Issue #16986 has been updated by duerst (Martin D=FCrst).
One more point: I haven't seen much examples of similar features in other l=
anguages. The only suggestion I saw was that of a similarity to Python tupl=
es. But tuples are much closer to Arrays than to Structs or hashes. The eas=
iest description for them may be "fixed-length Arrays".
(While not being available in (m)any other language(s) isn't by itself an a=
rgument against a feature, it definitely strengthens the need for careful e=
valuation and explanation of a new feature, including actual practical use =
cases.)
----------------------------------------
Feature #16986: Anonymous Struct literal
https://bugs.ruby-lang.org/issues/16986#change-87387
* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
----------------------------------------
# Abstract
How about introducing anonymous Struct literal such as `${a: 1, b: 2}`?
It is almost the same as `Struct.new(:a, :b).new(1, 2)`.
# Proposal
## Background
In many cases, people use hash objects to represent a set of values such as=
`person =3D {name: "ko1", country: 'Japan'}` and access its values through=
`person[:name]` and so on. It is not easy to write (three characters `[:]`=
!), and it easily introduces misspelling (`person[:nama]` doesn't raise an =
error).
If we make a `Struct` object by doing `Person =3D Struct.new(:name, :age)` =
and `person =3D Person.new('ko1', 'Japan')`, we can access its values throu=
gh `person.name` naturally. However, it costs coding. And in some cases, we=
don't want to name the class (such as `Person`).
Using `OpenStruct` (`person =3D OpenStruct.new(name: "ko1", country: "Japan=
")`), we can access it through `person.name`, but we can extend the fields =
unintentionally, and the performance is not good.
Of course, we can define a class `Person` with attr_readers. But it takes s=
everal lines.
To summarize the needs:
* Easy to write
* Doesn't require declaring the class
* Accessible through `person.name` format
* Limited fields
* Better performance
## Idea
Introduce new literal syntax for an anonymous Struct such as: `${ 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 =3D ${a: 1, b: 2, c: 3}
s2 =3D ${a: 1, b: 2, c: 3}
assert s1 =3D=3D s2
s3 =3D ${a: 1, c: 3, b: 2}
s4 =3D ${d: 4}
assert_equal false, s1 =3D=3D s3
assert_equal false, s1 =3D=3D s4
```
## Note
Unlike Hash literal syntax, this proposal only allows `label: expr` notatio=
n. No `${**h}` syntax.
This is because if we allow to splat a Hash, it can be a vulnerability by s=
platting outer-input Hash.
Thanks to this spec, we can specify anonymous Struct classes at compile tim=
e.
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 =3D Struct.new(:a, :b).new(1, 2)
hs =3D {a: 1, b: 2}
class C
attr_reader :a, :b
def initialize() =3D (@a =3D 1; @b =3D 2)
end
ob =3D 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 (26.25=
ns/i, 76clocks/i)
hs[:a] 37.845M i/s - 38.037M times in 1.005051s (26.42=
ns/i, 76clocks/i)
st.a 33.348M i/s - 33.612M times in 1.007904s (29.99=
ns/i, 87clocks/i)
Calculating -------------------------------------
ob.a 87.917M i/s - 114.300M times in 1.300085s (11.37=
ns/i, 33clocks/i)
hs[:a] 85.504M i/s - 113.536M times in 1.327850s (11.70=
ns/i, 33clocks/i)
st.a 61.337M i/s - 100.045M times in 1.631064s (16.30=
ns/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 im=
prove 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 nam=
e.
```ruby
s1 =3D ${a:1}
s2 =3D ${a:2}
p [s1, s2] #=3D> [#<struct a=3D1>, #<struct a=3D2>]
A =3D s1.class
p [s1, s2] #=3D> [#<struct A a=3D1>, #<struct A a=3D2>]
```
Maybe that is not a good behavior.
-- =
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