Feature #16986
Updated by sawa (Tsuyoshi Sawada) over 4 years ago
# Abstract How about introducing to introduce anonymous Struct literal such as `${a: 1, b: 2}`? It is almost the same as `Struct.new(:a, :b).new(1, 2)`. # Proposal ## Background In many cases, people use hash objects to represent represents a set of values such as `person = {name: "ko1", country: 'Japan'}` and access its values through accesses it with `person[:name]` and so on. It is not easy to write (three characters (3 letters `[:]`!), and it easily introduces easy to introduce misspelling (`person[:nama]` doesn't raise an error). If we make a `Struct` object by doing objects such as `Person = Struct.new(:name, :age)` and `person = Person.new('ko1', 'Japan')`, we can access its values through it with `person.name` naturally. However, it costs However making new `Struct` is a cost of coding. And in some cases, Some cases we don't want to name the class (such as `Person`). Using `OpenStruct` (`person = OpenStruct.new(name: "ko1", country: "Japan")`), we can access it through with `person.name`, but we can extend the fields unintentionally, and the performance is not good. Of course, we can define a the class `Person` with and attr_readers. But it takes several lines. lines we need. To summarize summaries the needs: issues: * Easy to write Write * Doesn't require declaring Don't need to declare the class * Accessible through with `person.name` format * Limited fields * Better performance ## Idea Introduce new literal syntax for to make an anonymous Struct literal such as: `${ a: 1, b: 2 }`. Similar to Hash syntax (with labels), but with `$` prefix to distinguish. recognize. Anonymous structs which have the has same member in the with same order share their 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 to for this spec, we can specify the anonymous Struct classes 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` similarly 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 the key names. ## Naming If we name an the anonymous class, literals with the same members 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 that it is not a good behavior.