Re: [Caml-list] What if exn was not an open type?

[Philippe Veber <philippe.veber@gmail.com>]

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Thanks a lot Leo and Oleg for all this very exciting material!

2017-11-01 8:16 GMT+01:00 Oleg <oleg@okmij.org>:

>
>
> > I thought one big selling argument of monads was also that the type of
> > the functions shows which effect it performs. As I understand it, it
> > is not the case for effects, at least not in existing implementations
> > like multicore ocaml. ... Also, would you know a reference that shows
> > that effects compose indeed a lot more easily than monads?
>
> How expressive are the types of effectful computations pretty much
> depends on a particular type system in use. Let me cite from the message
> that I received from Nick Palladinos (CCed) the other week, who implemented
> extensible effects in F#, a rather close relative of OCaml. You can
> write the code as below
>
> let example () =
>     eff {
>         do! put 1
>         let! x = get ()
>         let! y = ask ()
>         return x + y
>     }
>
> (Here, `eff' is a tag of so-called computational expressions of F#, a
> very handy feature).
>
> The *inferred* type is as follows
>   val example : unit -> Eff<'U,int> when 'U :> Reader<int> and 'U :>
> State<int>
>
> clearly stating that example is an effectful expression that uses at
> least Reader and State effects. The example also illustrates
> composability, your second question: put/get and ask are operations of two
> distinct effects (State and Reader, resp). You can combine them freely
> within the same program.
>
> (I hope Nick will write a paper explaining his implementation so that
> we can all learn from it.)
>
> To give more references (in addition to Leo's work), I should point to Koka
>         https://koka-lang.github.io/koka/doc/kokaspec.html
>
> which is an OCaml-like language based on effects and
> effect-typing. The language is mature enough to write a web server in
> it (as Daan Leijen described in his talk at the ML Family workshop
> this year).
>
>         Other references are the recent Effekt library in Scala
>         http://b-studios.de/scala-effekt/
>         and extensible-effect library
>         http://okmij.org/ftp/Haskell/extensible/
> which, although being first proposed for Haskell has also been ported
> to Scala (and now F#). Purescript's extensible effects are also
> similar.
>
>
>

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