Cookbook : fix in Lwt (type mismatch with iter_s/iter_p functions) (#…

…2127)

* Cookbook : fix in Lwt

* Update 00-lwt.ml

---------

Co-authored-by: Frédéric LOYER <[email protected]>

data/cookbook/concurrency/lwt/00-lwt.ml

@@ -23,15 +23,18 @@ let task n =
 let (_result1, _result2) =
   Lwt_main.run @@ Lwt.both (task 1) (task 2)
 
-(* When having a promise function which should be scheduled multiple times with different values, `iter_p`, `iter_s`, `map_p`, `map_s` schedule one promise per item from a given list. The `_s` versions schedule the promises sequentialy, and `_p` in parallel. `iter_*` return `()` while `map_*`return the list of results. *)
+(* When having a promise function which should be scheduled multiple times with different values, `iter_p`, `iter_s`, `map_p`, `map_s` schedule one promise per item from a given list. The `_s` versions schedule the promises sequentialy, and `_p` in parallel. `iter_*` return `()` (and expect tasks which return a unit Lwt) while `map_*`return the list of results. *)
 let _result_list =
   Lwt_main.run @@ Lwt_list.map_p task [1; 2; 3] 
 let _result_list =
-  Lwt_main.run @@ Lwt_list.map_s task [1; 2; 3] 
+  Lwt_main.run @@ Lwt_list.map_s task [1; 2; 3]
+let task' n =
+  let* _result = task n in
+  Lwt return ()
 let () =
-  Lwt_main.run @@ Lwt_list.iter_p task [1; 2; 3] 
+  Lwt_main.run @@ Lwt_list.iter_p task' [1; 2; 3] 
 let () =
-  Lwt_main.run @@ Lwt_list.iter_s task [1; 2; 3] 
+  Lwt_main.run @@ Lwt_list.iter_s task' [1; 2; 3] 
 
 (* `sleep seconds` pauses for then given seconds number.  *)
 let () =