Ruby Lambda Coalesce

lambda_coalesce.rb

#
# Given a bit of program logic:
#
#  x2 = y1 * x1 || y2 / exchange_rate || x2 * exchange_rate / y1 || another
#
#  Where y1, x1, y2 and x2 are function calls, which may yield nils
# 

def coalesce(*args)

  # each arg is a value or a lambda
  # keep going until there is a "true" value, 
  # ignoring any errors when calling the lambdas
  # the last arg is the default return (or sentinal)

  default = args.rpop

  args.each do |arg|
    value = arg.respond_to?(:call) ? arg.call rescue nil : arg
    return value unless value
  end

  default.respond_to?(:call) ? default.call : default

end

# now, a more elegant way to express the logic:

x2 = coalesce
  -> { y1 * x1 },
  -> { y2 / exchange_rate },
  -> { x2 * exchange_rate },
  -> { another },
  default