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From: =?ISO-8859-1?Q?J=E9r=E9mie?= Dimino <jeremie@dimino.org>
To: Dmitry Grebeniuk <gdsfh1@gmail.com>
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Subject: Re: [Caml-list] Lwt and exceptions

Le samedi 17 septembre 2011 à 09:38 +0300, Dmitry Grebeniuk a écrit : 
> When I began to use Lwt, I was badly surprised
> by the fact that Lwt.t values are not real I/O actions.
> So I had to get a new habit: if the top-level module
> value has type Lwt.t 'a, I have to defer its
> computation by wrapping it under "fun () -> <expr>".
> And I have to remember which of my functions return
> Lwt.t values, which of my abstract types really have
> type Lwt.t 'a or contain the Lwt.t 'a values, to defer
> their computations/sideeffects too.

You are comparing Lwt with the IO monad. They are two different monads:
IO deals with actions while Lwt deals with threads. If you write:

  let m = IO.read_char IO.stdin

then [m] is just the description of the action of reading a character
from [stdin]. On the contrary, if you write:

  let m = Lwt_io.read_char Lwt_io.stdin

then [m] is really a thread waiting for a character from [stdin]. If you
use "fun () -> ..." everywhere then this becomes actions but it is not
threads anymore and you loose most of the advantages of Lwt. For example
with Lwt you can write:

  let t1 = f1 ()
  and t2 = f2 ()
  ...
  and tn = fn () in 
  Lwt.bind t1 (fun x1 -> Lwt.bind t2 (fun x2 -> ... Lwt.bind tn (fun xn -> return (x1, x2, ..., xn)) ...))

and this will let [t1], ..., [tn] run concurrently. With the IO monad
this is just exactly the same as:

  let x1 = f1 () in
  let x2 = f2 () in
  ...
  let xn = fn () in
  (x1, x2, ..., xn)

in a non-monadic world. There is no concurrency at all.

Cheers,

-- 
Jérémie