proc_usage.rb

# source: https://openhome.cc/Gossip/Ruby/Proc.html
#         https://openhome.cc/Gossip/Ruby/IteratorBlock.html
#         https://tonytonyjan.net/2011/08/12/ruby-block-proc-lambda/

# self-built reduce function
class Array
  def my_reduce(value = 0)
    for i in 0...self.length
      value = yield(value, self[i])
    end
    value
  end
end


# each do vs each {}：
# normal 
puts [1, 2, 3].each { |element| print element }

# nothing printed, because it is received as 'puts ([1, 2, 3].each) do ...'
puts [1, 2, 3].each do |element|
  print element
end


# 
def def_without_block_in_param
  yield if block_given? # not adding condition will cause 'LocalJumpError: no block given (yield)' when no block is given.
end

def def_with_amp(&block)
  block.call  # 'yield' works as well
end

def def_without_amp(block)
  block.call
end

# proper usage 
b = Proc.new { puts 'block here' }

def_with_amp(&b)
def_with_amp(&proc { puts 'block here' }) # proc == Proc.new
def_with_amp { puts 'block here' }
def_without_amp(b)

# 實際上，& 會觸發物件的 to_proc 方法
# 例如 Symbol 類別有直接定義 to_proc 方法:
["amazing_proc"].each { |name| name.capitalize! }
== :capitalize.to_proc.call("amazing_proc")  # 先將 capitalize 變成 symbol, 再呼叫 to_proc 方法
== ["amazing_proc"].each(&:capitalize!)

# 如果不使用 symbol 的 to_proc 方法，可以在物件上面自己定義 to_proc，然後一樣用 & 來觸發
class Ball
  attr_reader :radius

  def initialize(radius)
    @radius = radiu
  end

  def self.to_proc
    Proc.new { |ball| ball.radius }
  end
end

print [Ball.new(10), Ball.new(20), Ball.new(30)].collect(&Ball) # [10, 20, 30]

# Symbol 中的 to_proc 設計
class Symbol
  def to_proc
      Proc.new { |o| o.send(self) }
  end
end

# Compare Proc with Lambda
def foo
  f = Proc.new { return "return from foo from inside proc" }
  f.call # control leaves foo here
  return "return from foo"
end

def bar
  f = lambda { return "return from lambda" }
  f.call # control does not leave bar here
  return "return from bar"
end

puts foo # prints "return from foo from inside proc"
puts bar # prints "return from bar"