How to compile Let_syntax of Base?

Learning
1

I’m reading RWO v2 and try to write my own demo as:

open Base
open Stdio

let f1 n =
  if n < 10 then None else Some n

let f2 n =
  if n > 15 then None else Some n

let combines n =
  let open Option.Let_syntax in
  let%bind r1 = f1 n in
  let%bind r2 = f2 n in
  Some r2

let _ = 
  match combines 13 with
  | Some x -> print_int x
  | None -> print_endline "nothing"

The jbuild file:

(executables
 ((names (t))
  (libraries (base stdio))
  ))

But there’s an error:

$ jbuilder build t.exe
File "t.ml", line 12, characters 6-10:
Error: Uninterpreted extension 'bind'.
2

AFAIK you should have ppx_let installed (via opam install ppx_let if haven’t transitively depended), and add the preprocess stanza on your jbuild file, as such:

(executables
 ((names (t))
  (libraries (base stdio))
  (preprocess (pps (ppx_let)))
  ))
2 Likes
3

In general, the ppx syntax extensions used in RWO are all transitively included by ppx_jane, so using that in your jbuild file should fix it for [@@deriving sexp], [%compare: int list], etc.

1 Like
4

I seem to have almost the exact same issue but updated for 2020

some code:

let _ =
  let open Option.Let_syntax in
  let%bind x = Some(1) in
  x

the dune:

(library
 (name sre)
 (inline_tests)
 (preprocess
  (pps ppx_let ppx_inline_test))
 (libraries str))

the dune build output:

File "lib/nth.ml", line 2, characters 11-28:
2 |   let open Option.Let_syntax in
               ^^^^^^^^^^^^^^^^^
Error: Unbound module Option.Let_syntax
1 Like
5

Base is not listed in the dune file. Try replacing (libraries str) with (libraries base str)

1 Like
6

Adding base didn’t work, same error. Could something be missing from opam?

7

You’ll also probably need to open Base to the top of the .ml file so that you’ll be using the Base version of the option module.

1 Like
8

That was it, thanks @mseri & @Levi_Roth !