Ruby Proc Syntax

Question

An answer to a question I posed yesterday on here was the following piece of Ruby code:

def overlap?(r1,r2)
  r1.include?(r2.begin) || r2.include?(r1.begin)
end

def any_overlap?(ranges)
  ranges.sort_by(&:begin).each_cons(2).any? do |r1,r2|
  overlap?(r1, r2)
  end
end

I get each_cons, but what's the strange &:begin notation? Save me from syntax hell!

Thanks!

Answer 1

When you prefix the last argument of a call with & you are making clear that you are sending a block and not a normal argument. Ok, in method(&:something), :something is a symbol, not a proc, so Ruby automatically calls the method to_proc to get a real block. And Rails guys (and now also vanilla Ruby) cleverly defined it as:

class Symbol
  def to_proc
    proc { |obj, *args| obj.send(self, *args) }
  end
end

That's why you can do:

>> [1, 2, 3].map(&:to_s) # instead of [1, 2, 3].map { |n| n.to_s }
=> ["1", "2", "3"]

[edit] Note: when you realize that this construction is no syntatic sugar but generic infrastructure that Ruby provides, nothing stops you from implementing your own to_proc for other classes. Never felt limited because &:method allowed no arguments?

class Array
  def to_proc
    proc { |obj, *args| obj.send(*(self + args)) }
  end
end

>> ["1", "F", "FF"].map(&[:to_i, 16])
=> [1, 15, 255]