Issue #16986 has been updated by retro (Josef imnek).


First of all, this is super cool idea!

I do have habit to use hash since it is seems to be elegant (as described in original proposal background section) and I end up having problems later (since I need to use fetch everywhere to get at least some kind of consistency and to avoid typos for example).

I think there's no need for new syntax. "Struct.new" and "Kernel.Struct()" should be enough (if possible to extend Struct.new and keep the same performance).

Regarding syntax used, it would be great to support also "nested" structs, which I'm not sure if possible for all current ideas. For example:

``` ruby
config = Struct(assets: Struct(reload: true))
config.assets.reload #=> true
```


For simple structs I think %t or %o notation would be handy as well.

As mentioned at [#10](https://bugs.ruby-lang.org/issues/16986#note-10), by introducing Struct(), extending Struct.new and allowing %o or %t it would just follow already common patterns used for Array and Hash.

----------------------------------------
Feature #16986: Anonymous Struct literal
https://bugs.ruby-lang.org/issues/16986#change-86367

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

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

# Proposal

## Background

In many cases, people use hash objects to represents a set of values such as `person = {name: "ko1", country: 'Japan'}` and accesses it with `person[:name]` and so on. It is not easy to write (3 letters `[:]`!), and easy to introduce misspelling (`person[:nama]` doesn't raise an error).
If we make a `Struct` objects such as `Person = Struct.new(:name, :age)` and `person = Person.new('ko1', 'Japan')`, we can access it with `person.name` naturally.
However making new `Struct` is a cost of coding. Some cases we don't  want to name (such as `Person`).
Using `OpenStruct` (`person = OpenStruct.new(name: "ko1", country: "Japan")`), we can access it with `person.name`, but we can extend the fields and the performance is not good.
Of course, we can define the class `Person` and attr_readers. But several lines we need.

To summaries the issues:

* Easy to Write
  * Don't need to declare the class
  * Accessible with `person.name` format
* Limited fields
* Better performance

## Idea

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

Anonymous structs which has same member with same order share the 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 for this spec, we can specify the anonymous Struct class 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` similar 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 key names.

## Naming

If we name the anonymous class, the same member literals 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 it is not good behavior.




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