This kind of expression is not allowed as right-hand side of `let rec' - ocaml

Question

I'm implementing the Raft protocol and my code is as follows:
let rec request_vote_loop: int =
match myState.myRole with
| Follower -> 0
| Leader -> 1
| Candidate ->
let trigger = Domain.spawn(fun _ -> Chan.send c TriggerEvent) in
let request_vote_daemon = Domain.spawn(fun _ ->
let rec loop n =
if n = 0 then 0
else let msg = Chan.recv votes in
match msg with
| (status, id) ->
Domain.join (Array.get !arr id);
if status = 1 then (Array.get votePeers id) := true; (Chan.send c ReceiveVoteEvent); loop (n - 1)
in loop ((Array.length (!peers)) / 2 + 1 - !current_vote)) in
let evt = Chan.recv c in
match evt with
| TimeoutEvent -> myState.myRole <- Follower; 3
| AppendEntriesEvent(_) ->
myState.myRole <- Follower; 4
| ReceiveVoteEvent ->
if !current_vote > (Array.length (!peers) / 2) then
begin current_vote := !current_vote + 1; myState.myRole <- Leader; 3 end
else current_vote := !current_vote + 1; request_vote_loop
| TriggerEvent ->
arr := Array.make (Array.length (!peers)) (Domain.spawn (fun i ->
if (!(Array.get votePeers i)) then 0
else
let conn = Array.get !peers i in
Lwt_main.run
(let+ resp = call_server conn
(RequestVoteArg({
candidateNumber = myState.myPersistentState.id;
term = myState.myPersistentState.currentTerm;
lastlogIndex = (Array.get myState.myPersistentState.logs ((Array.length myState.myPersistentState.logs) - 1)).index;
lastlogTerm = (Array.get myState.myPersistentState.logs ((Array.length myState.myPersistentState.logs) - 1)).term
})) in (match resp with
| Error(s) -> Chan.send votes (0, i); Printf.printf "requestVote: connection failed: %s" s; 1
| Ok(repl, s) ->
(match repl with
| RequestVoteRet(repl) ->
if repl.voteGranted then begin Chan.send votes (1, i); Printf.printf "requestVote: status: %s, currentVote: %d" s !current_vote; 2 end
else
if not (repl.term = (-1l)) then begin myState.myPersistentState.currentTerm <- repl.term; Chan.send votes (0, i);
Printf.printf "requestVote failed because of term: status: %s, currentVote: %d" s !current_vote; 3 end
else Chan.send votes (0, i); Printf.printf "requestVote failed: status: %s" s; 4
| _ -> failwith "Should not reach here" ))))); request_vote_loop
| _ -> failwith "Should not reach here"
in print_endline (Int.to_string request_vote_loop)
But there's an error that "This kind of expression is not allowed as right-hand side of `let rec'", it said my function is of type unit. I don't know what happened...
Thanks in advance.

Answer 1

Your definition starts like this:
let rec request_vote_loop: int = ...
This doesn't define a function, it defines a simple value of type int. The reason is that there are no parameters given.
There's too much code to process (and furthermore it's not self-contained). But I suspect you want to define a function that doesn't take any parameters. The way to do this is to pass () (known as unit) as the parameter:
let rec request_vote_loop () : int = ...
The recursive calls look like this:
request_vote_loop ()
The final call looks like this:
Int.to_string (request_vote_loop ())