I'm encountering an issue while using lwt_ppx and the let%lwt syntax as described in the CS3110 textbook's section on promises (https://cs3110.github.io/textbook/chapters/ds/promises.html).
In the code snippet provided:
open Lwt_io
let p =
let%lwt s1 = read_line stdin in
let%lwt s2 = read_line stdin in
Lwt_io.printf "%s\n" (s1^s2)
let _ = Lwt_main.run p
I'm trying to compile the code using dune. My dune file looks like this:
(executable
(name main)
(libraries lwt.unix lwt_ppx))
However, when I run the command dune exec ./main.exe, I'm getting the error message "Uninterpreted extension 'lwt'."
I've verified that I have the lwt_ppx package installed using opam list, and interestingly, the code works as expected in utop when I run:
#require "lwt_ppx";;
#require "lwt.unix";;
#use "main.ml";;
What might I be doing wrong with my dune configuration? Any help would be greatly appreciated.
lwt_ppx is a preprocessor, and so also needs to be added to the preprocess stanza:
(executable
(name main)
(libraries lwt.unix lwt_ppx)
(preprocess (pps lwt_ppx)))
But also note that since 4.08 you can use let operators for much the same purpose, without any preprocessor. For example, you can bind Lwt.bind to let*, then your example can be written as:
open Lwt_io
let (let*) = Lwt.bind
let p =
let* s1 = read_line stdin in
let* s2 = read_line stdin in
printf "%s\n" (s1^s2)
let _ = Lwt_main.run p