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

[Goswin von Brederlow <goswin-v-b@web.de>]

On Thu, Feb 06, 2014 at 11:10:21AM +0000, Ben Millwood wrote:
> On 6 February 2014 10:58, Goswin von Brederlow <goswin-v-b@web.de> wrote:
> 
> > How do you match any exception but not a successfull value?
> >
> > let f5 x =
> >   try None
> >   with
> >   | Empty -> ()
> >   | val x -> x
> >   | _ -> 1
> >
> > Or does "_" only match exceptions and "val _" any value?
> >
> 
> Interesting.
> 
> So, I would assume that the behaviour of _ would be unchanged, so it would
> match any exception and no value in a try and any value and no exception in
> a match.
> 
> The way I justify this is that "val x" or "exception x" aren't really
> patterns, but rather "x" is a pattern and val/exception are part of the
> try/match syntax. For example, "val (val x)" would presumably be a parse
> error. As such, "_" isn't a more general pattern than "val _", because
> they're different kinds of thing.
> 
> This makes me wonder if the current syntax is misleadingly uniform. Perhaps
> value and exception cases ought to be more clearly separated, e.g.
> 
> try (...)
> with val
> | Some x -> (...)
> | None -> (...)
> with exception
> | Not_found -> (...)

That would break every existing source using exceptions. Unless the
exception in "with exception" is only required when using "with val".
Maybe "with val" should better be a single keyword like

    try (...)
    val
    | Some x -> (...)
    | None -> (...)
    with
    | Not_found -> (...)

MfG
	Goswin