Feature #14973: Proposal of percent literal to expand Hash

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

[osyo (manga osyo)]

概要

変数名から { 変数名: 変数の値 } という Hash を定義するための %記法の提案です。
以前からちょくちょく提案されていた

x = 1
y = 2
h = {x:, y:}
p h #=> {:x=>1, :y=>2}

のような ES6 ライクな構文を {} 構文ではなくて %記法で定義するものになります。

仕様

hoge = 1
foo = 2
bar = 3

# スペース区切りの変数名をキー、変数の値を Hash の値として展開する
%h(hoge foo bar) # => { hoge: 1, foo: 2, bar: 3 }

これは以下と同等の処理になります。

hoge = 1
foo = 2
bar = 3

{ hoge: eval("hoge"), foo: eval("foo"), bar: eval("bar") } # => { hoge: 1, foo: 2, bar: 3 }

ローカル変数以外

内部で eval を使用しているので、そのコンテキストで評価できればローカル変数以外も使用することが出来ます。

def meth
	"meth"
end

@hoge = 42
Foo = "str"

p %h(meth @hoge Foo $stdout)
# => {:meth=>"meth", :@hoge=>42, :Foo=>"str", :$stdout=>#<IO:<STDOUT>>}

キーは変数名そのままです（$ や @ がついたまま。

重複したキーがある場合

キーが重複している場合、{} と同様に2回目以降は無視されます。

hoge = 42
foo = "str"

%h(hoge foo hoge)
# => {:hoge=>42, :foo=>"str"}

キーの変数が存在しない場合

hoge = 42
foo = "str"

%h(hoge foo bar)
# Error: undefined local variable or method `bar' for main:Object (NameError)

Hash 内で展開

Hash なので ** で展開できます。

hoge = 42
foo = "str"

{ bar: "bar", **%h(hoge foo) }
# => {:bar=>"bar", :hoge=>42, :foo=>"str"}

式展開

%I などと同様に式展開を行います。

hoge = 42
foo = "hoge"

%h(#{foo})
# => {:hoge=>42}

ユースケース

キーワード引数に渡す

Model = Struct.new(:tag, :name, :age, keyword_init: true)

def create_human(name:, age:)
	# ** で Hash を展開して渡す
	Model.new(tag: :human, **%h(name age))
	# Model.new(tag: :human, name: name, age: age)
end

name = "mami"
age = 15
# 変数をまとめて渡す
create_human(%h(name age)) # => #<struct Model tag=:human, name="mami", age=15>
# create_human(name: name, age: age)

デバッグ出力として利用する

class Hash
	# 適当なデバッグ出力用メソッド
	def debug_output
		each { |key, value|
			puts "#{key}: #{value}"
		}
	end
end

def create(name, age)
	# 引数を確認するためのデバッグ出力
	%h(name age).debug_output
end

name = "homu"
age = 14
create(name, age)
# output:
# name: homu
# age: 14

その他

• キーは Symbol で定義 → String 版もあったほうがよいだろうか
• 式展開のみ実装 → %i みたいな式展開を行わないでほしいユースケースが思いつかなかった
• なぜ %h という名前？ → Hash だから…

以上、任意の変数名から Hash を展開する %h 記法の提案でした。
ご意見等あればコメント頂けると助かります。

関連するチケット

https://bugs.ruby-lang.org/issues/11105
https://bugs.ruby-lang.org/issues/14579
https://bugs.ruby-lang.org/issues/13137

[shevegen (Robert A. Heiler)]

I can not read japanese but I believe I can understand part of this
proposal (the ruby code examples).

If I understood the examples correctly then one part of the proposal
is to add support for:

%h( ) 

(Possibly meaning to use a hash, and expanding the variables used
within itself, like in the example:)

%h( hoge foo bar )

If this is the case then I think this is a very interesting suggestion.
(I can't say much about the other examples given, so my comment
refers primarily to %h() alone and local variables. I have no particularly
strong opinion about using instance variables, constants or global
variables there - I think local variables alone are a quite interesting
example; it seems to fit to other shortcuts, such as %w() for constructing
Arrays ).

In one of the older linked in issues e. g. the one by Shugo Maeda, nobu
wrote that there were conflicts with the syntax used in the suggestion. I
do not know if this is the case here (if I understood it correctly,
the example here is not the same, e. g. uses another syntax).

It may be useful to see if there are any problems with the syntax
here; and if not whether this may have a chance to be implemented.

Since this keeps coming up in one way or another, even with different
syntax, it may be useful to discuss this eventually at a developer
meeting.

Personally I like the idea behind the suggestion here made by @osyo (manga osyo).

[knu (Akinori MUSHA)]

For debugging purposes, you could have a method like this:

class Binding
  def dump(*syms)
    pp syms.lazy.map { |sym| [sym, local_variable_get(sym)] }.to_h
  end
end

a = 1
b = 2
binding.dump(:a, :b) # prints "{:a=>1, :b=>2}"

[blakewest (Blake West)]

I just want to +1 this. I think it's a great idea, and had raised it independently in 15236. It doesn't seem like Matz has given an opinion on it though either here or in 15236. Can we bump this to see what his thoughts are? What's the best way to do that? Thanks - Blake

[hsbt (Hiroshi SHIBATA)]

• Status changed from Open to Closed

https://bugs.ruby-lang.org/issues/14579 was accepted and added for Ruby 3.1.

We don't need to % literal now.

[hsbt (Hiroshi SHIBATA)]

• Related to Feature #14579: Hash value omission added