Insta-block with Symbol#to_proc

    Tim Lucas

    Something that might have slipped your radar in your Ruby readings is Symbol#to_proc, something that’s been in Rails since 0.14.4.

    Using this nice little tidbit of Ruby tricky you can simplify code such as:

    articles.collect { |a| a.title }
    articles.sort { |a| a.created_at }



    …and just in case you’re wondering why &:ruby_is_the_new_black doesn’t work in the beautiful ruby shell script you just wrote, it’s because Symbol#to_proc isn’t a standard Ruby thang. To quote Mauricio Fernandez who noted it’s recent inclusion in the upcoming Ruby 1.9:

    Symbol#to_proc: first discovered by some anonymous Japanese Rubyist (fairly certain), rediscovered by Florian Groß (I saw that happen back when I wasted my time on #ruby-lang), then popula rized by Pragdave and Rails, finally adopted officially. It’s been a long journey.

    “That’s nice,” you say, “but I still don’t really understand what all this Proc and block stuff is about.” Well, let me indulge you in some Ruby block mumblings.

    I think the best way to describe a block if you’ve come from web-design-land is to think of it like an anonymous function in Javascript.

    For example, the following piece of Ruby:

    socceroos.each do |s|

    is the equivalent to the following Javascript:

    socceroos.each(function(s) {

    The block passed to a function is signified by an ampersand:

    class Team
      def each(&block)
        for player in @players


    In fact, the above pattern of calling the block is so common Ruby takes it one step further, allowing you to simply write:

    class MyTeam
      def each
        for member in @members
          yield member

    yield member is another way of saying “call the associated block passing it the member”. You don’t even need to add &block to the parameter list.

    You would have seen this pattern when dealing with ActiveRecord. The ActiveRecord::Base class allows you to specify a block when creating new instances. For example, if Player was an ActiveRecord subclass you could do the following:

    @player = do |p| = "Ronaldo"
      p.nickname = "Porky"

    How does ActiveRecord provide this neat API? Easy! It just checks whether you specified a block and, if so, calls the block, passing the new Player object to it.

    The bare minimum logic to accomplish this would be:

    class ActiveRecord::Base
      def initialize
        yield self if block_given?