重写 Enum#inject 以在 Ruby 中将符号作为参数

内置的inject非常多态，所以在try 从头开始实现它之前(不查看源代码) 我们需要探索它在不同情况下的表现.我跳过您已经知道的内容(例如，将第一个元素用作 初始值(如果未明确提供等)，除此之外:

[1,2,3].inject(0, :+, :foo) #=> ArgumentError: wrong number of arguments (given 3, expected 0..2)
# Max. arity is strict

[1,2,3].inject(0, :+) { 1 } #=> 6
# If both symbol _and_ block are provided, the former dominates

[1,2,3].inject(:+) { |acc, x| acc } #=> :+
# With only 1 parameter and a block the former will be treated as an init value, not a proc.

[1,2,3].inject("+") #=> 6
[1,2,3].inject("+") { |acc, x| acc } #=> "+"
# Strings work too. This is important, because strings _don't respond to `to_proc`_, so we would need smth. else

[1,2,3].inject #=> LocalJumpError: no block given
# Ok, local jump error means that we try to yield in the cases where the 2nd parameter is not provided

[1,2,3].inject(nil) #=> TypeError: nil is not a symbol nor a string
# But if it is provided, we try to do with it something that quacks pretty much like `send`...

考虑到这些内容，我们可以编写如下内容

module Enum
  # Just for convenience, you don't need it if you implement your own `each`
  include Enumerable

  def my_inject(acc = nil, sym = nil)
    # With a single attribute and no block we assume that the init value is in fact nil
    # and what is provided should be "called" on elements somehow
    if acc && !sym && !block_given?
      sym, acc = acc, nil
    end
  
    each do |element|
      if !acc
        # If we are not initialized yet, we just assign an element to our accumulator
        # and proceed
        acc = element
      elsif sym
        # If the "symbol" was provided explicitly (or resolved as such in a single parameter case)
        # we try to call the appropriate method on the accumulator. 
        acc = acc.send(sym, element)
      else
        # Otherwise just try to yield
        acc = yield acc, element
      end
    end
    
    acc
  end
end

听我说，我们快到了:)让我们来看看它是如何嘎嘎作响的:

class Ary < Array
  include Enum
end

ary = Ary.new.push(*[1,2,3])

ary.my_inject #=> LocalJumpError: no block given
ary.my_inject(0) #=> TypeError: 0 is not a symbol nor a string
ary.my_inject("+") #=> 6
ary.my_inject(:+) #=> 6
ary.my_inject(0, :+) #=> 6
ary.my_inject(1, :+) #=> 7
ary.my_inject(1, :+) { 1 } #=> 7
ary.my_inject(:+) { |acc, x| acc } #=> :+

因此，情况大致相同.可能还有其他一些我的实现不能满足的边缘情况，但我把它们留给您:)