Value Struct

A value struct is a subclass of normal Ruby struct that behaves almost the same. However, it has a major difference:

Value structs are immutable, i.e. they don't have setters (although, not recursively*)

Additionally, this gem provides the following optional mixins to make life easier when using immutable structs:

  • :dup_with_changes #dup takes a optional hash for setting new values in the duplicate

  • :to_h #to_h for converting into a valuestruct into a hash (if Ruby version below < 2.0)

  • :strict_arguments Value structs need to be initialized with the exact amount of arguments

  • :freeze Automatically freezes new instances

  • :no_clone #clone does not clone, but return the same object

By default, only :dup_with_changes and :to_h get included.

Why?

Sometimes you want to eliminate state. See [this blog article] for more information.

Performance

Without mixins, ValueStructs are as fast as normal structs. Some (optional) mixins add noticable overhead, e.g. strict_arguments

Example

require 'value_struct'

Point = ValueStruct.new(:x, :y) do # methods defined in the block will be available in your new value struct class
  # ...
end

Please refer to the [documentation of Ruby's struct] for more details on usage.

How to use structs with mixins

Point = ValueStruct.new_with_mixins :x, :y, [
  ValueStruct::ToH,
  ValueStruct::Freeze,
  ValueStruct::DupWithChanges,
  ValueStruct::StrictArguments,
]

p = Point.new(1,2)
p.to_h       #=> { :x => 1, :y => 2 }
p.frozen?    #=> true
p.dup(x: 0)  #=> #<ValueStruct Point x=0, y=2>
Point.new(1) # ArgumentError

*

Because of the nature of Ruby, most things are not really immutable. So if you have an attribute :by and initialize it with an array, you cannot change the value struct anymore, but still the array:

Ru = ValueStruct.new(:by)
ruby = Ru.by([1,2,3])
ruby.by # => [1,2,3]

ruby.by = [1,2,3,4] # not possible
ruby.by << 4        # possible

Install

$ gem install value_struct

Influenced by / Thanks to

J-_-L