Question

In the code below, I read from a redis queue and trigger callbacks on a local deferrable variable d when the redis operation completes.

It works fine, but I expected to get an error because when the method f returns the variable d goes out of scope. Or does it?

why does this work? is it because the blocks that use d create closures and these closures keep d alive?

require 'em-hiredis'

def f
  d = EM::DefaultDeferrable.new

  EM::Hiredis.connect.blpop('abcdef', 5).
    callback {|x| d.succeed x}.
    errback {|e| d.fail e}

  d.
    callback {|x| puts "value=#{x}"}.
    errback {|e| puts "error #{e}"}

  return nil
end

EM.run {f}

EDIT: Please notice that the EM::Hiredis instance is local too, and the blocks are attached to it. Are not the blocks supposed to go away with the redis connection when f returns?

Answer 1

Ruby blocks are interesting, because they include a binding to the scope they are defined within.

This means that your deferrable is bound within the block and available to the block, even when the block is executed elsewhere/later.