This gem is come from Promising

A glimpse of a promising future in which Ruby supports lazy evaluation.

Promises and futures both transparently defer the execution of a block. Promises evaluate the given block if and when its result is first needed. Futures evaluate the given block optimistically in another thread.

require 'promising'
require 'future'    # you can just require 'future' if using both

x = promise { 1 + 2 }
y = future  { sleep 10 && 6 * 7 }

puts x      #=> 3
sleep 5     # ... do work for 5 seconds ...
puts y      #=> 42, after blocking 5 seconds

Note that this is pretty useless in Ruby's interactive shell irb, as it will eagerly evaluate everything as part of its read-eval-print loop, forcing promises and futures to yield their results.

The library has been tested with Ruby 1.8.7, 1.9.1 and JRuby 1.5.0. YARD documentation is available at http://promising.rubyforge.org/

• {Promise}
• {Future}

The library is distributed via RubyGems:

$ gem install promising

The source is available at http://github.com/fsword/promising

Fsword

Promising Future is free and unencumbered public domain software. For more information, see http://unlicense.org/ or the accompanying UNLICENSE file.