I should explain a little more. You never actually see the lambda method, the RewriteRails plugin works a little like Lisp's macros. You write: returning [

I should explain a little more. You never actually see the lambda method, the RewriteRails plugin works a little like Lisp's macros. You write: returning [1] do |numbers| numbers << 2 numbers += [3] end And that's all you see in your source code. RewriteRails rewrites that into: lambda do |numbers| numbers << 2 numbers += [3] numbers end.call([1]) And that's what the ruby interpreter sees. But you need never look at the "ugly" rewritten version, just what you wrote.