[ruby-core:98986] [Ruby master Feature#16986] Anonymous Struct literal

Issue #16986 has been updated by ko1 (Koichi Sasada).


Other syntax ideas, by others:

* Other prefixes
  * `::{a: 1}`
  * `\{a: 1}`
* `<>` to indicate
  * `{<> a:1}` for anonymous Struct.
  * `{<A> a:1}` for named Struct, the name is `A`.
* Similar with `%`
  * `{% a:1}` for anonymous Struct.
  * `{%A a:1}` for  named Struct, the name is `A`.

----------------------------------------
Feature #16986: Anonymous Struct literal
https://bugs.ruby-lang.org/issues/16986#change-86366

* 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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