dream/example/2-middleware

Middleware is just functions that take handlers and wrap them, producing, again handlers. 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!"

The @@ is just the standard OCaml operator for calling functions without nesting tons of parentheses. In fact, the above code is the same as:

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

But you can probably see that if you stack multiple middlewares like this, you will end up with way too many parens! So, we opt for @@.

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

$ make
08.02.21 22:19:21.126                       Running on http://localhost:8080
08.02.21 22:19:21.126                       Press ENTER to stop
08.02.21 22:19:24.927       dream.log  INFO REQ 1 GET / 127.0.0.1:58549 Mozilla/5.0 ...
08.02.21 22:19:24.928       dream.log  INFO REQ 1 200 in 2 μs

You can write your own messages to the log using Dream.log.

Where to go from here?

• 3-counter calls Dream.log while incrementing its little counter.
• 4-catch shows how to centralize error handling and enable the debugger.