madlep / Values

Simple immutable value objects for ruby (the readme is longer than the code)

Values

Summary

Values is a tiny library for creating value objects in ruby.

Classes created using Value mostly look like classes created using Struct or OpenStruct, but fix two problems with those:

Problems with Struct and OpenStruct

Struct and OpenStruct constructors can take fewer than the default number of arguments and set other fields as nil:

Point = Struct.new(:x, :y)
Point.new(1)
# => #<struct Point x=1, y=nil>

p = OpenStruct.new(x: 1)
# => #<OpenStruct x=1>
p.y
# => nil

Struct and OpenStruct objects are mutable:

p = Point.new(1, 2)
p.x = 2
p.x
# => 2

p = OpenStruct.new(x: 1, y: 2)
p.x = 2
p.x
# => 2

Values is Better

Values fixes both of the above problems.

Constructors require expected arguments:

Point = Value.new(:x, :y)
Point.new(1)
# => ArgumentError: wrong number of arguments, 1 for 2
# from /Users/tcrayford/Projects/ruby/values/lib/values.rb:7:in `block (2 levels) in new
# from (irb):5:in new
# from (irb):5
# from /usr/local/bin/irb:12:in `<main>

Instances are immutable:

p = Point.new(1, 2)
p.x = 1
# => NoMethodError: undefined method x= for #<Point:0x00000100943788 @x=0, @y=1>
# from (irb):6
# from /usr/local/bin/irb:12:in <main>

Features

Values also provides an alternative constructor which takes a hash:

p = Point.with(x: 3, y: 4)
p.x
# => 3

Values can copy and replace fields using a hash:

p = Point.with(x: 1, y: -1)
q = p.with(y: 2)
# => #<Point x=1, y=2>

Value classes can be converted to a hash, like OpenStruct:

Point.with(x: 1, y: -1).to_h
# => {:x=>1, :y=>-1}

Values also supports customization of value classes inheriting from Value.new:

class Point < Value.new(:x, :y)
  def to_s
    "<Point at (#{x}, #{y})>"
  end
end

p = Point.new(1, 2)
p.to_s
# => "<Point at (1, 2)>"

Values does NOT have all the features of Struct or OpenStruct (nor is it meant to).