Harness the power of functions to build composable rack applications by Marc Busqué What's a function? A function is a black box that takes an input and returns an output. Similarly, from the outside, HTTP requests take some request data to give it back as a response. Functions may compose if the output of one matches the input of the next one. web_pipe helps you build rack applications by plugging small process units that progressively create a response from a given request.