Feature #16986
Updated by ko1 (Koichi Sasada) over 4 years ago
# 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.