Hamster
Efficient, immutable, and thread-safe collection classes for Ruby.
Hamster provides 6 Persistent Data
Structures: Hash
, Vector
, Set
, SortedSet
, List
, and Deque
(which works as an immutable queue or stack).
Hamster collections are immutable. Whenever you modify a Hamster 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.
While Hamster collections are immutable, you can still mutate objects stored in them. We recommend that you don't do this, unless you are sure you know what you are doing. Hamster collections are thread-safe and can be freely shared between threads, but you are responsible for making sure that the objects stored in them are used in a thread-safe manner.
Hamster collections are almost always closed under a given operation. That is, whereas Ruby's collection methods always return arrays, Hamster collections will return an instance of the same class wherever possible.
Where possible, Hamster collections offer an interface compatible with Ruby's
built-in Hash
, Array
, and Enumerable
, to ease code migration. Also, Hamster methods accept regular Ruby collections as arguments, so code which uses Hamster
can easily interoperate with your other Ruby code.
And lastly, Hamster lists are lazy, making it possible to (among other things) process "infinitely large" lists.
Using
To make the collection classes available in your code:
require "hamster"
Or if you prefer to only pull in certain collection types:
require "hamster/hash"
require "hamster/vector"
require "hamster/set"
require "hamster/sorted_set"
require "hamster/list"
require "hamster/deque"
Hash (API Documentation)
Constructing a Hamster Hash
is almost as simple as a regular one:
person = Hamster::Hash[name: "Simon", gender: :male]
# => Hamster::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") # => Hamster::Hash[:name => "James", :gender => :male]
person # => Hamster::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) # => Hamster::Hash[:gender => :male]
person # => Hamster::Hash[:name => "Simon", :gender => :male]
male.key?(:name) # => false
person.key?(:name) # => true
Since it is immutable, Hamster's 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 = Hamster::Hash[evens: 0, odds: 0] # => Hamster::Hash[:evens => 0, :odds => 0]
counters.put(:odds) { |value| value + 1 } # => Hamster::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 = Hamster::Vector[1, 2, 3, 4] # => Hamster::Vector[1, 2, 3, 4]
vector[0] # => 1
vector[-1] # => 4
vector.put(1, :a) # => Hamster::Vector[1, :a, 3, 4]
vector.add(:b) # => Hamster::Vector[1, 2, 3, 4, :b]
vector.insert(2, :a, :b) # => Hamster::Vector[1, 2, :a, :b, 3, 4]
vector.delete_at(0) # => Hamster::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 = Hamster::Set[:red, :blue, :yellow] # => Hamster::Set[:red, :blue, :yellow]
set.include? :red # => true
set.add :green # => Hamster::Set[:red, :blue, :yellow, :green]
set.delete :blue # => Hamster::Set[:red, :yellow]
set.superset? Hamster::Set[:red, :blue] # => true
set.union([:red, :blue, :pink]) # => Hamster::Set[:red, :blue, :yellow, :pink]
set.intersection([:red, :blue, :pink]) # => Hamster::Set[:red, :blue]
Like most Hamster 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 Hamster collections, Hamster::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 = Hamster::SortedSet['toast', 'jam', 'bacon'] # => Hamster::SortedSet["bacon", "jam", "toast"]
set.first # => "bacon"
set.last # => "toast"
set[1] # => "jam"
You can also specify the sort order using a block:
Hamster::SortedSet.new(['toast', 'jam', 'bacon']) { |a,b| b <=> a }
Hamster::SortedSet.new(['toast', 'jam', 'bacon']) { |str| str.chars.last }
See the API documentation for details on all SortedSet
methods.
List (API Documentation)
Hamster List
s have a head (the value at the front of the list),
and a tail (a list of the remaining items):
list = Hamster::List[1, 2, 3]
list.head # => 1
list.tail # => Hamster::List[2, 3]
Add to a list with List#add
:
original = Hamster::List[1, 2, 3]
copy = original.add(0) # => Hamster::List[0, 1, 2, 3]
Notice how modifying a list actually returns a new list.
That's because Hamster List
s are immutable.
Laziness
List
is lazy where possible. It tries to defer processing items until
absolutely necessary. For example, the following code will only call
Prime.prime?
as many times as necessary to generate the first 3
prime numbers between 10,000 and 1,000,000:
require 'prime'
Hamster.interval(10_000, 1_000_000).select do |number|
Prime.prime?(number)
end.take(3)
# => 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|
Prime.prime?(number)
end.take(3)
# => 10s
Construction
Besides Hamster::List[]
there are other ways to construct lists:
Hamster.interval(from, to)
creates a lazy list equivalent to a list containing all the values betweenfrom
andto
without actually creating a list that big.Hamster.stream { ... }
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 Hamster.stream { count += 1 }
Hamster.repeat(x)
creates an infinite list withx
the value for every element.Hamster.replicate(n, x)
creates a list of sizen
withx
the value for every element.Hamster.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:Hamster.iterate(1) { |i| i + 1 }
or even more succinctly:
Hamster.iterate(1, &:next)
Hamster::List.empty
returns an empty list, which you can build up using repeated calls to#add
or otherList
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 Hamster.
IO#to_list
enables lazy processing of huge files. For example, imagine the
following code to process a 100MB file:
require 'hamster/core_ext'
File.open("my_100_mb_file.txt") do |file|
lines = []
file.each_line do |line|
break if lines.size == 10
lines << line.chomp.downcase.reverse
end
end
Compare to the following more functional version:
File.open("my_100_mb_file.txt") do |file|
file.map(&:chomp).map(&:downcase).map(&:reverse).take(10)
end
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.
File.open("my_100_mb_file.txt") do |file|
file.to_list.map(&:chomp).map(&:downcase).map(&:reverse).take(10)
end
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 = Hamster::Deque[1, 2, 3] # => Hamster::Deque[1, 2, 3]
deque.first # 1
deque.last # 3
deque.pop # => Hamster::Deque[1, 2]
deque.push(:a) # => Hamster::Deque[1, 2, 3, :a]
deque.shift # => Hamster::Deque[2, 3]
deque.unshift(:a) # => Hamster::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.
Transformations
Hamster arrays, hashes, and nested structures of arrays and hashes may be transformed with the update_in
method.
c = Hamster.from({
people: [{name: 'Chris', city: 'Lagos'}, {name: 'Pat', city: 'Madrid'}],
places: [{name: 'Lagos', population: 1}, {name: 'Madrid', population: 1}]})
c2 = c.update_in(:people, 1, :city) { |old_city| 'Lagos' }
c3 = c2.update_in(:places, 1, :population) { |old_population| old_population - 1 }
c4 = c3.update_in(:places, 0, :population) { |old_population| old_population + 1 }
Hamster.to_ruby(c4)
# => {:places=>[{:population=>2, :name=>"Lagos"}, {:population=>0, :name=>"Madrid"}], :people=>[{:name=>"Chris", :city=>"Lagos"}, {:name=>"Pat", :city=>"Lagos"}]}
Naturally, update_in
never mutates your collections.
See Hamster::Hash#update_in
, Hamster::Vector#update_in
, and Hamster::Associable#update_in
for details.
Installing
Add this line to your application's Gemfile:
gem "hamster"
And then execute:
$ bundle
Or install it yourself as:
$ gem install hamster
Contributing
- Read the Code of Conduct
- Fork it
- Create your feature branch (
git checkout -b my-new-feature
) - Commit your changes (
git commit -am "Add some feature"
) - Push to the branch (
git push origin my-new-feature
) - Create new Pull Request
Other Reading
- The structure which is used for Hamster's
Hash
andSet
: Hash Array Mapped Tries - An interesting perspective on why immutability itself is inherently a good thing: Matthias Felleisen's Function Objects presentation.
- The Hamster FAQ
- Changelog
Licensing
Copyright (c) 2009-2015 Simon Harris
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.