Re: [Caml-list] Question about Lwt/Async

["Milan Stanojević" <milanst@gmail.com>]

> One of the advantages of OCaml over Haskell (which I'm not crazy about) is
> the fact that you don't have to constantly be stuck inside a monad. However,
> once you want to use these user-level threading libraries, you're
> essentially tied to a monad. It also means that the usage of any other monad
> from Lwt/Async code is out -- OCaml doesn't have the monad transformer
> infrastructure to layer monads easily as far as I can tell (am I wrong?). I
> mean, even in Haskell using Monad Transformers is a pain (IMO).

Are there any particular monads you have in mind?

My experience is that it's not a big problem in practice with Async (I
don't have experience with lwt but I guess it is similar) but I don't
really use a lot of monads (mostly just Deferred and Or_error)

With async, a good programming style is to keep asynchronous code at
the fringes of your application and make as much of your program's
logic synchronous. This is good for performance but more importantly
makes your code easier to reason about because you don't have to worry
about any interleaving and races since async runs code without binds
and maps as an atomic job.

This means that large parts of your code are just usual synchronous
ocaml code (you might not even open Async in a module) and you are
free to easily use any monad you want.

Of course, there are cases when you will run into issues, usually
combining Or_error and Deferred monads. Common case is when handling
rpcs in async. There indeed you are stuck for a bit in
Deferrred.Or_error monad but at some point you need to handle two
cases (Error and Ok) and you escape it.