Module Ppx_lwt
module Ppx_lwt : sig..end
Ppx syntax extension for Lwt
Ppx extensions ¶
This extension adds the following syntax:
- lwt-binding:
let%lwt ch = get_char stdin in
code
is the same as bind (get_char stdin) (fun ch -> code).
Moreover, it supports parallel binding:
let%lwt x = do_something1 ()
and y = do_something2 in
code
will run do_something1 () and do_something2 (), then bind their results to x and y. It is the same as:
let t1 = do_something1
and t2 = do_something2 in
bind t1 (fun x -> bind t2 (fun y -> code))
- exception catching:
try%lwt
<expr>
with
<branches>
For example:
try%lwt
f x
with
| Failure msg ->
prerr_endline msg;
return ()
is expanded to:
catch (fun () -> f x)
(function
| Failure msg ->
prerr_endline msg;
return ()
| exn ->
Lwt.fail exn)
Note that the exn -> Lwt.fail exn branch is automatically added when needed.
- finalizer:
(<expr>) [%finally <expr>]
You can use [%lwt.finally ...] instead of [%finally ...].
- assertion:
assert%lwt <expr>
- for loop:
for%lwt i = <expr> to <expr> do
<expr>
done
and:
for%lwt i = <expr> downto <expr> do
<expr>
done
- while loop:
while%lwt <expr> do
<expr>
done
- pattern matching:
match%lwt <expr> with
| <patt_1> -> <expr_1>
...
| <patt_n> -> <expr_n>
Exception cases are also supported:
match%lwt <expr> with
| exception <exn> -> <expr_1>
| <patt_2> -> <expr_2>
...
| <patt_n> -> <expr_n>
- exception raising:
For all other expression, the construct
[%lwt <expr>]
is expanded to:
Lwt.catch (fun () -> <expr>) Lwt.fail
It allows to encode the old raise_lwt <e> as [%lwt raise <e>], and offers a convenient way to interact with non-Lwt code.
Debug ¶
By default, the debug mode is enabled. This means that the backtrace versions of the bind, finalize and catch functions are used, enabling proper backtraces for the Lwt exceptions.
The debug mode can be disabled with the option -no-debug.
Sequence ¶
It is also possible to sequence Lwt operations with the >> operator:
write stdio "Hello, " >> write stdio "world!"
By default, each operation must return unit Lwt.t. This constraint can be lifted with the option -no-strict-sequence. The operator can be disabled with the option -no-sequence.
Logging ¶
The syntax extension will replace expression of the form:
Lwt_log.info_f ~section "x = %d" x
by
if Lwt_log.Section.level section <= Lwt_log.Info then
Lwt_log.info_f ~section "x = %d" x
else
return ()
Notes:
- The application must be complete. For example: Log.info "%d" will make compilation fail.
- Debug messages are removed if the option -no-debug is passed.
- The log syntax extension can be disabled with the option -no-log.