What does to_proc method mean in Ruby?

Question

I am learning rails and following this thread. I am stuck with the to_proc method. I consider symbols only as alternatives to strings (they are like strings but cheaper in terms of memory). If there is anything else I am missing for symbols, then please tell me. Please explain in a simple way what to_proc means and what it is used for.

Answer 1

Some methods take a block, and this pattern frequently appears for a block:

{|x| x.foo}

and people would like to write that in a more concise way. In order to do that they use a combination of: a symbol, the method Symbol#to_proc, implicit class casting, and & operator. If you put & in front of a Proc instance in the argument position, that will be interpreted as a block. If you combine something other than a Proc instance with &, then implicit class casting will try to convert that to a Proc instance using to_proc method defined on that object if there is any. In case of a Symbol instance, to_proc works in this way:

:foo.to_proc # => ->x{x.foo}

For example, suppose you write:

bar(&:foo)

The & operator is combined with :foo, which is not a Proc instance, so implicit class cast applies Symbol#to_proc to it, which gives ->x{x.foo}. The & now applies to this and is interpreted as a block, which gives:

bar{|x| x.foo}

Answer 2

The easiest way to explain this is with some examples.

(1..3).collect(&:to_s)  #=> ["1", "2", "3"]

Is the same as:

(1..3).collect {|num| num.to_s}  #=> ["1", "2", "3"]

and

[1,2,3].collect(&:succ)  #=> [2, 3, 4]

Is the same as:

[1,2,3].collect {|num| num.succ}  #=> [2, 3, 4]

to_proc returns a Proc object which responds to the given method by symbol. So in the third case, the array [1,2,3] calls its collect method and. succ is method defined by class Integer. So this parameter is a short hand way of saying collect each element in the array and return its successor and from that create a new array which results in [2,3,4]. The symbol :succ is being converted to a Proc object so it call the Array's succ method.

Answer 3

For me the clearest explanation is seeing a simple implementation of it. Here's what it might look like if I were reimplementing Symbol#to_proc:

class Symbol  # reopen Symbol class to reimplement to_proc method
  def to_proc
    ->(object) { object.send(self) }
  end
end

my_lambda = :to_s.to_proc

puts my_lambda.(1)  # prints '1'; .() does the same thing as .call()
puts my_lambda.(1).class  # prints 'String'

puts [4,5,6].map(&:to_s)  # prints "4\n5\n6\n"
puts [4,5,6].map(&:to_s).first.class  # prints 'String'

Answer 4

For anybody still a bit stumped, running the following code might make things a little clearer:

class Symbol
  def to_proc
    proc do |obj|
      puts "Symbol proc: #{obj}.send(:#{self})"
      obj.send(self)
    end
  end
end

class Array
  def map(&block)
    copy = self.class.new
    self.each do |index|
      puts "Array.map:   copy << block.call(#{index})"
      copy << block.call(index)
    end
    copy
  end
end

remapped_array = [0, 1, 2].map &:to_s
puts "remapped array: #{remapped_array.inspect}"

These are not the actual implementations of Symbol.to_proc or Array.map, they are just simplified versions which I'm using to demonstrate how map &:to_s and similar calls work.