From: masafumi.o1988@...
Date: 2020-08-14T05:17:02+00:00
Subject: [ruby-core:99587] [Ruby master Feature#16986] Anonymous Struct literal
Issue #16986 has been updated by okuramasafumi (Masafumi OKURA).
> hoge {{a: 1, b: 2}} is not syntax error. {{a: 1, b: 2}} is block argument.
> That means {{ itself is not a syntax error and conflicts with a nested hash.
Thank you for pointing it out. Obviously it doesn't work.
What about `?{a: 1, b: 2}`?
Question mark (`?`) is not be able to a method by itself.
Character literal follows only one character, but in this form at least two characters follow.
So I think this form will not interpreted other ways but a Struct literal.
----------------------------------------
Feature #16986: Anonymous Struct literal
https://bugs.ruby-lang.org/issues/16986#change-87064
* 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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