Middleware is just functions that take handlers and wrap them, producing handlers that do a little bit more. This example takes the handler from 1-hello and wraps it in one of the most useful middlewares, the logger:

let () =
  Dream.run
    (Dream.logger (fun _ ->
      Dream.respond "Good morning, world!"))

However, as you can see, the more middlewares we stack on top of each other like this, the more parentheses and indentation we will end up with! To keep the code tidy, we use @@, the standard OCaml operator for calling functions without parentheses. So, the actual code in this example looks like this:

let () =
  Dream.run
  @@ Dream.logger
  @@ fun _ ->
    Dream.respond "Good morning, world!"

When you run this server and visit http://localhost:8080, you get much more interesting (and colorful!) output:

$ dune exec --root . ./middleware.exe
08.03.21 22:19:21.126                       Running on http://localhost:8080
08.03.21 22:19:21.126                       Press ENTER to stop
08.03.21 22:19:24.927       dream.log  INFO REQ 1 GET / 127.0.0.1:58549 Mozilla/5.0 ...
08.03.21 22:19:24.928       dream.log  INFO REQ 1 200 in 217 μs

You can write your own messages to the log using Dream.log. See example 9-logging for more logging options. Now that we have the logger, we will use it in all other examples, even though it's not really necessary — it just makes it much easier to see what is going on.

There's not much else to middlewares — they are really just functions from handlers to handlers, so you can create them anywhere. Example 4-counter already shows a simple custom middleware. There are also more complicated middlewares defined in

Next steps:

The next example, 3-router, shows routes, the other way to build up handlers in Dream.
4-counter builds the first custom middleware.

Up to the example index