Feature #16986: Anonymous Struct literal

• Status: Assigned
• Assignee: matz (Yukihiro Matsumoto)
• Target version: -

[ko1 (Koichi Sasada)]

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.

    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.

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.

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.

[ko1 (Koichi Sasada)]

• Description updated (diff)

[shevegen (Robert A. Heiler)]

First, I like the idea, so +1 for the idea. It also reminds me of
a more "prototypic-based approach" in general with structs, so if
the syntax could be made simpler to type, that seems to be a
possible improvement. And it fits into other "shortcut" variants,
such as %w() %I() and so forth. I make use of %w() and so forth
a LOT - it really helps when you have to create longer arrays;
we can avoid all the middle "','" parts.

What I dislike a bit about the suggestion here is the proposed
syntax. There are two aspects as to why that is the case for me:

(1) While I do sometimes use $ variables (in particular the regex
variants $1 $2 are so convenient to use), sometimes I use $stdin
as well, but I don't quite like global variables in general. In
particular remembering $: $? $_ (if these even exist) is a bit
tedious. So most of the time when we can avoid $, I think this
is better than having to use $. But this is just one part.

(2) The other, perhaps slightly more problematic thing, is that
we would introduce a new variant of how people have to
understand $ variables.

Specifically this variant:

${a: 1, b: 2}

May look like regular variable substitution such as:

cat = 'Tom'
puts "#{cat} is hunting Jerry."

Or, if people use global variables:

$cat = 'Tom'
puts "#{$cat} is hunting Jerry."
puts "#$cat is hunting Jerry." # ok perhaps not quite as much, since we can omit {} in that case.

Possibly the {} may have ruby user associate this with a regular
substitution.

The other aspect is that this would be the first global variable
use that combines {} upon "definition" time. We used to have
something like this:

$abc = 'def'

That feels a bit different to:

abc = ${a: 1, b: 2}

Hmm.

Matz said he thought about {|a: 1, b: 2 |} syntax.

Do you mean without '$' there? If so then I think the syntax by
matz is better. Though people could assume it is a block. :)

However had, having said that, I do not have a good alternative
suggestion; and even then I think the feature may quite useful.

Perhaps:

struct a: 1, b: 2

Would be somewhat short too.

Implying:

struct {a: 1, b: 2}

Though, admittedly the struct variant is still longer to type than
the $ suggestion here. While I don't quite like the $, it is arguably
short to type. And I suppose one requirement for this proposal is
that it should be shorter than:

Struct.new(:a, :b).new(1, 2)

Even if made shorter, like:

Struct.new(a: 1, b: 2)

Admittedly the $ is shortest:

${a: 1, b: 2}

matz' variant:

foobar = {|a: 1, b: 2 |}
foobar = {|a: 1, b: 2|}
foobar = ${|a: 1, b: 2 |}

(I was not sure which variant was the one; I assume matz' variant is
actually without the $, so perhaps the second variant.)

If we name the anonymous class, the same member literals share the
name.

s1 = ${a:1}
s2 = ${a:2}

Hmmmmm. The matz' variant would then be like this?

s1 = {|a: 1|}

To be honest, even if that is longer, I think it is better than the
variant with $. I am not sure if I am the only one disliking the $
there but I think it would be better to not have to use $, even
if the second variant is longer. But as said, I think the idea
itself is fine.

[osyo (manga osyo)]

hi, I like this idea :)
I was wondering if ${} can do the following things compared to Hash literals:

# Can a value that is not a Symbol be used as a key (Symbol only?)
${ "a" => 1 }
${ 1 => 1 }

# Can variables be used as keys
key = :a
${ key => 1 }

# Can Hash be expanded with `**`
hash = { a: 1, b: 2 }
${ c: 3, **hash }

Thank you.

[zverok (Victor Shepelev)]

WDYT about half-solution (without syntax change)?
E.g. for me, the problem with Struct.new(:a, :b).new(1, 2) is not that it is "too long to write" but just that it is looks "hacky" (like, "you are using Struct against its expectations/best practices"), and non-atomic.
So, may be this would be enough for most cases:

Struct.anonymous(a: 1, b: 2)
# method name is debatable.
# or, IDK, maybe just
Struct(a: 1, b: 2)

Also, I'd say that maybe the value produced this way should be immutable? (as in #16122)
Otherwise, one might just use OpenStruct (if the value has the same amount of mutability as hash), or normal Type = Struct.new(:a, :b) (if the structure of value is fixed, but content is mutable — it means structure of type has some fixed semantic and probably should have a name).

[zverok (Victor Shepelev)]

Another (unrelated, but conflicting) matter: I am not sure we have/had a discussion for this, but I remember Bozhidar Batsov in his "Ruby 4: To Infinity and Beyond" proposed ${} as a literal for set (which for me seems more important and potentially more widespread).

[byroot (Jean Boussier)]

Matz said he thought about {|a: 1, b: 2 |} syntax.

Might be just me but {| on first sight makes me think: this is a block.

What about a % syntax? %s and %S already exists, but %O could make sense: %O{foo: 1, bar: 42}. Or yeah just a simple Struct() or Struct[] with no additional syntax as suggested just above.

[byroot (Jean Boussier)]

Or yeah just a simple Struct() or Struct[] with no additional syntax as suggested just above.

Actually I just realized that it can't really work as it would allow for **kwargs.

[zverok (Victor Shepelev)]

Thinking a bit more about it, I see more of a conceptual problem.

Introducing new literal in a language, we kinda introduce a new concept of the core collection. But what's that collection is, how'd you explain it to a novice?
It is:

• "a Struct", but it can't be created with Struct.new (which, already confusingly enough, creates something that not is_a?(Struct))
• which has keys/values like Hash,
• ...but allows access by .<key>... oh, but also [:key] and ["key"]. And [0] :)
• ...and does not allow adding/removing keys
• ...but is not fully immutable, as it allows assigning new values to keys
• ...also it, for example, unpacks (*${a: 1}) in just values? (at least structs currently behave so)

So, what core concept it represents?..

[ttanimichi (Tsukuru Tanimichi)]

struct = (a: 1, b: 2) would be a great syntax, because I think anonymous struct is something like Tuple of Python.
But, there are two problems:

1. () returns nil, not an empty struct.
2. The meaning of p(a: 1) and p (a: 1) are different. The former prints {:a=>1}, but the latter prints #<struct a=1>

[bkuhlmann (Brooke Kuhlmann)]

I like the idea of being able to quickly create an anonymous Struct but am concerned about the use of $ for the syntax since that hinders readability and causes confusion due to $ generally denoting a global variable. Why not allow structs to have similar behavior to existing objects in Ruby like hashes, arrays, etc in order to remain consistent? Example:

# Kernel.Hash
Hash a: 1, b: 2 # => {a: 1, b: 2}
# Hash.[]
Hash[a: 1, b: 2] # => {a: 1, b: 2}

# Kernel.Array
Array 1 # => [1]
# Array.[]
Array[1] # => [1]

With the suggestion above, we could implement the same for Structs too. Example:

# Kernel.Struct
Struct a: 1, b: 2 # => #<struct a=1, b=2>
# Struct.[]
Struct[a: 1, b: 2] # => #<struct a=1, b=2>

[ko1 (Koichi Sasada)]

Q&A

Splat like Hash literal and method arguments

https://bugs.ruby-lang.org/issues/16986#note-3

I was wondering if ${} can do the following things compared to Hash literals:

Not allowed because of vulnerability concerns (please read a ticket for more details).

Syntax

• (1) ${a:1, b:2} # original
• (2) {|a:1, b:2|} # matz's idea ... conflict with block parameters.
• (3) struct a: 1, b: 2 # #1 ... introducing new struct keyword can introduce incompatibility.
• (4) %o{a:1, b:2} # #6
• (5) (a:1, b:2) # #9
• (6) Methods
  • (6-1) Struct.anonymous(a:1, b:2) # #4
  • (6-2) Struct(a:1, b:2) # #4, #10
  • (6-3) Struct[a:1, b:2] # #10

Some support comments on ${...}:

• I can recognize it is not global variables.
• $ is seems as an initial letter of Struct ... S !!! (50% joking)
• If we can introduce ${ ... }, we can also consider about $[...] (Set?) and $(...). I agree it can introduce further chaotic.
• We can replace $ with @, but no reason to choose @ ... ah, not a support comment.

I thought similar idea on "(4) %o{a:1, b:2} # #6", and my idea was %t (Struct).
However, there is no % notation which allows Ruby's expression in it.
In other words, existing % notation defines different language in % (%w, %i for example).
This is why I gave up this idea. But I don't against it (new language can accept Ruby's expression).

For "(6) Methods", my first proposal was Struct(a;1, b:2). https://twitter.com/_ko1/status/1276055259241046016?s=20

However, there are several advantages by introducing new syntax which are described in a ticket.
And real reason I make a demonstrate moving code https://github.com/ruby/ruby/pull/3259 is I want to modify parse.y to escape from Ractor's debugging.

The biggest advantage of choosing a method approach is simplicity. No new syntax and only a few learning cost.
It is easy to introduce same method for older versions (~2.7).

(For performance of object creation, we can introduce specialization for Struct() method to prepare an anonymous class at compile time)

"(5) `(a:1, b:2)" seems interesting, but I agree there are issues which are pointed in the comment 9.

[ko1 (Koichi Sasada)]

Other syntax ideas, by others:

• Other prefixes
  • ::{a: 1}
  • \{a: 1}
• <> to indicate
  • {<> a:1} for anonymous Struct.
  • {<A> a:1} for named Struct, the name is A.
• Similar with %
  • {% a:1} for anonymous Struct (it can conflict with % notation).
  • {%A a:1} for named Struct, the name is A.

[retro (Josef Šimánek)]

First of all, this is super cool idea!

I do have habit to use hash since it is seems to be elegant (as described in original proposal background section) and I end up having problems later (since I need to use fetch everywhere to get at least some kind of consistency and to avoid typos for example).

I think there's no need for new syntax. "Struct.new" and "Kernel.Struct()" should be enough (if possible to extend Struct.new and keep the same performance).

Regarding syntax used, it would be great to support also "nested" structs, which I'm not sure if possible for all current ideas. For example:

config = Struct(assets: Struct(reload: true))
config.assets.reload #=> true

For simple structs I think %t or %o notation would be handy as well.

As mentioned at #10, by introducing Struct(), extending Struct.new and allowing %o or %t it would just follow already common patterns used for Array and Hash.

[sawa (Tsuyoshi Sawada)]

• Description updated (diff)

[Hanmac (Hans Mackowiak)]

i think this is more of a confusing feature

IF ${a: 1, b: 2} is like Struct.new(:a, :b).new(1, 2) then my gut is telling me that

  s1 = ${a: 1, b: 2, c: 3}
  s2 = ${a: 1, b: 2, c: 3}
  assert s1 == s2

should not be the same because their class is different

[sobrinho (Gabriel Sobrinho)]

+1 for the idea, I'm using my_hash.fetch(:my_key) to ensure consistency in a lot of places.

Concerns:

1. Struct is slow (should not be that hard to improve, though)
2. ${} will be hard to read, search and explain about
3. It would be useful to have nested support like Struct(a: Struct(b: 1)) as mentioned before, using ${a: ${b: 1}} is a bit confusing to read

I'm also with the Kernel.Struct method, it seems the best proposal so far.

[calebhearth (Caleb Hearth)]

I'm also +1 for this. The utility of such a feature is obvious and would improve a lot of Ruby code both in readability and in being more bug-free as it helps with the Hash#[] problem of missing/misspelled keys.

I immediately understood the ${} meaning, and so I would have to disagree with those who suggest it might be harder to read. $ for Struct was my first thought, and {} makes the hash-iness of it obvious so I think this is good syntax.

Perhaps as a compromise ${} could be shorthand for some longform method such as we see in ->() for lambda {} or .() for .call.

The longform might simply be Struct.new().new(), but ko1 mentioned that it was "almost the same" - what differences are there?

[byroot (Jean Boussier)]

it was "almost the same" - what differences are there?

The actual implementation is more like this:

$structs = {}
def Struct(**kwargs)
  klass = $structs[kwargs.keys.sort] ||= Struct.new(*kwargs.keys, keyword_init: true)
  klass.new(**kwargs).freeze
end

Two literal structs with the same set of fields, will share the same class, that's the subtility.

I believe this also answers @Hanmac's interrogation.

[ko1 (Koichi Sasada)]

klass.new(**kwargs).freeze

This proposal doesn't include freeze.
I agree it is one option, but need a discussion.

[marcandre (Marc-Andre Lafortune)]

Without expressing an opinion on the proposal (yet), I'd like to point an easy way to avoid having to use fetch all the time: the default_proc.

my_hash = Hash.new { |h, k| raise "Invalid key: #{k}" }.merge(default: 42)

my_hash[:defautl] # => Invalid key: defautl

[dimasamodurov (Dima Samodurov)]

Accessing object methods via dots is more convenient than via brackets. And I tend to think of {a: 1, b: 2} as of object with properties.
That is why I like ${} syntax: I see the same object in square brackets and can easily copy/paste. Using of $ does not confuse, as global variables are used rarely.

I would rather think of concept differences between Hash and Struct classes which would have similar literals. Hashes are extendable, using hash.merge(hash) is great. This is kind of symbols vs strings. Both are great, but using symbols was a bit less understood. Collecting more common use cases and antipatterns would help adopting the feature.

[esquinas (Enrique Esquinas)]

Hello, this is a great idea. I would just love to have this feature in Ruby, so I thought I may provide some helpful input:

First, I would like to read more ideas about the exact implementation, specially:

1. Will the literal produce a frozen struct by default or not?
2. Will other objects (hashes, for instance) be allowed to be included by reference or will they be copied by value? Recursively?

I think some hint about the implementation will make the case for the usefulness and, more important, the purpose of the new syntax/feature and help us all argue and decide.
I have not enough experience or knowledge to debate about the implementation problems of any of the options, so I apologize in case I propose a non-sensical approach in this regard.

My comment about the ${} syntax would be that it feels very arbitrary. Others have proposed even more arbitrary notations, but if I had to choose one, I would vote for a .{} notation, the dot is an obvious reminder of how you will access the struct literal later on. Again, sorry if this syntax is non-sensical and unfeasible, just trying to come up with a short syntax which is less arbitrary.

Another notation that I liked was the proposed %o{} or %t{}. They feel very familiar and even allow the capital letter variation. The meaning of %O{} or %T{} could depend on the first point I made, the purpose behind it.

A new option I didn't see yet is to extend the Hash class to have a to_struct method, so we avoid the new syntax altogether: { a: 1, b: 2 }.to_struct. I wrote a quick & dirty implementation to explore some ideas around this concept in this Gist gist.github.com/esquinas/6c47046b1557a7b372466032187b152f

To summarize, if I had to rank my top 5 of proposed notations:

1. { a: 1, b: 2 }.to_struct no need for a new syntax. Very "Ruby" IMO. The Hash#to_struct method could take arguments for the class name, the frozen state, etc.
2. Struct a: 1, b: 2 or similar, very obvious and readable.
3. %o{ a: 1, b: 2 } or %t{ a: 1, b: 2 } again, very familiar and allow the capital-letter variation.
4. .{ a: 1, b: 2 } very short syntax alternative and a little less arbitrary than ${} or \{}.
5. ::{ a: 1, b: 2} same as the previous one, my_struct::a just works, so why not?

Thanks!

[Hanmac (Hans Mackowiak)]

the problem i have with that is that each time it creates a (cached) struct class.

after taking so long to make frozen literals and making even such frozen objects GC able,
something that creates new (class) objects which then can never be cleared sounds like a problem that would bite you in the butt sooner or later

other than that, Struct a: 1, b: 2 would probably be the most clean variant

[jrochkind (jonathan rochkind)]

Why is more special syntax needed, when it can just be a method?

def Kernel.AStruct(**key_values)
  Struct.new(key_values.keys).new(key_values.values)
end

AStruct(a: 1, b: 2)

If that implementation isn't efficient enough, implement it in C with a high-performance memoizing implementation or something, why not.

But additional syntax makes the language larger, harder to implement, harder to learn. It's hard to google for punctuation. At its best Ruby is just objects and methods, if we can provide this functionality just fine with a plain old method, why the need for new syntax?

[zliang (Gimi Liang)]

Maybe {a = 1, b = "hello world"}?

[ko1 (Koichi Sasada)]

Matz said: "good to have, but current proposed syntax are not acceptable. If there is good syntax, I can consider to accept."

[okuramasafumi (Masafumi OKURA)]

I found that

{{a: 1, b: 2}}

is a syntax error and could be a good candidate for this feature.

[osyo (manga osyo)]

okuramasafumi (Masafumi OKURA) wrote in #note-27:

I found that

{{a: 1, b: 2}}

is a syntax error and could be a good candidate for this feature.

hi.
hoge {{a: 1, b: 2}} is not syntax error. {{a: 1, b: 2}} is block argument.

def hoge(&block)
  block.call
end

# OK
hoge {{a: 1, b: 2}}
# => {:a=>1, :b=>2}

[nobu (Nobuyoshi Nakada)]

okuramasafumi (Masafumi OKURA) wrote in #note-27:

I found that

{{a: 1, b: 2}}

is a syntax error and could be a good candidate for this feature.

Read the error message.

$ ruby -e '{{a: 1, b: 2}}'
-e:1: syntax error, unexpected '}', expecting =>
{{a: 1, b: 2}}
             ^

That means {{ itself is not a syntax error and conflicts with a nested hash.

[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 be 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.

[nobu (Nobuyoshi Nakada)]

okuramasafumi (Masafumi OKURA) wrote in #note-30:

What about ?{a: 1, b: 2}?
Question mark (?) is not be able to be a method by itself.
Character literal follows only one character, but in this form at least two characters follow.

Do you mean a: by "two characters"?
As { is a punctuation, the a cannot be the part of that character literal.

[sawa (Tsuyoshi Sawada)]

nobu (Nobuyoshi Nakada) wrote in #note-31:

Do you mean a: by "two characters"?
As { is a punctuation, the a cannot be the part of that character literal.

I think he meant the characters { and a (or even :). I think his point is that having them in succession causes a syntax error, which means that it does not conflict with existing code.

?{ # => "{"
?a # => "a"
?{a # >> Syntax error

[esquinas (Enrique Esquinas)]

Issue #16122 gave me this idea:

I we already had Struct::Value, then I think it would make a lot of sense to allow the following syntax:

%v{ a: 1, b: 2, c: 3 } v short for Struct.Value => immutable: true, enumerable: false, hash_accessors: false

I think the above syntax is great because %v{ a: 1, b: 2, c: 3 } is memorable ("v" for value) and friendly to people coming from JavaScript/Nodejs: "Same as JS, just start with %v". This will cover the use-case of having a very quickly way to create an efficient, safe, well structured, value-like object to store data, which'd be amazing.

For other variations, as some have said on Issue #16122 , it may be good enough to use keyword arguments for Struct.new().

However, if we wanted to add a short syntax for good old regular Structs, I propose yet another option, apart from %o{...} :

%a{ a: 1, b: 2, c: 3 } a for Anonymous Struct?

Sorry for the brainstorming undertone and the slight diversion.

[ko1 (Koichi Sasada)]

how about %struct{a: 1, b: 2} (and %value{...} if needed)?
(znz-san's idea and it seems nice)

[esquinas (Enrique Esquinas)]

ko1 (Koichi Sasada) wrote in #note-34:

how about %struct{a: 1, b: 2} (and %value{...} if needed)?
(znz-san's idea and it seems nice)

I like it! IMO, it's one of the most "natural to read" options and is undoubtedly easier to write than Struct.new(:a, :b, :c).new(1, 2, 3).

My only tiny concern is, shouldn't be %Struct{a: 1, b: 2} also valid?

Cannot wait to know what others think about this.

---

PS: I love the idea this would allow more syntax in the future which could be short, explicit and readable, for instance:

# Alternative to `%q{Hello, World}`?
%string{Hello, World} #=> "Hello, World"

# Alternative to `%w[one two three]`?
%array[one two three] #=> ["one", "two", "three"]

# Extra nice if Issue #16122/#18 were implemented. An even shorter alternative to the "Value" helper `Struct.Value(a: 1, b: 2, c: 3)`!
quick_data = %value{ a: 1, b: 2, c: 3 } #=> #<value a=1, b=2, c=3>
quick_data.a #=> 1

# Or going even crazier:
obj = %Object{ a: 1, b: 2, c: 3 } #=> #<Object:0x0000xxxxxxxxxxxx>
obj.a #=> 1

%BasicObject{ inspect: "This is why we can't have nice things..." }
#=> This is why we can't have nice things...

[mame (Yusuke Endoh)]

At first, I misunderstood this feature. I thought that this would be a useful substitute of symbol-key Hash (like JSON data) which we can read the values by s.name instead of s[:name].

However, it is not. This use is potentially dangerous.

# Dangerous usage

json = JSON.parse(external_input) # { :name => "John", :age => 20 }

data = Struct(**json) # or something

data.name #=> "John"
data.nme #=> NoMethodError: useful to detect typo or wrong data?

Because method-name symbols are never GC'ed, so converting arbitrary external input to anonymous Struct is vulnerable against Symbol DoS.

To make it foolproof, adding a new literal instead of method like Struct() is very important. And for the same reason, note that we will never have Hash#to_anonymous_struct or something.

Also, we should not use this feature when keys are optional. It creates a struct class for all unique set of keys.
So ${ name: "John" } and ${ name: "John", age: 20 } will create two different class objects. This may cause combinational explosion. You need to always write ${ name: "John", age: nil, (and other keys if any...) }.

Now, I'm unsure if and when this feature is actually useful.

[Dan0042 (Daniel DeLorme)]

I thought that this would be a useful substitute of symbol-key Hash (like JSON data) which we can read the values by s.name instead of s[:name].

This is the same thing stated in the Feature proposal, and I would also like this, and it seems a lot of other people here too. So even if the solution is not exactly an Anonymous Struct literal let's find a way to be able to write this kind of nice-looking s.name code.

Because method-name symbols are never GC'ed, so converting arbitrary external input to anonymous Struct is vulnerable against Symbol DoS.

It should be possible to work around this. Let say we create an anonymous struct with Struct(**json), instead of creating an actual Struct.new it could be something like AnonStruct.new where only already-existing method-name symbols are created as methods, and the other keys are handled via method_missing.

It creates a struct class for all unique set of keys.

Assuming that the struct class can be garbage-collected when there are no longer any instances, I think this should not be a big problem?

[mame (Yusuke Endoh)]

Dan0042 (Daniel DeLorme) wrote in #note-37:

I thought that this would be a useful substitute of symbol-key Hash (like JSON data) which we can read the values by s.name instead of s[:name].

This is the same thing stated in the Feature proposal, and I would also like this, and it seems a lot of other people here too.

I thought so. Thus, I wanted to let all of you know what this proposal is not.
BTW, the proposal includes the following note that states the problem:

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.

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(...).

Because method-name symbols are never GC'ed, so converting arbitrary external input to anonymous Struct is vulnerable against Symbol DoS.

It should be possible to work around this. Let say we create an anonymous struct with Struct(**json), instead of creating an actual Struct.new it could be something like AnonStruct.new where only already-existing method-name symbols are created as methods, and the other keys are handled via method_missing.

In my opinion, it is too hacky to include into the core.

It creates a struct class for all unique set of keys.

Assuming that the struct class can be garbage-collected when there are no longer any instances, I think this should not be a big problem?

Theoretically, yes. But it will require much work.

[ko1 (Koichi Sasada)]

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(...).

Yes. This proposal is strict one.

[nobu (Nobuyoshi Nakada)]

ko1 (Koichi Sasada) wrote in #note-34:

how about %struct{a: 1, b: 2} (and %value{...} if needed)?
(znz-san's idea and it seems nice)

Is %struct"a: 1, b: 2" same?

[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).

[marcandre (Marc-Andre Lafortune)]

I'm also unconvinced of a good use case and why creating a MyResult = Struct.new(:some_value, :name) is really something that should be "saved".

[duerst (Martin Dürst)]

One more point: I haven't seen much examples of similar features in other languages. The only suggestion I saw was that of a similarity to Python tuples. But tuples are much closer to Arrays than to Structs or hashes. The easiest description for them may be "fixed-length Arrays".

(While not being available in (m)any other language(s) isn't by itself an argument against a feature, it definitely strengthens the need for careful evaluation and explanation of a new feature, including actual practical use cases.)

[chrisseaton (Chris Seaton)]

This will be very useful for pattern matching - I don't think you can usefully destructure a struct at the moment can you? This leads me to do all my pattern matching code using Hash and Array which isn't ideal.

[marcandre (Marc-Andre Lafortune)]

chrisseaton (Chris Seaton) wrote in #note-44:

This will be very useful for pattern matching - I don't think you can usefully destructure a struct at the moment can you?

You can, like a hash...

X = Struct.new(:foo, :bar, keyword_init: true)
obj = X.new(foo: 1, bar: 2)

obj in {foo:, bar:}
foo # => 1
bar # => 2

[esquinas (Enrique Esquinas)]

duerst (Martin Dürst) wrote in #note-43:

One more point: I haven't seen much examples of similar features in other languages. The only suggestion I saw was that of a similarity to Python tuples. But tuples are much closer to Arrays than to Structs or hashes. The easiest description for them may be "fixed-length Arrays".

(While not being available in (m)any other language(s) isn't by itself an argument against a feature, it definitely strengthens the need for careful evaluation and explanation of a new feature, including actual practical use cases.)

Apart from Structs, examples have been given: regular Classes, Hashes, OpenStructs, and indirectly through the reference to this related Issue #16122 about using Struct::Value as native Value objects. I would also add the convenience of Javascript objects as "value objects" or quick duck-typed mocks, for testing, refactoring or other purposes:

// Original Point class may be complex but in JS we can do this instead of instantiating Point:
let point = {
  x: 12,
  y: 34,
  z: 56
};

console.log(calculationOn3D(point));

Here calculationOn3D expects the point variable to implement the 3D interface and be accessed like point.x,point.y,point.z so it just works.

In Ruby we currently have the option to Struct.new(:x, :y, :z).new(12, 34, 56) or use OpenStruct.new({ x: 12, y: 34, z; 56 }) which we have to require and has other problems already addressed at the top by @ko1 (Koichi Sasada)

It's my understanding that we want a feature to easily, quickly, efficiently, natively, naturally create a "value object"-like intance so we could do something like:

point = %struct{ x: 12, y: 34, z: 56 } # ^1

puts "Our point is #{point.z} units deep"
puts calculation_on_3D(point) # ...

I hope this issue doesn't start to go in circles and lose focus. However, there's a discussion to have and find whether using %struct is a good choice or not. Specially when, at the end, the instance created by the literal notation %struct{...} may not resemble a regular struct very much... Or %struct{...} may be more than perfect if will be strictly equivalent to Struct.new(:x, :y, :z).new(12, 34, 56).

^1 One of the new syntaxes proposed by @ko1 (Koichi Sasada)

[marcandre (Marc-Andre Lafortune)]

esquinas (Enrique Esquinas) wrote in #note-46:

Here calculationOn3D expects the point variable to implement the 3D interface and be accessed like point.x,point.y,point.z so it just works.

In Ruby we currently have the option to Struct.new(:x, :y, :z).new(12, 34, 56) or use OpenStruct.new({ x: 12, y: 34, z; 56 }) which we have to require and has other problems already addressed at the top by @ko1 (Koichi Sasada)

I don't find this example convincing at all. In Ruby, we would have an actual class Point3D with specialized +, -, as well as extra methods, and there's no way to know which of these calculationOn3D would use (now or in the future). There's also no gain in passing %struct{...} instead of Point3D.new(...).

I wish someone would point out a few examples in a popular gem or a Rails app say where this would be useful and actually improve actual code.

[tenderlovemaking (Aaron Patterson)]

marcandre (Marc-Andre Lafortune) wrote in #note-47:

esquinas (Enrique Esquinas) wrote in #note-46:

Here calculationOn3D expects the point variable to implement the 3D interface and be accessed like point.x,point.y,point.z so it just works.

In Ruby we currently have the option to Struct.new(:x, :y, :z).new(12, 34, 56) or use OpenStruct.new({ x: 12, y: 34, z; 56 }) which we have to require and has other problems already addressed at the top by @ko1 (Koichi Sasada)

I don't find this example convincing at all. In Ruby, we would have an actual class Point3D with specialized +, -, as well as extra methods, and there's no way to know which of these calculationOn3D would use (now or in the future). There's also no gain in passing %struct{...} instead of Point3D.new(...).

I wish someone would point out a few examples in a popular gem or a Rails app say where this would be useful and actually improve actual code.

I frequently use Struct.new().new() in test cases where I need fakes. Here is some real code that I think %struct{...} would improve:

https://github.com/rails/rails/blob/6fca0f31f14db4e8b73a6c1d89afd16d51e6b6f3/activerecord/test/cases/reflection_test.rb#L422-L423
https://github.com/rails/rails/blob/6fca0f31f14db4e8b73a6c1d89afd16d51e6b6f3/activerecord/test/cases/reflection_test.rb#L437-L438
https://github.com/rails/rails/blob/6fca0f31f14db4e8b73a6c1d89afd16d51e6b6f3/activerecord/test/cases/reflection_test.rb#L452-L453
https://github.com/rails/rails/blob/6fca0f31f14db4e8b73a6c1d89afd16d51e6b6f3/actionview/test/template/url_helper_test.rb#L69

Here is a non-test case:

https://github.com/rails/rails/blob/6fca0f31f14db4e8b73a6c1d89afd16d51e6b6f3/actionview/lib/action_view/template/types.rb#L9

An example in comments:

https://github.com/rails/rails/blob/6fca0f31f14db4e8b73a6c1d89afd16d51e6b6f3/actionview/lib/action_view/helpers/rendering_helper.rb#L87

Actually it's pretty trivial to find these places in the Rails codebase. git grep 'Struct.new([^)]*)\.new' will give you quite a few examples of real world code that could be improved with this syntax.

I didn't want to limit myself to just Rails, so here are some examples from random projects I have checked out:

https://github.com/rspec/rspec-expectations/blob/87376eed1f580682c86ea88448ab0411be238c07/spec/rspec/expectations/syntax_spec.rb#L7-L17
https://github.com/erikhuda/thor/blob/09ae06628ab6b20d4abdea6400bcbc2cffcec52a/spec/command_spec.rb#L13-L32
https://github.com/rubygems/rubygems/blob/ea2376928988c8124bba661e0754d7a00dcab347/lib/rubygems/requirement.rb#L24
https://github.com/sinatra/sinatra-contrib/blob/e1d920fa5dc09aea277db10de2a025a673a4f54a/spec/respond_with_spec.rb#L180

Anyway, I think a syntax for the Struct.new().new() pattern would be nice. I've most commonly seen it in tests, but I don't think that makes it any less useful.

[marcandre (Marc-Andre Lafortune)]

tenderlovemaking (Aaron Patterson) wrote in #note-48:

I frequently use Struct.new().new() in test cases where I need fakes.
Here is some real code that I think %struct{...} would improve:

https://github.com/rails/rails/blob/6fca0f31f14db4e8b73a6c1d89afd16d51e6b6f3/activerecord/test/cases/reflection_test.rb#L422-L423
https://github.com/rails/rails/blob/6fca0f31f14db4e8b73a6c1d89afd16d51e6b6f3/activerecord/test/cases/reflection_test.rb#L437-L438
https://github.com/rails/rails/blob/6fca0f31f14db4e8b73a6c1d89afd16d51e6b6f3/activerecord/test/cases/reflection_test.rb#L452-L453

Thanks for your reply and looking into the Rails codebase.

I actually think these examples might actually benefit from a helper method to create a fake table schema. It would be clearer and if ever the minimal implementation of an activerecord schema was change, say to require an extra method, you'd have to change all these different calls instead of one.

That being said, I understand that creating test fakes is a circumstance where one might want to do Struct.new().new(), maybe the only one actually. I'm not convinced that this warrants a dedicated syntax, but maybe that's because I try my best to avoid fakes in tests.

Here is a non-test case:

https://github.com/rails/rails/blob/6fca0f31f14db4e8b73a6c1d89afd16d51e6b6f3/actionview/lib/action_view/template/types.rb#L9

Actually it's pretty trivial to find these places in the Rails codebase. git grep 'Struct.new([^)]*)\.new' will give you quite a few examples of real world code that could be improved with this syntax.

Actually I couldn't find any other non-test example in Rails than the one you provided above. I don't know how many LOC are in the lib of Rails, but if in the whole implementation there is only a single use for it, I think that this only validates that there is no need for a special syntax.

Let us remember there is a cost for a special syntax. Tooling needs to be updated (parser, rubocop, ...) and more importantly, this increases the cognitive load. In this case, the gain does not justify it.

We could settle for Struct.[]:

Struct[name: 'Joe', id: 42]  # => Struct.new(:name, :id, keyword_init: true).new(name: 'Joe', id: 42)

No new syntax required. Less of cognitive load as it is a simple shortcut. A note in the doc that this won't be particularly performant and best reserved for test fakes or static constants.

[tenderlovemaking (Aaron Patterson)]

marcandre (Marc-Andre Lafortune) wrote in #note-49:

We could settle for Struct.[]:

Struct[name: 'Joe', id: 42]  # => Struct.new(:name, :id, keyword_init: true).new(name: 'Joe', id: 42)

No new syntax required. Less of cognitive load as it is a simple shortcut. A note in the doc that this won't be particularly performant and best reserved for test fakes or static constants.

I think that's pretty reasonable. Struct.new.new is not something I would do in a hot path. But maybe that's why we don't see more of it in library code? 🤷🏻‍♀️

[mame (Yusuke Endoh)]

tenderlovemaking (Aaron Patterson) wrote in #note-50:

marcandre (Marc-Andre Lafortune) wrote in #note-49:

We could settle for Struct.[]:

Struct[name: 'Joe', id: 42]  # => Struct.new(:name, :id, keyword_init: true).new(name: 'Joe', id: 42)

No new syntax required. Less of cognitive load as it is a simple shortcut. A note in the doc that this won't be particularly performant and best reserved for test fakes or static constants.

I think that's pretty reasonable. Struct.new.new is not something I would do in a hot path. But maybe that's why we don't see more of it in library code? 🤷🏻‍♀️

As I said above, this is a bit dangerous API. Some users will write Struct[**untrusted_user_input_hash], which is vulnerable against Symbol DoS.

[marcandre (Marc-Andre Lafortune)]

mame (Yusuke Endoh) wrote in #note-51:

As I said above, this is a bit dangerous API. Some users will write Struct[**untrusted_user_input_hash], which is vulnerable against Symbol DoS.

Right, in the same way users can already do OpenStruct.new(untrusted_user_input_hash)... I should probably another entry in the docs of the caveats of that library 😅

If it's documented that it's both not performant and note that Symbol DOS is possible and should be reserved for tests it doesn't seem too bad...

I'll repeat that I'm not enthusiastic about it. It's also very trivial to implement in Ruby, it doesn't need to make it into core.

def Struct.[](**hash)
  new(*hash.keys, keyword_init: true).new(**hash)
end

[Eregon (Benoit Daloze)]

mame (Yusuke Endoh) wrote in #note-36:

Because method-name symbols are never GC'ed, so converting arbitrary external input to anonymous Struct is vulnerable against Symbol DoS.

Could that be solved?
In TruffleRuby methods don't hold to a Symbol, so I believe there is no such issue.

[ko1 (Koichi Sasada)]

Named tuple example:

On my scripting sometimes I use an array as a tuple like that:

s = [0, 0] # s is "statistics" and [A's count, B's count]
           # I want to prohibit extension of `s` (s[3] = 0), but there is no way.
data.each{|e|
  case e
  when /A's expression/
    s[0] += val1
  when /B's expression/
    s[1] += val2
  end
}

pp s

Of course, sometimes I use s = {a: 0, b: 0}, but sometimes I mistype as s[:aa] += val #=> error because of nil+val

With an anonymous struct,

s = ${a: 0, b: 0}
data.each{|e|
  case e
  when /A's expression/
    s.a += val1
  when /B's expression/
    s.b += val2
  end
}

pp s

is bit readable and writable ...? And it is useful when I want to add c and d (maybe I forget the index).

[marcandre (Marc-Andre Lafortune)]

ko1 (Koichi Sasada) wrote in #note-54:

Of course, sometimes I use s = {a: 0, b: 0}, but sometimes I mistype as s[:aa] += val #=> error because of nil+val

Seems to me that s = {a: 0, b: 0} is a good solution here. If you mistype aa, you want an error, so I don't see the issue.

FWIW, it's not clear to me why you don't simply use 2 variables.

There is nothing simpler and more efficient than a += 1

a = b = 0
# ...

pp [a, b]
# or just
p {a: a, b: b}

More importantly, the question is not whether there exist a case where one might like to use it. The question is if it's a compelling use, if it significantly improves our programming life.

[ko1 (Koichi Sasada)]

marcandre (Marc-Andre Lafortune) wrote in #note-55:

Seems to me that s = {a: 0, b: 0} is a good solution here. If you mistype aa, you want an error, so I don't see the issue.

In this case, yes. So IDE support in future? (cleaver IDE seems to inspect the hash keys, though...)

FWIW, it's not clear to me why you don't simply use 2 variables.

A set of values is easy to pass to other methods and so on (pp s on this example).

The question is if it's a compelling use, if it significantly improves our programming life.

I believe this usecase improves my life :p
At least if I can say it is a fixed set of values (only a and b) is a good reason.
Also writing s.a and s.b make me feeling good, than writing s[:a] and s[;b].
"feeling good" is enough for me, and I believe stacking "feeling good" is a history of Ruby.

Of course, if disadvantage is bigger than "feeling good", we should not introduce the feature. In this case, disadvantage is introducing new syntax will increase the language spec and complexity. Also break the compatibility with Ruby 2.7 and earlier.

Introducing new method such as Struct(a: 1, b: 2) doesn't have an issues because of new syntax, but is able to becomes vulnerability. implementation technique (for example, using method_missing like ostruct) can solve. No time to implement it before 3.0 release for me.
A method approach can reduce performance improvement chance, but it depends implementation technique.

I think there are other cases we can utilize it.
Maybe it is not used on (well-maintained) library interface because they can define a struct or a class.

---

BTW I don't think it is good way to say "significantly improves our programming life" is required to introduce/discuss about new feature.

For example, I don't think making Set literal doesn't improve our programming life significantly because I never use Set (I use Hash). But I think I may change my mind when I use it for some best fit case.

[ko1 (Koichi Sasada)]

From the aspect of "writing-well", ${a: 0, b: 0} was happy with me (but I understand some (many?) people doesn't like $), but %struct{a: 0, b: 0} is longer. Struct(a:0, b:0) is shorter and clearer...

[Eregon (Benoit Daloze)]

I think a new syntax for this is way too heavy for such a small thing.

s = Struct.new(:a, :b).new(0, 0)

is easy enough and already works.

And refactoring that to make it efficient for many instances is just giving it a name, so all trade-offs are clear.

Also almost every time I use Struct.new, I want to define an extra method. That's not possible for the feature proposed here.

Something that magically creates the class does not seem nice, because it hides the cost of class creation (which is high, but doesn't matter if just used a couple times).

No matter the syntax, if we add a new way and we provide caching (members => StructClass), it should be a weak cache, so that if there are no instances of that particular combination of members anymore, and no method which can create it (e.g., if used in top-level code which can GC very quickly), we can GC such unnecessary classes.

Given that constraint I think Struct(a: 0, b: 0) is the nicer of the 3 in #57. But I think the existing Struct.new(members).new is enough.

[shan (Shannon Skipper)]

A slick syntax like ${a: 0, b: 0} would encourage many to use it when they otherwise might turn to a Hash literal. Performance aside, a dollar sign Struct literal looks relatively lovely.

[briankung (Brian Kung)]

duerst (Martin Dürst) wrote in #note-43:

One more point: I haven't seen much examples of similar features in other languages.

Sorry to resurrect an old thread, but I just wanted to mention that this is a feature of the Zig programming language: https://ziglang.org/documentation/0.6.0/#Anonymous-Struct-Literals I think it's quite innovative, but it's true that it's not a common language feature.

[Bumppoman (Brendon Stanton)]

In a Ruby 3.2 context, I wonder how this would look in the mindset of the new Data class instead of a struct?

[matz (Yukihiro Matsumoto)]

If I have to pick one from either Struct or Data, I'd pick Data. But still don't like the notation proposed.

Matz.

[shyouhei (Shyouhei Urabe)]

JFYI JavaScript is having a similar proposal which now proposes #{…} syntax.
https://github.com/tc39/proposal-record-tuple

# is not a comment marker in that language though.

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If I have to pick one from either Struct or Data, I'd pick Data. But still don't like the notation proposed.

What about something like:

%t[a: 1, b: 2]

for Struct (can default to frozen/immutable Struct or use %T to create it frozen) and

%d[a: 1, b: 2]

for Data (already immutable). An idea based on %w, %i, etc.

Btw, Happy New Year, people. :-)

[hsbt (Hiroshi SHIBATA)]

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[hsbt (Hiroshi SHIBATA)]

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[hsbt (Hiroshi SHIBATA)]

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