[ruby-core:100215] [Ruby master Feature#16986] Anonymous Struct literal

Issue #16986 has been updated by esquinas (Enrique Esquinas).


duerst (Martin Dst) wrote in #note-43:
> One more point: I haven't seen much examples of similar features in other languages. The only suggestion I saw was that of a similarity to Python tuples. But tuples are much closer to Arrays than to Structs or hashes. The easiest description for them may be "fixed-length Arrays".
> 
> (While not being available in (m)any other language(s) isn't by itself an argument against a feature, it definitely strengthens the need for careful evaluation and explanation of a new feature, including actual practical use cases.)

Apart from Structs, examples have been given: regular Classes, Hashes, OpenStructs, and indirectly through the reference to this related Issue #16122 about using Struct::Value as native Value objects. I would also add the convenience of Javascript objects as "value objects" or quick duck-typed mocks, for testing, refactoring or other purposes:

``` javascript
// Original Point class may be complex but in JS we can do this instead of instantiating Point:
let point = {
  x: 12,
  y: 34,
  z: 56
};

console.log(calculationOn3D(point));
```

Here `calculationOn3D` expects the `point` variable to implement the 3D interface and be accessed like `point.x`,`point.y`,`point.z` so it just works.

In Ruby we currently have the option to `Struct.new(:x, :y, :z).new(12, 34, 56)` or use `OpenStruct.new({ x: 12, y: 34, z; 56 })` which we have to require and has other problems already addressed at the top by @Ko1

It's my understanding that we want a feature to easily, quickly, efficiently, natively, **naturally** create a *"value object"-like* intance so we could do something like:


``` ruby
point = %struct{ x: 12, y: 34, z: 56 } # ^1

puts "Our point is #{point.z} units deep"
puts calculation_on_3D(point) # ...
```

I hope this issue doesn't start to go in circles and lose focus. However, there's a discussion to have and find whether using `%struct` is a good choice or not. Specially when, at the end, the instance created by the literal notation `%struct{...}` may not resemble a regular struct very much... Or `%struct{...}` may be more than perfect if will be strictly equivalent to `Struct.new(:x, :y, :z).new(12, 34, 56)`.


**^1** One of the new syntaxes proposed by @ko1


----------------------------------------
Feature #16986: Anonymous Struct literal
https://bugs.ruby-lang.org/issues/16986#change-87805

* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
----------------------------------------
# Abstract

How about introducing anonymous Struct literal such as `${a: 1, b: 2}`?
It is almost the same as `Struct.new(:a, :b).new(1, 2)`.

# Proposal

## Background

In many cases, people use hash objects to represent a set of values such as `person = {name: "ko1", country: 'Japan'}` and access its values through `person[:name]` and so on. It is not easy to write (three characters `[:]`!), and it easily introduces misspelling (`person[:nama]` doesn't raise an error).

If we make a `Struct` object by doing `Person = Struct.new(:name, :age)` and `person = Person.new('ko1', 'Japan')`, we can access its values through `person.name` naturally. However, it costs coding. And in some cases, we don't want to name the class (such as `Person`).

Using `OpenStruct` (`person = OpenStruct.new(name: "ko1", country: "Japan")`), we can access it through `person.name`, but we can extend the fields unintentionally, and the performance is not good.

Of course, we can define a class `Person` with attr_readers. But it takes several lines.

To summarize the needs:

* Easy to write
  * Doesn't require declaring the class
  * Accessible through `person.name` format
* Limited fields
* Better performance

## Idea

Introduce new literal syntax for an anonymous Struct such as: `${ a: 1, b: 2 }`.
Similar to Hash syntax (with labels), but with `$` prefix to distinguish.

Anonymous structs which have the same member in the same order share their class.

```ruby
    s1 = ${a: 1, b: 2, c: 3}
    s2 = ${a: 1, b: 2, c: 3}
    assert s1 == s2

    s3 = ${a: 1, c: 3, b: 2}
    s4 = ${d: 4}

    assert_equal false, s1 == s3
    assert_equal false, s1 == s4
```

## Note

Unlike Hash literal syntax, this proposal only allows `label: expr` notation. No `${**h}` syntax.
This is because if we allow to splat a Hash, it can be a vulnerability by splatting outer-input Hash.

Thanks to this spec, we can specify anonymous Struct classes at compile time.
We don't need to find or create Struct classes at runtime.

## Implementatation

https://github.com/ruby/ruby/pull/3259

# Discussion

## Notation

Matz said he thought about `{|a: 1, b: 2 |}` syntax.

## Performance

Surprisingly, Hash is fast and Struct is slow.

```ruby
Benchmark.driver do |r|
  r.prelude <<~PRELUDE
  st = Struct.new(:a, :b).new(1, 2)
  hs = {a: 1, b: 2}
  class C
    attr_reader :a, :b
    def initialize() = (@a = 1; @b = 2)
  end
  ob = C.new
  PRELUDE
  r.report "ob.a"
  r.report "hs[:a]"
  r.report "st.a"
end
__END__
Warming up --------------------------------------
                ob.a    38.100M i/s -     38.142M times in 1.001101s (26.25ns/i, 76clocks/i)
              hs[:a]    37.845M i/s -     38.037M times in 1.005051s (26.42ns/i, 76clocks/i)
                st.a    33.348M i/s -     33.612M times in 1.007904s (29.99ns/i, 87clocks/i)
Calculating -------------------------------------
                ob.a    87.917M i/s -    114.300M times in 1.300085s (11.37ns/i, 33clocks/i)
              hs[:a]    85.504M i/s -    113.536M times in 1.327850s (11.70ns/i, 33clocks/i)
                st.a    61.337M i/s -    100.045M times in 1.631064s (16.30ns/i, 47clocks/i)
Comparison:
                ob.a:  87917391.4 i/s
              hs[:a]:  85503703.6 i/s - 1.03x  slower
                st.a:  61337463.3 i/s - 1.43x  slower
```

I believe we can speed up `Struct` similarly to ivar accesses, so we can improve the performance.


BTW, OpenStruct (os.a) is slow.

```
Comparison:
              hs[:a]:  92835317.7 i/s
                ob.a:  85865849.5 i/s - 1.08x  slower
                st.a:  53480417.5 i/s - 1.74x  slower
                os.a:  12541267.7 i/s - 7.40x  slower
```


For memory consumption, `Struct` is more lightweight because we don't need to keep the key names.

## Naming

If we name an anonymous class, literals with the same members share the name.

```ruby
s1 = ${a:1}
s2 = ${a:2}
p [s1, s2] #=> [#<struct a=1>, #<struct a=2>]
A = s1.class
p [s1, s2] #=> [#<struct A a=1>, #<struct A a=2>]

```

Maybe that is not a good behavior.




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