Functional Composition in Ruby

Let’s explore functional thinking and what it entails wrt Ruby.

Anjali Jaiswal
Apr 19 · 5 min read
Photo by Valeria Bold on Unsplash

We all know that Ruby is an Object-oriented language, and while that’s true we can still use Ruby in a purely functional way. It won’t be much Rubish if we did that though, so let’s see what bits we can do with Ruby that are inspired from functional programming.

Functional programming is essentially like writing a mathematical program. Let’s understand this with an example, consider that I want to find out first odd number in an array. Now we will write two functions with same objective, one in Ruby way, another in a functional way.

# Ruby way
def first_odd(array)
for i in 0...array.size
break i if i.odd?

In Ruby function we are reassigning value of i with each iteration of the loop, which would not be possible in a mathematical function, because there we define a variable, it’s the definition and not an assignment. That’s the major difference between functional and non-functional programming, functional programming stresses on immutability and referential transperancy(meaning if you pass same value to a function again and again the output will always be same.) Now, let’s see how we can write the same method in a functional way:

# Functional way
def first_odd(array)
i = array.shift
return nil if array.empty?
if i.odd?
return i

In Functional way we wrote a recursive program, which is like a mathematical function ie it checked if the first element is odd and if not it called itself with remaining elements. From this we can derive three important points about functional programming which we need to remember:

  1. Functions are definitions, not list of instructions.
  2. We don’t assign variables, we define things.
  3. We can pass functions in another function, just like a variable.

Let’s dive more into how functional composition can rescue us in daily life problems. Suppose we have a dog adoption application and we have been asked for a report on how well the dogs are getting adopted. You already have a list of dogs and their attributes.

# We have:
$ dogs.sample
=> #<struct Dog adoption_fees: 12, rating: 4, breed: :golden, adopted: 22>

First thing we can ask for in a report is pretty simple, just average adoption fees. To do this we can create a Report class, and then add an inititalize method to take and store a list of all the dogs and then add a run method to calculate the average adoption fees.

class Report
def initialize(dogs)
@dogs = dogs
# inject is an enumerable function
def run
money_taken = @dogs.inject(0) do |total, dog|
(dog.adoption_fees * dog.adopted) + total

total_adoption = @dogs.inject(0) do |total, dog|
dog.adopted + total

money_taken / total_adoption
=> 20

Recently a lot of Golden’s got adopted because of COVID, and now we need a new report to show the average adoption fees the organisation got from it. We can tweak the previous code to add a new parameter which selects only a given breed and do calculations. Next we require top three breeds who are getting adopted. These request will make our Report class long and as new requests will come, this approach will become harder to maintain and will introduce a lot of flags and conditional logic. There are tons of ways wherein we can refactor such case, but since we are looking at functional composition lets see what can be done here with some of Ruby’s functional programming ideas.

# Note: This section will use lambdas heavily, so if you need a revision: Ruby Blocks, Procs and Lambdas

# lambda for summing up using inject
sum = -> list { list.inject(&:+) }
# lambda for calculating average adoption fee
total_adoption_fee = -> dogs do
adoption_fee = { |dog| dog.adoption_fees * dog.adopted }

# lambda for calculating average adoption fee
avg_adoption_fee = -> dogs do
total_adoption_fee[dogs]/ total_adoptions[dogs]
# lambda for getting number of dogs adopted
total_adoption = dogs { sum[] }
$ avg_adoption_fee[DOGS]
=> 20
# now, we need to get adoption fees only for golden's
# hence we will add a new lambda for getting jut one breed
golden = -> dogs { { |dog| dog.breed == :golden }}

Let’s rewrite our Reports class where we can use the lambdas we just created.

class FunctionalReport

# splated argument will take a list of functions generated at
# each stage of report
def initialize(dogs, *fns)
@dogs = dogs
@fns = fns
def run
@fns.inject(@dogs) do |last_result, fn|
$, avg_adoption_fee).run
=> 20
$, golden, avg_adoption_fee).run
=> 12
# We can create more lambdas and work accordingly, lets say someone # wants to know adoption fees of dogs which have rating of three and # higherhigh_rating = -> dogs { { |d| d.rating >= 3 }}$, high_rating, avg_adoption_fee).run
=> 15

As seen above, we can safely deduce with the use of functional programming:

  1. We can add new features without modifying existing code
  2. Create each step of report with very little code
  3. No need to support unrelated code from other steps

But the thing is this is not very Rubish, and ignores a lot expressiveness and power that Ruby has to offer. So, what things we can use from Functional Programming in Ruby?

Now the takeaway from this article should be the approach, we wrote lambdas that implemented some small specific tasks and found a way of making them work together. We can do the same thing with objects implementing small specific rules and finding a way to compose them together.

class BreedFilter
def initialize(breed)
@breed = breed
def apply(dogs) { |dog| dog.breed == @breed }

This Object-Oriented style of coding is much easier to read and think, feels much more natural while writing Ruby and most importantly it treats data as being immutable. Functional Programming has a lot to teach us, we don’t have to write purely functional code to reap the benefits. Hope this helped you in understanding how you can write better code. Sayonara for now, will catch you later with new tidbits. Stay happy, stay safe.

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Anjali Jaiswal

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Programmer, avid Reader, artist, exploring different trades, fun-loving, adventurous and most of the time a hell-yes person!

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