Project

General

Profile

Feature #16122

Updated by matz (Yukihiro Matsumoto) 2 months ago

**Value Object** is a useful concept, introduced by Martin Fowler ([his post](https://martinfowler.com/bliki/ValueObject.html), [Wikipedia Entry](https://en.wikipedia.org/wiki/Value_object)) with the following properties (simplifying the idea): 

 * representing some relatively simple data; 
 * immutable; 
 * compared by type & value; 
 * nicely represented. 

 Value objects are super-useful especially for defining APIs, their input/return values. Recently, there were some movement towards using more immutability-friendly approach in Ruby programming, leading to creating several discussions/libraries with value objects. For example, [Tom Dalling's gem](https://github.com/tomdalling/value_semantics), [Good Ruby Value object convention](https://github.com/zverok/good-value-object) (disclaimer: the latter is maintained by yours truly). 

 I propose to introduce **native value objects** to Ruby as a core class. 

 **Why not a gem?** 

 * I believe that concept is that simple, that nobody *will even try* to use a gem for representing it with, unless the framework/library used already provides one. 
 * Potentially, a lot of standard library (and probably even core) APIs could benefit from the concept. 

 **Why `Struct` is not enough** 

 Core `Struct` class is "somewhat alike" value-object, and frequently used instead of one: it is compared by value and consists of simple attributes. On the other hand, `Struct` is: 
 * mutable; 
 * collection-alike (defines `to_a` and is `Enumerable`); 
 * dictionary-alike (has `[]` and `.values` methods). 

 The above traits somehow erodes the semantics, making code less clear, especially when duck-typing is used. 

 For example, this code snippet shows why `to_a` is problematic: 

 ```ruby 
 Result = Struct.new(:success, :content) 

 # Now, imagine that other code assumes `data` could be either Result, or [Result, Result, Result] 
 # So, ... 

 data = Result.new(true, 'it is awesome') 

 Array(data) # => expected [Result(true, 'it is awesome')], got [true, 'it is awesome'] 

 # or... 
 def foo(arg1, arg2 = nil) 
 p arg1, arg2 
 end 

 foo(*data) # => expected [Result(true, 'it is awesome'), nil], got [true, 'it is awesome'] 
 ``` 

 Having `[]` and `each` defined on something that is thought as "just value" can also lead to subtle bugs, when some method checks "if the received argument is collection-alike", and value object's author doesn't thought of it as a collection. 

 **Concrete proposal** 

 * Class name: `Struct::Value`: lot of Rubyists are used to have `Struct` as a quick "something-like-value" drop-in, so alternative, more strict implementation, being part of `Struct` API, will be quite discoverable; *alternative: just `Value`* 
 * Class API is copying `Struct`s one (most of the time -- even reuses the implementation), with the following exceptions *(note: the immutability is **not** the only difference)*: 
   * Not `Enumerable`; 
   * Immutable; 
   * Doesn't think of itself as "almost hash" (doesn't have `to_a`, `values` and `[]` methods); 
   * Can have empty members list (fun fact: `Struct.new('Foo')` creating member-less `Struct::Foo`, is allowed, but `Struct.new()` is not) to allow usage patterns like: 

 ```ruby 
 class MyService 
   Success = Struct::Value.new(:results) 
   NotFound = Struct::Value.new 
 end 
 ``` 

 `NotFound` here, unlike, say, `Object.new.freeze` (another pattern for creating "empty typed value object"), has nice inspect `#<value NotFound>`, and created consistently with the `Success`, making the code more readable. And if it will evolve to have some attributes, the code change would be easy. 

 **Patch is provided** **Consensus update, Sep 2022** 

 * API agreed 
 * Name for the class: `Data` 
 * [Pull Request](https://github.com/ruby/ruby/pull/6353) 
 * [Sample rendered RDoc documentation](https://zverok.github.io/ruby-rdoc/Struct-Value.html) documentation rendering](https://zverok.space/ruby-rdoc/Data.html)

Back