Functional equivalent of decorator pattern?

Question

What is the functional programming equivalent of the decorator design pattern?

For example, how would you write this particular example in a functional style?

Answer 1

Here's an example using JSGI, a web server API for JavaScript:

function Log(app) {
    return function(request) {
         var response = app(request);
         console.log(request.headers.host, request.path, response.status);
         return response;
     };
 }

 var app = Logger(function(request) {
     return {
         status: 200,
         headers: { "Content-Type": "text/plain" },
         body: ["hello world"]
     };
  }

Compliant middleware can be stacked, of course (e.x. Lint(Logger(ContentLength(app))))