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

[Ben Millwood <bmillwood@janestreet.com>]

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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 -> (...)

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