[Nix-dev] Re: tryCatch function: tryRethow ?

[Nicolas Pierron]

On Sat, Jan 3, 2009 at 11:39, Michael Raskin <7c6f434c at mail.ru> wrote:
> Nicolas Pierron wrote:
>> Many people have reported that message are not very helpful and this
>> is really the case inside NixOS where everything is merged and you are
>> not able to figure out what is the problem.  So I have made a patch
>> for nix which add a builtin called "tryCatch".  The tryCatch function
>> expect a value and a function which expect one string argument.
>>
>> |   v1 = builtins.throw "v1: throw message";
>> |   v2 = tryCatch v1 (msg: builtins.throw "v2: ${msg}");
>
> I would prefer having tryRethrow here.
>
> v2 = tryRethrow v1 (msg: "v2: ${msg}");

Ok:

tryRethrow = try: edit: tryCatch try (msg: throw (edit msg));

> It looks like you haven't analyzed the benefits and drawbacks
> of your syntax solution for that task (and if you have, you don't give
> us your findings).

Your are right, I've submitted this proposal without looking much at
the drawbacks.  Here is the list:
- throw only throw strings and not any nix expression. (improvement)
- the position of the first throw is lost.
- ..?

In fact, I don't see much drawback.

> So I vote for reducing functionality to force
> rethinking implementation if a need arises.

What do you mean ?
I have made this because I need this trick to handle error messages
inside NixOS.  I think this feature is general, very small and provide
separation of concerns like somebody told me in an other post (why is
there a name argument to the merge function ?).

-- 
Nicolas Pierron
http://www.linkedin.com/in/nicolasbpierron
- If you are doing something twice then you should try to do it once.