Immutable Ruby

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Efficient, immutable, and thread-safe collection classes for Ruby.

The immutable-ruby gem provides 6 Persistent Data Structures: Hash, Vector, Set, SortedSet, List, and Deque (which works as an immutable queue or stack).

Whenever you "modify" an Immutable collection, the original is preserved and a modified copy is returned. This makes them inherently thread-safe and shareable. At the same time, they remain CPU and memory-efficient by sharing between copies. (However, you can still mutate objects stored in these collections. We don't recommend that you do this, unless you are sure you know what you are doing.)

Immutable collections are almost always closed under a given operation. That is, whereas Ruby's collection methods always return arrays, Immutable collections will return an instance of the same class wherever possible.

Where possible, Immutable collections offer an interface compatible with Ruby's built-in Hash, Array, and Enumerable, to ease code migration. Also, Immutable methods accept regular Ruby collections as arguments, so code which uses Immutable can easily interoperate with your other Ruby code.

And lastly, Immutable lists are lazy, making it possible to (among other things) process "infinitely large" lists.


Immutable was forked from Simon Harris' Hamster library, which is no longer maintained. It features some bug fixes and performance optimizations which are not included in Hamster. Aside from the name of the top-level module, the public API is virtually identical.


To make the collection classes available in your code:

require "immutable"

Or if you prefer to only pull in certain collection types:

require "immutable/hash"
require "immutable/vector"
require "immutable/set"
require "immutable/sorted_set"
require "immutable/list"
require "immutable/deque"

Hash (API Documentation)

Constructing an Immutable::Hash is almost as simple as a regular one:

person = Immutable::Hash[name: "Simon", gender: :male]
# => Immutable::Hash[:name => "Simon", :gender => :male]

Accessing the contents will be familiar to you:

person[:name]                       # => "Simon"
person.get(:gender)                 # => :male

Updating the contents is a little different than you are used to:

friend = person.put(:name, "James") # => Immutable::Hash[:name => "James", :gender => :male]
person                              # => Immutable::Hash[:name => "Simon", :gender => :male]
friend[:name]                       # => "James"
person[:name]                       # => "Simon"

As you can see, updating the hash returned a copy leaving the original intact. Similarly, deleting a key returns yet another copy:

male = person.delete(:name)         # => Immutable::Hash[:gender => :male]
person                              # => Immutable::Hash[:name => "Simon", :gender => :male]
male.key?(:name)                    # => false
person.key?(:name)                  # => true

Since it is immutable, Immutable::Hash doesn't provide an assignment (Hash#[]=) method. However, Hash#put can accept a block which transforms the value associated with a given key:

counters = Immutable::Hash[evens: 0, odds: 0]
counters.put(:odds) { |n| n + 1 }   # => Immutable::Hash[:odds => 1, :evens => 0]

Or more succinctly:

counters.put(:odds, &:next)         # => {:odds => 1, :evens => 0}

This is just the beginning; see the API documentation for details on all Hash methods.

Vector (API Documentation)

A Vector is an integer-indexed collection much like an immutable Array. Examples:

vector = Immutable::Vector[1, 2, 3, 4] # => Immutable::Vector[1, 2, 3, 4]
vector[0]                              # => 1
vector[-1]                             # => 4
vector.set(1, :a)                      # => Immutable::Vector[1, :a, 3, 4]
vector.add(:b)                         # => Immutable::Vector[1, 2, 3, 4, :b]
vector.insert(2, :a, :b)               # => Immutable::Vector[1, 2, :a, :b, 3, 4]
vector.delete_at(0)                    # => Immutable::Vector[2, 3, 4]

Other Array-like methods like #select, #map, #shuffle, #uniq, #reverse, #rotate, #flatten, #sort, #sort_by, #take, #drop, #take_while, #drop_while, #fill, #product, and #transpose are also supported. See the API documentation for details on all Vector methods.

Set (API Documentation)

A Set is an unordered collection of values with no duplicates. It is much like the Ruby standard library's Set, but immutable. Examples:

set = Immutable::Set[:red, :blue, :yellow] # => Immutable::Set[:red, :blue, :yellow]
set.include? :red                          # => true
set.add :green                             # => Immutable::Set[:red, :blue, :yellow, :green]
set.delete :blue                           # => Immutable::Set[:red, :yellow]
set.superset? Immutable::Set[:red, :blue]  # => true
set.union([:red, :blue, :pink])            # => Immutable::Set[:red, :blue, :yellow, :pink]
set.intersection([:red, :blue, :pink])     # => Immutable::Set[:red, :blue]

Like most Immutable methods, the set-theoretic methods #union, #intersection, #difference, and #exclusion (aliased as #|, #&, #-, and #^) all work with regular Ruby collections, or indeed any Enumerable object. So just like all the other Immutable collections, Immutable::Set can easily be used in combination with "ordinary" Ruby code.

See the API documentation for details on all Set methods.

SortedSet (API Documentation)

A SortedSet is like a Set, but ordered. You can do everything with it that you can do with a Set. Additionally, you can get the #first and #last item, or retrieve an item using an integral index:

set = Immutable::SortedSet['toast', 'jam', 'bacon'] # => Immutable::SortedSet["bacon", "jam", "toast"]
set.first                                           # => "bacon"
set.last                                            # => "toast"
set[1]                                              # => "jam"

You can also specify the sort order using a block:['toast', 'jam', 'bacon']) { |a,b| b <=> a }['toast', 'jam', 'bacon']) { |str| str.chars.last }

See the API documentation for details on all SortedSet methods.

List (API Documentation)

Immutable::Lists have a head (the value at the front of the list), and a tail (a list of the remaining items):

list = Immutable::List[1, 2, 3]
list.head                    # => 1
list.tail                    # => Immutable::List[2, 3]

Add to a list with List#add:

original = Immutable::List[1, 2, 3]
copy = original.add(0)      # => Immutable::List[0, 1, 2, 3]

Notice how modifying a list actually returns a new list.


Immutable::List is lazy where possible. It tries to defer processing items until absolutely necessary. For example, the following code will only call as many times as necessary to generate the first 3 prime numbers between 10,000 and 1,000,000:

require 'prime'

Immutable.interval(10_000, 1_000_000).select do |number|
  # => 0.0009s

Compare that to the conventional equivalent which needs to calculate all possible values in the range before taking the first three:

(10000..1000000).select do |number|
  # => 10s


Besides Immutable::List[] there are other ways to construct lists:

  • Immutable.interval(from, to) creates a lazy list equivalent to a list containing all the values between from and to without actually creating a list that big.

  • { ... } allows you to creates infinite lists. Each time a new value is required, the supplied block is called. To generate a list of integers you could do:

    count = 0 { count += 1 }
  • Immutable.repeat(x) creates an infinite list with x as the value for every element.

  • Immutable.replicate(n, x) creates a list of size n with x as the value for every element.

  • Immutable.iterate(x) { |x| ... } creates an infinite list where the first item is calculated by applying the block on the initial argument, the second item by applying the function on the previous result and so on. For example, a simpler way to generate a list of integers would be:

    Immutable.iterate(1) { |i| i + 1 }

    or even more succinctly:

    Immutable.iterate(1, &:next)
  • Immutable::List.empty returns an empty list, which you can build up using repeated calls to #add or other List methods.

Core Extensions

Enumerable#to_list will convert any existing Enumerable to a list, so you can slowly transition from built-in collection classes to Immutable.

IO#to_list enables lazy processing of huge files. For example, imagine the following code to process a 100MB file:

require 'immutable/core_ext'"my_100_mb_file.txt") do |file|
  lines = []
  file.each_line do |line|
    break if lines.size == 10
    lines << line.chomp.downcase.reverse

Compare to the following more functional version:"my_100_mb_file.txt") do |file|

Unfortunately, though the second example reads nicely, it takes many seconds to run (compared with milliseconds for the first) even though we're only interested in the first ten lines. Using #to_list we can get the running time back comparable to the imperative version."my_100_mb_file.txt") do |file|

This is possible because IO#to_list creates a lazy list whereby each line is only ever read and processed as needed, in effect converting it to the first example.

See the API documentation for details on all List methods.

Deque (API Documentation)

A Deque (or "double-ended queue") is an ordered collection, which allows you to push and pop items from both front and back. This makes it perfect as an immutable stack or queue. Examples:

deque = Immutable::Deque[1, 2, 3] # => Immutable::Deque[1, 2, 3]
deque.first                       # 1
deque.last                        # 3
deque.pop                         # => Immutable::Deque[1, 2]
deque.push(:a)                    # => Immutable::Deque[1, 2, 3, :a]
deque.shift                       # => Immutable::Deque[2, 3]
deque.unshift(:a)                 # => Immutable::Deque[:a, 1, 2, 3]

Of course, you can do the same thing with a Vector, but a Deque is more efficient. See the API documentation for details on all Deque methods.


Add this line to your application's Gemfile:

gem "immutable-ruby"

And then execute:

$ bundle

Or install it yourself as:

$ gem install immutable-ruby

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