Feature #11925: Struct construction with kwargs

• Status: Closed
• Assignee: -
• Target version: -

[prijutme4ty (Ilya Vorontsov)]

Propose to make Struct subclass constructors which accept keyword arguments. Not sure, if it's reasonable to allow .new accept kwargs, so may be should use different method named like .create:

  Point = Struct.new(:x,:y, :color)
  pt_1 = Point.create(x: 1, y: 2) # => Point<x: 1, y: 2, color: nil>
  pt_2 = Point.create!(x: 1, y: 2) # => ArgumentError, color not specified.

It will greatly simplify work with big structures, especially in cases when struct layout changes and for cases when structure can have lots of non-significant values. It also allows simpler ways to use implement default values for struct members.

[ksss (Yuki Kurihara)]

Hi.

I have thought the same thing that want to use kwargs in Struct class.

But I can't come up with a good API.

Finally, I made a gem. https://github.com/ksss/type_struct

[naruse (Yui NARUSE)]

• Assignee deleted (core)

[shyouhei (Shyouhei Urabe)]

We looked at this issue in yesterday's developer meeting. Nobody there was against the functionality -- but the name. create! doesn't sound appropriate at all. create also not that obvious for non-English speakers like us that it expects keywords.

[herwinw (Herwin Quarantainenet)]

What about new_from_kwargs(**kwargs) ? It's a bit long, but it describes the functionality exactly.

[herwin (Herwin W)]

https://github.com/ruby/ruby/pull/1468

A proposal for an implementation.

irb(main):001:0> MyClass = Struct.new(:a, :b, :c)
=> MyClass
irb(main):002:0> MyClass.new_from_kwargs(a:1, c: 3)
=> #<struct MyClass a=1, b=nil, c=3>
irb(main):003:0> MyClass.new_from_kwargs(1, 2, 3, b: 3)
=> #<struct MyClass a=1, b=3, c=3>
irb(main):004:0> MyClass.new_from_kwargs(d: 4)
NameError: no member 'd' in struct
	from (irb):4:in `new_from_kwargs'
	from (irb):4
	from ./irb:11:in `<main>'
irb(main):005:0> MyClass.new_from_kwargs(1, 2, 4, 5, b: 3)
IndexError: offset 3 too large for struct(size:3)
	from (irb):5:in `new_from_kwargs'
	from (irb):5
	from ./irb:11:in `<main>'
irb(main):006:0> 

[nobu (Nobuyoshi Nakada)]

Herwin W wrote:

irb(main):003:0> MyClass.new_from_kwargs(1, 2, 3, b: 3)
=> #<struct MyClass a=1, b=3, c=3>

Why does new_from_kwargs accept other than keyword arguments?

[herwin (Herwin W)]

To be prepared for "the great unification of constructors" of course.

It looked like a pretty logical step to support while I was coding this. It's also pretty easy to remove again

[shyouhei (Shyouhei Urabe)]

• Has duplicate Feature #9209: Struct instances creatable with named args added

[ko1 (Koichi Sasada)]

Another idea is introducing another method to define own struct, such as T = Struct.define(:a, :b); T.new(a: 1, b: 2) and so on.
(just idea) Moreover we can extend Struct with some properties, like: Struct.define(:a, b: :read_only).

These ideas are provided by another person.

[nobu (Nobuyoshi Nakada)]

• Has duplicate Feature #13272: Keyword argument to instantiate a subclass of Struct added

[k0kubun (Takashi Kokubun)]

Similar to one commented by ko1, how about this interface?

T = Struct.new(:a, :b, keyword_argument: true)
T.new(a: 1, b: 2)

As keyword_argument is long, another option is:

Struct.new(:a, :b, keyword_args: true)

[k0kubun (Takashi Kokubun)]

In case that my suggestion of the name is accepted, I wrote a patch for Struct.new(:a, :b, keyword_args: true).
https://github.com/ruby/ruby/pull/1771

[knu (Akinori MUSHA)]

What if Struct.new([:a, :b]) created a class with the desired constructor?

[matz (Yukihiro Matsumoto)]

I vote for the keyword argument (e.g. keyword_init:) to Struct#new.

Matz.

[herwin (Herwin W)]

knu (Akinori MUSHA) wrote:

What if Struct.new([:a, :b]) created a class with the desired constructor?

If you'd compare the two possible constructors:

Struct.new(:a, :b)
Struct.new([:a, :b])

There is nothing in the second one that would indicate the second creates a keyword constructor. If I hadn't read this discussion, I would just expect them to behave the same.

[k0kubun (Takashi Kokubun)]

• Status changed from Open to Closed

Applied in changeset trunk|r61137.

---

struct.c: add keyword_init option to Struct.new

to initialize struct with keyword arguments.

[Feature #11925] [close GH-1771]

[k0kubun (Takashi Kokubun)]

As Matz approved, I committed only keyword_init option which is equivalent to "Point.create(x: 1, y: 2)" in original suggestion. If you still want "Point.create!" version which raises ArgumentError (keyword_init initializes unspecified fields with nil), please file another ticket.

[nobu (Nobuyoshi Nakada)]

• Related to Feature #15076: Struct to raise error when keyword arguments used but not enabled added

[k0kubun (Takashi Kokubun)]

• Related to Feature #15222: Add a way to distinguish between Struct classes with and without keyword initializer added

[k0kubun (Takashi Kokubun)]

• Related to Feature #16806: Struct#initialize accepts keyword arguments too by default added