what happened when pass a method to iterator method

Question

As we know, wo can pass a method to a iterator method by a &: prefix.
For example:

[\"a\", \"b\"].map(&:upcase) #=> [\"A\", \"B         


        

Answer 1

Here's what works. Explanation below.

class String
  def rettwo
    self + self
  end
end

def a_simple_method &proc
  proc.call('a')
end

def a_iterator_method
  yield 'b'
end

a_simple_method(&:rettwo) # => "aa"
a_iterator_method(&:rettwo) # => "bb"

The &: construct is called Symbol#to_proc. It turns symbol into a proc. This proc expects a receiver as a first argument. The remaining arguments are used to call the proc. You're not passing any arguments, hence the "receiver not given" error.

Here's a demonstration of additional arguments:

class String
  def say name
    "#{self} #{name}"
  end
end

def a_simple_method &proc
  proc.call('hello', 'ruby')
end


a_simple_method(&:say) # => "hello ruby"

Here's a definition of Symbol#to_proc from some blog post from 2008. Modern Symbol#to_proc seems to be implemented in C, but this can still help the understanding.

class Symbol
  def to_proc
    Proc.new { |*args| args.shift.__send__(self, *args) }
  end
end