What is the explanation for &:hex in ruby? [duplic

Question

This question already has an answer here:

• What does map(&:name) mean in Ruby? 15 answers
• What do you call the &: operator in Ruby? [duplicate] 1 answer

I am trying to figure out what &:hex mean in this code

    sort_by{|x|x.scan(/\d*/).map &:hex}

The full code looks like this

 class Array
  def version_sort
   sort_by{|x|x.scan(/\d*/).map &:hex}
  end
 end

I know that map does an action to the scanned part , so I am guessing it replaces numbers

 (/\d*/) 

with

&:hex

but I dont know what that means

Answer 1

These two expressions are equivalent:

foo.map {|x| x.hex}
foo.map &:hex

Answer 2

In this case & takes an object and if the object is not already a Proc, as it is the case with the Symbol :hex it will try to invoke the method to_proc on it. In the Symbol documentation you will find the implementation detail for the to_proc method:

to_proc

Returns a Proc object which respond to the given method by sym.

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

In your case through &:hex the symbol :hex will be converted into the Proc object which is equivalent to { |item| item.hex() }

What is a Proc exactly? Basically the Proc class is a basic anonymous function. In Ruby the notion of a callable object is embodied in Ruby through objects to which you can send the message call. The main representatives of these kind are Proc and Lambda.

Proc objects are self-contained code sequences, that can be created, stored, passed around as method arguments and executed at some point through call. A method like map can also take a block as an argument which is the case if you pass &:hex.

In the method definition of map a kind of implicit call to Proc.new is made using the very same block. Then the Proc is executed through its call method executing the code embodied by the Proc object.