Like many other developers, I’ve been intrigued with functional programming for a long while. I remember myself reading articles promising programming heaven for those who are brave enough to go functional. I bought a used Real World Haskell on Ebay, but sadly never finished it. I then bought Scala for the Impatient, but this time had the persistence to finish the book.

All these years functional programming seemed like a holy grail, but as a true holy grail, I was afraid it was meant to stay undiscovered.

All these years I paid my bills writing Ruby-on-Rails and JavaScript code and never made the functional leap. I never became a full-time Haskell or Scala developer and probably will never become one.

But you know what? It’s possible to be slightly more functional with normal languages we’re using every day. This article will try to demonstrate several concrete examples where functional programming is useful or elegant. I will show you the old way of doing things in Ruby and the new, more functional way of doing similar things in Ruby again.

Let me start by saying that this article assumes you’re interested in functional programming. It also assumes that you’ve probably seen other examples of functional code before.

I’m going to split this article into several parts and each part will elaborate upon a specific example.

Part 1: Immutability

What is immutability? When people speak about immutability they usually mean immutable objects. Quoting from wikipedia:

an immutable object is an object whose state cannot be modified after it is created. This is in contrast to a mutable object, which can be modified after it is created.

A very simple concept with far reaching consequences.

First let’s define what ‘whose state cannot be modified’ really means. At first you may think that such an object is useless. How can we possibly use an object if we cannot change it? Usually an immutable object creates a copy of itself with desired modifications. The original object remains unchanged. You will see the examples of it further in the article.

Immutability and functional programming

Now, another foundational question: why does functional programming favor immutable values and data structures over mutable ones? Is real functional programming possible with mutable values? You probably know that functional programming is more than ‘programming with functions’. It also requires the functions to be pure. I’m not a mathematician and my explanation of pure functions may not be scientifically correct, but you can think of them simply as functions that: always accept an argument, always return a result and the computing of the result depends solely on the input. In other words, a pure function cannot depend on some other data, existing elsewhere, called state, to influence how the result is computed. The only thing that dictates how the result is computed is the function’s argument. Pure functions cannot change the external state either. This is called creating side effects.

Sometimes programmers call the external state “the world” and refer to pure functions as functions that cannot depend on “the world” and read “the world” state, nor change “the world” while making its job.

Why worry at all about the purity of functions? Composability. When your functions are pure, you can compose large programs from small functions. Knowing that a function is pure provides guarantees that it will not change the external state.

Is it possible to write a real program using only pure functions? How can you talk to the database, write to files, charge credit cards and do everything else real programs do? Functional applications are usually built using a pure core (where the bulk of the logic lives) and a thin, impure shell (that provides access to the pure core from the outside world). This way you have a large part of the code that is easy to reason about, easy to test and easy to understand.

Example of a pure function:

1 2 3 def sum_two_numbers ( a , b ) a + b end

You can see that this function computes the result only using its arguments.

Example of an impure function:

1 2 3 4 def sum_two_numbers ( a , b ) logger . info ( "calculating sum of two numbers" ) a + b end

This function writes to the file system in addition to computing the result. In other words, this function changes “the world” by creating side effects.

Using v2 of this function you hurt composability; you limit yourself in the ways you can use this function in other parts of your program.

Immutability and purity

Now let’s look why function purity demands immutability with a concrete example. We all know that strings in ruby are mutable. You can mutate the string with:

1 2 3 4 5 6 7 s = "Hello" # mutating with '<<' s << ", world" # mutating with bang methods s . upcase! puts s # => "HELLO, WORLD"

This code fragment modifies the string in-place, mutating it. Now let’s use the string as a function argument:

1 2 3 4 def upcase_string ( input ) input . upcase! input end

This method mutates the argument and returns it. On the surface, this looks OK, but we have just inadvertently created a side effect. Any external code that relies on this string may break.

Let’s create an example of this:

1 2 3 4 5 6 7 8 9 10 11 12 13 def upcase_string ( input ) input . upcase! input end current_user_name = get_current_user . name upcased_user_name = upcase_string ( current_user_name ) # ... # ... # somewhere else still thinking that current_user_name is downcased if current_user_name == 'admin' # this will never be true # ... end

You see now that in order to keep function pure we should never mutate its arguments, but create new objects and return them instead. Same function, but this time implemented as pure:

1 2 3 def upcase_string ( input ) input . upcase end

Just a minor modification gives us many benefits: we’re no longer modifying “the world” and only return a new string with the required modifications.

How can we guarantee that functions never mutate their arguments? By making the arguments immutable, of course!

The key thing to take away here is that by making each object immutable, we can guarantee that functions do not create side effects and remain pure.

Hopefully, by now I have convinced you that immutable objects are useful. Now you probably understand that by limiting the “reach” of the function to only the local function’s scope you automatically decrease the number of potential bugs and unpleasant surprises. However, you may still be unsure about the performance of immutable objects, and think that it is wasteful to create a copy of an object each time it needs to be modified. The following part of the article will hopefully make everything clear.

Immutability and primitives

Let’s define what primitives are. For our purposes, we can refer to primitives as data types, that serve as basic building blocks of the language. Usually the primitives are directly supported by the language. Ints, floats, characters and booleans are primitives and are usually treated in a special way by languages.

You don’t need to do something like:

1 2 # in fact you can't do this in Ruby num = Integer . new ( 99 )

You can use primitives directly:

1 2 num = 99 fnum = 3 . 14

Why does a language usually divide objects into, well, objects and primitives? The reason is performance. Primitives are closer to computer hardware and creating an object for every number is slow.

However, Ruby does not have true primitives, because in Ruby, everything is an object. You can call methods and properties on numbers and extend them with user-defined methods. I will still call them primitives, because it’s what they are on a conceptual level.

On one hand, primitives behave like immutable objects in Ruby:

1 2 3 4 5 6 i = 99 puts i . object_id # => 7 i += 1 puts i . object_id # => 12

This snippet demonstrates that you cannot modify a number. In real life this doesn’t make sense either, if you have the number 4 it’s the number 4 — eternal and beautiful. If you add 1 to it, you get completely different number 5, the old 4 stays the same.

On the other hand, you can define your own methods and properties:

1 2 3 4 5 class TrueClass attr_accessor :name end true . name = "one" false . name = "two"

Integers and floats are frozen by default, while booleans are not.

1 2 3 1 . frozen? # true 3 . 14 . frozen? # true true . frozen? # false

So while some primitives are not frozen, Ruby does not provide mutation methods for them and they usually can be treated as immutable objects. You should remember that this is easily overridden (as is everything in Ruby) and can cause potential problems.

Strings

Before diving into the specifics of Ruby strings, let’s talk about string mutability in general. In most languages strings are immutable: string concatenation or upcasing produces a new string rather than modifying it in-place.

Why do language designers usually make their string implementations immutable? To answer that we need to remember that strings are one of the most used data structures in any programming language.

Let’s consider the cases when string immutability is useful.

Concurrency.

This is a complex topic and I will talk about it later in the article. What you should know at this point is that when any data structure is immutable, it can be freely shared across threads without any locking or synchronization. Immutable data structures don’t need synchronisation at all when used in multithreaded environments.

Modern programming languages are designed from the ground up to be concurrent (go, rust), and having a single string instance to be shared across multiple threads helps to save a lot of memory and avoid the necessity of defensive copying when passing immutable strings around.

Hash table keys

More often than other data types, strings are used as keys in hash tables. This usage demands for strings to return the same hash code after the key and value were added to the hash table. With mutable strings a hash table would need to copy the string in order to guarantee the hash code staying the same. With immutable strings this is not needed.

Security

As I’ve mentioned, strings are used very frequently in any program. This entails a special treatment in terms of security. Strings are used when comparing user-names and passwords, storing credit card numbers and much more. Immutable strings guarantee that a malicious party is unable to tamper with the string after creation.

However, there is a performance downside of immutable strings. Mutable strings allow fast indexing and modifying in-place, as with regular arrays.

String immutability and Ruby

As with primitives, Ruby has no real immutable strings. To be precise, Ruby strings are mutable behind an immutable facade. That is, most operations on strings return new strings, while some of them still allow in-place modification.

Consider these examples:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 # immutable operations s = "hello" puts s . object_id # 70093095097920 s += ", world" puts s . object_id # 70093096228400 s = s . upcase puts s . object_id # 70093096177000 # mutable operations s = "hello" puts s . object_id # 70093096113460 s << ", world" puts s . object_id # 70093096113460 s = s . upcase! puts s . object_id # 70093096113460

As you can see mutable operations do not create new strings but rather modify existing strings in-place.

Strings as hash keys

Earlier I mentioned that mutable strings do not make good hash keys. Let me prove this:

1 2 3 4 5 bad_key = "hal9000" h = { bad_key => "Odyssey" } h [ bad_key ] # "Odyssey" bad_key << "!" h [ bad_key ] # nil

After I modified the string key we can no longer find the value, because the key’s hashcode has changed! Since it’s so easy to mutate the Ruby string, you can end up with a useless hash. This is why it is not recommended to use mutable strings as hash keys.

How can we remedy it? The first option is to freeze the string:

1 2 3 4 better_key = "hall9000" . freeze h = { better_key => "Odyssey" } better_key << "!" # RuntimeError: can't modify frozen String

A second and better option is to use symbols, which are immutable versions of strings often used as identifiers.

1 2 3 4 best_key = :hal9000 h = { best_key => "Odyssey" } best_key << :a NoMethodError : undefined method `<<' for :hal9000:Symbol

When using literal symbols as hash keys, Ruby provides a shorter syntax:

1 2 h = { hal9000 : "Odyssey" } # hal9000: gets converted to :hal9000 =>

You might say at this point, “Why can’t I just use symbols instead of strings if they’re immutable equivalents?”. The short answer is you may not be able to, depending on your use case. One reason is that symbols don’t have immutable equivalents of string’s many methods, so it’s inconvenient to use symbols as an immutable replacement. Just compare the number of methods in Symbol and String to see the difference.

Immutable data structures

So far my discussion was around built-in data types and their relationships with immutability. Real-life applications, however, require using data structures in order to be efficient.

What is a data structure?

It’s a complex question, but you can think of it as a way to organize other, simpler data structures in a convenient or efficient way. Some data structures are designed for ease of use, while others are built solely with efficiency in mind.

We all know about lists, queues, hash tables, arrays, trees and many, many more. These data structres can have both mutable and immutable implementations.

Mutable implementations are considered ‘classic’, because they are more widely used, have been around for longer and generally are easier to implement. Immutable counterparts offer advantages in concurrency and security.

While some people use ‘immutable’ and ‘persistent’ interchangeably, they are not the same. Persistent data structure is immutable and keeps and reuses large parts of itself while constructing an immutable copy. As an example, you can think of a persistent linked list that reuses its tail when appending a new node. If you’re interested in functional, persistent data structures, have a look at Purely functional data structures by Chris Okasaki.

Let me also add that many modern programming languages that focus on concurrency have their data structures implemented in an immutable fashion: Scala offers both immutable and mutable collections. Clojure and C# offer immutable collections as well.

Let’s go ahead and implement a classic, mutable stack in Ruby and then reimplement it as immutable. A stack is a data structure that follows this interface:

1 2 3 4 self push(item) self pop() item peek() bool empty?

Here is mutable implementation that uses a Ruby array as a backing store:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 class MutableStack def initialize @store = [] end def push ( item ) @store . push ( item ) self end def pop @store . pop self end def peek @store [- 1 ] end def empty? @store . empty? end end

This implementation is basically a thin wrapper around array. Whenever you call stack.push(item) , you’re modifying this array in-place. This implementation possesses all the weaknesses that we discussed previously.

Now to an immutable implementation:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 class ImmutableStack class EmptyStack def empty? true end def push ( item ) ImmutableStack . new ( item , self ) end def pop raise 'Cannot pop empty stack' end def peek raise 'Cannot peek empty stack' end end def self . empty EmptyStack . new end def initialize ( head , tail ) @head = head @tail = tail end attr_reader :head , :tail def peek head end def push ( item ) ImmutableStack . new ( item , self ) end def pop tail end def empty? false end end

Usage pattern:

1 2 3 4 5 6 7 s = ImmutableStack . empty s = s . push ( 99 ) s = s . push ( 100 ) puts s . peek # 100 s = s . pop puts s . peek # 99 s . peek # Cannot peek empty stack (RuntimeError)

Each destructive operation does not mutate the stack but rather returns a copy of itself with the required modifications. What’s more, it reuses a large portion of itself while doing so, thus making this stack a persistent data structure.

Unfortunately, Ruby does not allow you to directly create private constructors and users can potentially call

1 ImmutableStack . new ( 1 , ImmutableStack . empty )

If you want a good ruby library of immutable collections, I suggest using hamster.

Immutable data structures and multithreading

When writing a multi-threaded applications, follow these rules:

Avoid sharing data across threads. If you have to share your data across threads, make this data immutable. If you can’t avoid sharing mutable data, synchronize access to that data with synchronization constructs, such as Mutex.

In our two stack implementations it is safe to share an immutable version across multiple threads, because they will not be able to modify it in place. Whenever a thread makes a push or a pop , a new instance of the stack is created and returned so that the existing instance is never changed.

Conclusion

Now that you’ve read the article, you might have the impression that immutability is a silver bullet. It is not. It is only one possible way to design software and has its own strengths and weaknesses. Immutability let’s you design your functions and data structures in a new way, gaining much and losing much too. We’ve all been living in a world where the sequential computation was the de-facto standard. In the past immutability was not worth it. For a single core computer, immutability has too much overhead. You must carefully control the state and pay close attention to reusing and copying in order to be efficient. The performance impact that some of the immutable data structures incur can be too significant.

But the world is changing and the sequential model is disappearing. We all have smartphones with 2 or 4 cores. Our smart watches will have 8 cores in a couple of years, which means that concurrent will become the new sequential. If we want to exploit the power of modern hardware, we need to embrace the concurrent way of doing things. This is where immutability advantages outweigh the bad parts. I think that immutability is the way you should design your software now in order to be prepared for the concurrent future.