much simpler the code is! even_filter = -> (n) { n.even? } odd_filter = -> (n) { n.odd? } def filter_one_to_ten(filter) (1 .. 10).select { |n| filter.(n) } end puts filter_one_to_ten(even_filter).to_s # [2, 4, 6, 8, 10]

47 Predicates A function that returns either true or false. -> { true } -> { false } -> (n) { n.even? }

48 Lambdas as Filter Predicates This method takes a message fi lter as a parameter, defaulting to a fi lter that returns true for all messages. def get_messages(count, timeout, filter = -> (_message) { true }) messages = [] while messages.size < count message = get_message(timeout) messages << message if filter.(message) end messages end

49 Code Blocks as Filter Predicates Unlike the lambda approach, the code block approach is cryptic and not intention-revealing. The parameter list does not reveal that a fi lter is being passed. def get_messages(count, timeout) messages = [] while messages.size < count message = get_message(timeout) messages << message if (! block_given?) || yield(message) end messages end

50 Multiple Variable Behaviors When multiple variable behaviors are needed, code blocks don’t work, but callables do. class BufferedEnumerable # ... def self.create_with_callables(chunk_size, fetcher, fetch_notifier = nil) # ... end # ... end

51 Separating Unrelated Concerns The Bu ff eredEnumerable class can be seen at https://github.com/keithrbennett/trick_bag/blob/master/lib/trick_bag/enumerables/bu ff ered_enumerable.rb. # Creates an instance with lambdas for fetch and fetch notify behaviors. # @param chunk_size the maximum number of objects to be buffered # @param fetcher lambda to be called to fetch to fill the buffer # @param fetch_notifier lambda to be called to when a fetch is done def self.create_with_lambdas(chunk_size, fetcher, fetch_notifier = nil)

52 Classes Can Be De fi ned in a Lambda module M CREATE_CLASS = -> do class ::IWasCreatedByALambda # ... end end def self.create_class CREATE_CLASS.() end end M.create_class if some_condition # some real condition would go here puts IWasCreatedByALambda.new # => #

53 Transform Chains — ETL “Lite” We can use Ruby's Enumerable functionality to simplify passing an object through multiple transformations. tripler = -> (n) { 3 * n } squarer = -> (n) { n * n } transformers = [tripler, squarer] #144 start_val = 4 transformers.inject(start_val) do |value, transform| transform.(value) end # 144

54 Eliminating Duplication doubler = -> (n) { 2 * n } tripler = -> (n) { 3 * n } quadrupler = -> (n) { 4 * n }

55 Partial Application “Partial application (or partial function application) refers to the process of fi xing a number of arguments to a function, producing another function of smaller arity.” - Wikipedia fn_multiply_by = -> (factor) do -> (n) { factor * n } end tripler = fn_multiply_by.(3) tripler.(123) # => 369

56 This is the same, except it is a method and not a lambda that performs the partial application: def fn_multiply_by(factor) -> (n) { factor * n } end tripler = fn_multiply_by(3) tripler.(123) # => 369 Partial Application

57 The ‘curry’ method returns a lambda that, when called with arguments, returns another lambda in which those arguments are pre fi lled (i.e. no longer need to be passed). Currying multiply_2_numbers = -> (x, y) { x * y } tripler = multiply_2_numbers.curry.(3) tripler.(123) # => 369 # or multiply_2_numbers = -> (x, y) { x * y } currier = multiply_2_numbers.curry tripler = currier.(3) tripler.(123) # => 369

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