Double Splat operator

分享两篇文章，介绍Splat operator和Ruby2.0引入的Double Splat operator.

Splat goes Ruby
Drat! - Ruby has a Double Splat

Splat的用法：

• 使用在方法参数中，转化list为Array

def ahoy(*args)
  p args
end

ahoy :a, 345, hello: :world # => [:a, 345, {:hello=>:world}]

• 将Array参数转化为list

def ahoy(from, to)
  puts "#{to.capitalize}, #{from.capitalize} says ahoy!"
end

even_stephens = %w(steven stephen)
ahoy even_stephens # Array is interpreted as the first argument, and to_matey won’t be set. An ArgumentError is raised.

ahoy *even_stephens # Array is reversed to a list and all the arguments are filled out.
# => Stephen, Steven says ahoy!

• splat后面不能有required参数

def hello(name, *args, options, &block)
  p name
  p args
  p options
  p block
end

hello('denny', :a, :b, upcase: true) { 'block this!' }
# name: ”denny”
# args: [:a, :b]
# options: {:upcase=>true}
# => #<Proc:0x007fc1a10afd60>

# Works fine:
def hello(name = nil, *args)

# Throws SyntaxError:
def hello(*args, name = nil)

# Using several splats in a definition SyntaxErrors too:
def hello(a, *args, b, *brgs)

• 可以在yield参数里

ActiveSupport::Notifications.subscribe('render') do |name, start, finish, id, payload|
  puts payload
end

ActiveSupport::Notifications.subscribe('render') do |*, payload|
  puts payload
end

# It's also easier than:
ActiveSupport::Notifications.subscribe('render') do |*args|
  payload = args.last
  puts payload
end

• 赋值时的使用

a, b = [:a, :b]
a # => :a
b # => :b

a, b = [:a, :b, :c] # :c is lost
a # => :a
b # => :b

a, *rest = [:a, :b, :c]
a # => :a
rest # => [:b, :c]

a, *= [:a, :b, :c]
a # => :a

a ,= [:a, :b, :c]
a # => :a

• 如果不关心这个参数就不需要为这个参数命名

class WhipperSnapper
  def initialize(snap_count, whip_size)
    @snap_count, @whip_size = snap_count, whip_size
  end
end

class SupremeSnapper < WhipperSnapper
  def initialize(*)
    super
    @agility = 100_000
  end
end

Double Splat的用法

• Obligatorily Optional(如何翻译？)

def hello(**options)
  p options
end

hello
# options: {}

hello name: 'Kasper'
# options: { :name => 'Kasper' }

类似于optional arguments

def hello(options = {})
end

• Hash

def hello(name = nil, **options)
  p name
  p options
end

hello 'Kasper'
# name: 'Kasper'
# options: {}

hello upcase: true
# name: nil
# options: { :upcase => true }

**之前的参数必须是可选的！

def hello(name, **options)
  p name
  p options
end

hello upcase: true
# name: { :upcase => true }
# options: {}

• Off Key Splat
  使用了double splats 之后可以传入或多或少的keys

def hello(name:)
  p name
end

hello # raises "ArgumentError: missing keyword: name" as expected

hello name: 'Kasper', play_style: :laces_out 
# raises "ArgumentError: unknown keyword: play_style"

def hello(name:, **options)
  p name
  p options
end

hello name: 'Kasper', play_style: :laces_out
# name: 'Kasper'
# options: { :play_style => :laces_out }

hello play_style: :laces_out
# ArgumentError: missing keyword: name

# 加入默认值后

def hello(name: nil, **options)
  p name
  p options
end

hello play_style: :laces_out
# nil
# {:play_style=>:laces_out}

• Exploit a Splat, Expand a Hash
  可以展开一个hash

options = { a: 'b' }
{ c: 'd', **options } # => { :c => "d", :a => "b" }

*可以展开一个数组，两个用法还是挺接近的。

a=[1,3]
# => [1, 3]
[2, *a]
# => [2, 1, 3]

• Yield to Splat
  之前可以这么写

def hello
  yield name: 'Kasper'
end

hello { |options| p options[:name] }
# Outputs "Kasper"

可以简写

hello { |name:| p name }
# Outputs "Kasper"

def hello
  yield name: 'Kasper', play_style: :laces_out
end

hello { |name:, **| p name }
# Outputs "Kasper"