From: Russ McBride <russ@...>
Date: 2009-07-05T09:50:55+09:00
Subject: to_proc and Proc/block conversion with &


map requires a block, right?  So this works:

   words = %w(ardvark   anteater   llama)

   arr = words.map { |x| x + "!" }
   arr     #--> [ardvark!, anteater!, llama!]

But this doesn't:

   arr2 = words.map lambda{ |x|  x + "!"}       #--> ArgumentError:  
wrong number of arguments (1 for 0)


We can trigger a call to some object's to_proc method with the  
ampersand.  For example, this
will trigger the symbol's to_proc method, assuming there is one:

   upcase_words = words.map(&:upcase)

and if we have already have this:

   class Symbol
     def to_proc
       lambda{ |x| x.send(self) }
     end
   end

then we get:
   #--> [ARDVARK, ANTEATER, LLAMA]

But, the to_proc method here is returning a Proc, not a block!  So why  
does this work?
As far as I can tell, the ampersand must be doing 2 things:
1- triggering the call to to_proc
2- converting the returned Proc object into a block for map to handle.

Is this right?