[ruby-core:99836] [Ruby master Feature#16986] Anonymous Struct literal

[duerst@it.aoyama.ac.jp]

Issue #16986 has been updated by duerst (Martin Dürst).


mame (Yusuke Endoh) wrote in #note-38:

> So, external input like JSON data is not the target of this proposal.  Rather, this proposal is just a variant of Struct, which allows to omit the definition line: `Foo = Struct.new(...)`.

I'm still struggling to understand the actual uses of this proposal. I understand that many people like it, but can we see *actual real use cases*, i.e. code that not only gets shorter but also continues to be at least as easy to understand as before?

I'm trying to compare these "anonymous Structs" to anonymous functions. The success of anonymous functions would suggest that "anonymous structures" are also a good idea. But I'm not so sure about this.

For anonymous functions (blocks, lambdas), there's no need to discuss whether two of them that look identical are identical or not; it's rather rare to have the same thing twice anyway (something like {|a,b| a+b} might be an example that may turn up multiple times). Functions also don't have a second level of instantiation with actual data. And there's not much of a question about the semantics of such functions (in the above example, one could wonder whether it's an addition or a concatenation, but duck typing mostly makes that unnecessary/impossible).

For "anonymous Structs", the question of when two of these are the same seems to have several different expectations with different implementation and runtime consequences, but no widely satisfying solution. But it doesn't seem rare to have an "anonymous Struct" with the same components, because the actual instance data can differ. But the more instances I have, the stronger the need for a name becomes. If I have something like `${ name: 'foo', number: 'abcde' }`, I'm starting to wonder what that is: A person in a company? A room with name and number? Anything else of a lot of different choices?

So as you might guess, at this point, I'm very much not convinced. Examples with `a: 1, b: 2` don't really count as actual use cases, and are not concrete enough to help understand the actual pros and cons (except for syntax, which should be secondary).

----------------------------------------
Feature #16986: Anonymous Struct literal
https://bugs.ruby-lang.org/issues/16986#change-87360

* 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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