ReactiveX/Rx.rb

require 'rx'

#  Using an observable sequence
source = Rx::Observable.defer {
    Rx::Observable.return(42)
}

subscription = source.subscribe(
    lambda {|x|
        puts 'Next: ' + x.to_s
    },
    lambda {|err|
        puts 'Error: ' + err.to_s
    },
    lambda {
        puts 'Completed'
    })

# => Next: 42
# => Completed