OCaml 5

• Domain: Parallelism
  • Threads (but not the usual kind??)
• Concurrency: Effect handlers
  • Overlapped execution. Yield-restart kind of thing.

Effect handlers

No primitives for concurrency till Ocaml5.

• 3rd party libs: Lwt and Async
  • Not primitive. Essentially, syntactic sugar.
  • Monadic syntax, but doesn't obey monadic laws ??
  • Less readable errors. No backtrace. No exceptions.

Can think of effect handlers as 'first-class, restartable exceptions'

(* When [E] is performed, we will get a value of type [string] *)
effect E: string

(* A computation. Like a thunk *)
let comp () = 
  print_string "0: start";
  print_string (perform E);
  print_string "3: end";

let main () = 
  try
    comp ()
  (* Effect handler. [k] is a delimited continuation *)
  with effect E k ->
    print_string "1: EH start";
    continue k "2: EH cont";
    print_string "4: EH end";

Here control goes like:

• main
• comp: start
• E k: EH start
• comp: EH cont, end
• E k: EH end