From: daniel@dan42.com
To: ruby-core@ruby-lang.org
Subject: [ruby-core:95586] [Ruby master Feature#16122] Struct::Value: simple immutable value object
Date: Wed, 30 Oct 2019 02:46:38 +0000 (UTC) [thread overview]
Message-ID: <redmine.journal-82374.20191030024637.365b6d837c699028@ruby-lang.org> (raw)
In-Reply-To: redmine.issue-16122.20190823174036@ruby-lang.org
Issue #16122 has been updated by Dan0042 (Daniel DeLorme).
zverok (Victor Shepelev) wrote:
> So, considering all the points above, it could be either _multiple_ settings: `immutable: true, enumerable: false, hash_accessors: false`
I think that's a great idea. That way it's possible for everyone to mix and match the behavior they want in their structs. For example let say I want a struct to be mutable but not enumerable (because of the `Array(mystruct)` bug shown above), the `Struct::Value` approach doesn't work. If you find yourself always repeating the same options, it's trivial to write your own `ValueStruct` helper function.
----------------------------------------
Feature #16122: Struct::Value: simple immutable value object
https://bugs.ruby-lang.org/issues/16122#change-82374
* Author: zverok (Victor Shepelev)
* Status: Feedback
* Priority: Normal
* Assignee:
* Target version:
----------------------------------------
**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**
[Sample rendered RDoc documentation](https://zverok.github.io/ruby-rdoc/Struct-Value.html)
---Files--------------------------------
struct_value.patch (18.6 KB)
--
https://bugs.ruby-lang.org/
next prev parent reply other threads:[~2019-10-30 2:46 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <redmine.issue-16122.20190823174036@ruby-lang.org>
2019-08-23 17:40 ` [ruby-core:94508] [Ruby master Feature#16122] Struct::Value: simple immutable value object zverok.offline
2019-08-24 6:55 ` [ruby-core:94526] " eregontp
2019-08-24 10:17 ` [ruby-core:94527] " zverok.offline
2019-08-24 10:37 ` [ruby-core:94528] " zverok.offline
2019-08-29 8:01 ` [ruby-core:94661] " matz
2019-08-29 9:35 ` [ruby-core:94662] " zverok.offline
2019-08-29 12:50 ` [ruby-core:94665] " daniel
2019-08-29 12:55 ` [ruby-core:94666] " zverok.offline
2019-08-29 14:14 ` [ruby-core:94667] " mame
2019-08-29 14:46 ` [ruby-core:94668] " zverok.offline
2019-08-29 20:33 ` [ruby-core:94672] " zverok.offline
2019-08-30 0:09 ` [ruby-core:94674] " naruse
2019-08-30 7:05 ` [ruby-core:94679] " zverok.offline
2019-08-30 13:15 ` [ruby-core:94683] " daniel
2019-08-30 14:55 ` [ruby-core:94685] " zverok.offline
2019-08-30 18:26 ` [ruby-core:94690] " daniel
2019-09-03 15:56 ` [ruby-core:94762] " dementiev.vm
2019-09-07 19:56 ` [ruby-core:94832] " zverok.offline
2019-10-30 2:46 ` daniel [this message]
2019-11-28 8:40 ` [ruby-core:96017] " matz
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