let () =
  Dream.run
  @@ Dream.logger
  @@ Dream.router [
    Dream.post "/echo" (fun request ->
      Lwt.map Dream.response (Dream.body request));
  ]
  @@ Dream.not_found

$ dune exec --root . ./echo.exe

You can test it with curl:

$ curl http://localhost:8080/echo --data foo
foo

Or try HTTPie:

$ echo -n foo | http POST :8080/echo
HTTP/1.1 200 OK
Content-Length: 3

foo

The code uses Lwt.map. That's because Dream.body returns the body string inside a promise, and we want to transform that string promise into a response promise with Dream.response. This is just a touch of Lwt, because we need it here! Example a-promise introduces Lwt promises fully.

We usually want to do something more interesting with the request body than just echo it, and there are several examples for that!

d-form parses request bodies as forms.
e-json parses them as JSON.
g-upload receives file upload forms.
i-graphql receives GraphQL queries!
j-streaming streams huge bodies.

We delay these examples a bit, so we can squeeze in a couple security topics first. These examples do take client input, after all! So, it's better to present them the right way.

Next steps:

6-template renders responses from templates and guards against injection attacks (XSS).
7-debug renders error information in responses.

Up to the tutorial index