Module Pa_lwt

module Pa_lwt : sig..end

Syntactic sugars for lwt



This extension add the following sugars:

  • anonymous bind:

write stdio "Hello, " >> write stdio "world!"


  • lwt-binding:

lwt ch = get_char stdin in
         code


is the same as bind (get_char stdin) (fun ch -> code)

Moreover it supports parallel binding:

lwt x = do_something1 ()
         and y = do_something2 in
         code


will let do_something1 () and do_something2 () runs then bind their result 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>

,

try_lwt
           <expr>
         with
           <branches>

,

try_lwt
           <expr>
         finally
           <expr>


and:

try_lwt
           <expr>
         with
           <branches>
         finally
           <expr>


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 addedd when needed.

The construction try_lwt <expr> just catch regular exception into lwt exception. i.e. it is the same as catch (fun _ -> <expr>) fail.

  • exception raising:

raise_lwt <expr>


This allow exception to be traced when the -lwt-debug switch is passed to the syntax extension.

  • for loop:

for_lwt i = <expr> to <expr> do
          <expr>
        done


and:

for_lwt i = <expr> downto <expr> do
          <expr>
        done


  • iteration over streams:

for_lwt <patt> in <expr> do
          <expr>
        done


  • while loop:

while_lwt <expr> do
          <expr>
        done


  • pattern mattching:

match_lwt <expr> with
          | <patt_1> -> <expr_1>
          ...
          | <patt_n> -> <expr_n>