Re: [Caml-list] Proposal: extend try to handle success

[Jeremy Yallop <yallop@gmail.com>]

On 6 February 2014 10:58, Goswin von Brederlow <goswin-v-b@web.de> wrote:
> Examples from the blog:
[...]
> let rec iter_stream f s =
>   try MyStream.get s
>   with
>   | End_of_stream -> ()
>   | val (x, s') ->
>       f x;
>       iter_stream f s'
>
> Isn't that the same as this?
>
> exception Success of (char * MyStream.t)
> let rec iter_stream f s =
>   try raise (Success (MyStream.get s))
>   with
>   | End_of_stream -> ()
>   | Success (x, s') ->
>       f x;
>       iter_stream f s'

Yes, it's essentially the same if your streams are monomorphic.
However, if MyStream.t has a type parameter then you'll need something
more like this:

  let rec iter_stream : type a. (a -> unit) -> a MyStream.t -> unit =
    fun f s ->
      let module M = struct exception Success of (a * a MyStream.t) end in
      try raise (M.Success (MyStream.get s)) with
      | End_of_stream -> ()
      | M.Success (x, s') ->
        f x;
        iter_stream f s'

The code becomes quite cluttered with irrelevant details, but things
can get even worse.  In some cases there's no way to write down the
type of the expression at all, which makes defining the exception
rather tricky.

> And what does this do?
>
> let f1 x =
>   try None
>   with
>   | Empty -> ()
>   | val Some x -> x
>   | val _ -> 1
>
> let f2 x =
>   try raise Not_found
>   with
>   | Empty -> ()
>   | val Some x -> x
>   | val _ -> 1
>
> let f3 x =
>   try raise Not_found
>   with
>   | Empty -> ()
>   | val Some x -> x
>   | _ -> 1
>
> let f4 x =
>   try None
>   with
>   | Empty -> ()
>   | val Some x -> x
>   | _ -> 1
>
> My guesses:
> - f1 returns 1
> - f2 throws Not_found
> - f3 returns 1
> - f4 returns 1

Change '()' to '0' and your first three guesses are correct.  The last
function behaves as follows:

   # #use "f4.ml";;
   File "f4.ml", line 2, characters 2-63:
   Warning 8: this pattern-matching is not exhaustive.
   Here is an example of a value that is not matched:
   None
   val f4 : 'a -> int = <fun>
   # f4 ();;
   Exception: Match_failure ("f4.ml", 2, 2).

> Or does "_" only match exceptions and "val _" any value?

Right.  The behaviour of '_' is unchanged: it matches any exception.
The pattern 'val _' matches any value.