proc

options = { transform: -> s {"<#{s}>"}, upcase: true }

filters = []
if options[:upcase]
  filters << -> string { string.upcase }
end
if options[:transform]
  filters << options[:transform] # callback function
end

results = %w(foo bar)
filters.each do |filter|
  results = results.map(&filter)
end

puts results # <FOO>
             # <BAR>