In Racket 5.0.99.4 and before, the Web Server supported implicit
conversion of X-expressions and lists with the format (cons/c bytes?
(listof (or/c string? bytes?))) into response data structures for output.

After 5.0.99.4, this implicit conversion has been generalized into
current-response/c and response/c. In the process, implicit conversion
has been completely removed from some internal plumbing AND the
response structures have been streamlined---primarily for efficiency.

This document describes the incompatible changes and how to restore
the old behavior when that is possible.

--- Coercion behavior ---

The old coercion behavior will be dynamically introduced by requiring
web-server/compat/0/coerce or parameterizing current-response/c to
(coerce/c normalize-response) after requiring
web-server/compat/0/http/response-structs.

Since Xexpr conversion is so convenient, xexpr-response/c is now
provided by http/xexpr for use with current-response/c.

--- New response structures ---

The http/response-structs module has changed to be almost
unrecognizable. http/response-structs was provided through http and
servlet, so many Web applications implicitly rely on it.

web-server/compat/0/http/response-structs provides a version that
implements the old behavior without interfering with new version
(i.e., they can both be required simultaneously.) [N.B. response/port
was only present for a few weeks, so it is not provided in the
compatibility library.]

--- xexpr-response/cookies ---

The http/cookie module provided xexpr-response/cookies, but that
functionality is now part of response/xexpr. The old function can be
required from web-server/compat/0/http/cookie (without interfering
with any other bindings from the new cookie module.)

--- Internal APIs ---

Many internal APIs are restricted to receive only actual response?
structures:

configuration-table responders
authentication responder on dispatchers/dispatch-passwords
dispatchers/dispatch-lift --- make
dispatchers/dispatch-pathprocedure --- make
http/response --- output-response and output-response/method

No compatible interface is provided for these functions and data structures.

--- External APIs ---

Most external APIs now use the new response/c. Specifically:

dispatch/serve --- serve/dispatch
servlet-dispatch --- dispatch/servlet
servlet-env --- serve/servlet
servlet/web --- send/*, with-errors-to-browser
dispatchers/dispatch-servlets  --- servlet-loading responder argument
#lang web-server/insta
lang/web --- make-stateless-servlet
private/servlet --- handler field
servlet/servlet-structs --- response-generator/c, expiration-handler/c
servlet/setup --- make*servlet

These represent nearly every place where servlets typically interact
with the server.

However, one function: send/formlet from formlets used to allow any
old response/c as a wrapper result, while the new version requires
that the wrapper returns an Xexpr. This changes is justified in that
formlets already bake in support for Xexpr as a fundamental part of
their syntax.