Effect Handler

Author: mohanr

_posts/2024-11-05-Raft.md

@@ -414,3 +414,52 @@ I have shown only that part of the code below.
 
 This code facilitates a mechanism that allows use to _pause_ the timer when we don't need it and start
 it again.
+
+# Using effect handlers
+
+This is from the documentation.
+
+**Effect handlers are a mechanism for modular programming with user-defined effects. Effect handlers allow the programmers to describe
+computations that perform effectful operations, whose meaning is described by handlers that enclose the computations**
+
+And I use _Early_return_ as my effect to return from my function.
+
+{% highlight ocaml %}
+
+type _ Effect.t += Early_return :  int -> int Effect.t
+
+let rec effective_request_vote
+  (mutex : Eio.Mutex.t)
+  (id : int)
+  (term : int)
+  (votes_received : int) : int Lwt.t =
+
+  match_with (fun () -> requestvote mutex id term votes_received)
+    ()
+  { effc = (fun (type c) (eff1: c Effect.t) ->
+      match eff1 with
+      | Early_return v  -> Some (fun (k:  (c,_) continuation) ->
+              Printf.printf "Early Return" ;
+              continue k v
+          )
+      | _ -> None
+  );
+  exnc = (function
+        | e -> raise e
+  );
+  retc = (fun _ ->  Lwt.return votes_received)
+
+ }
+ {% endhighlight %}
+
+
+But there is a complex interplay between the effect, _LWT_ and _EIO_ as the older LWT _capnp-rpc-lwt_ is used. Eio can integrate with it.
+This is done because the code started using Lwt-Eio integration. The newer _capnp-rpc_ based entirely on Eio will be used later.
+
+This necessitates  the line in the calling functiion which produces the effect.
+ 
+ {% highlight ocaml %}
+    Lwt.return (perform (Early_return votes_received));
+ {% endhighlight %}
+
+At this stage the code compiles but has to be tested more.