Re: [review][LEAF] Review of LEAF starts today : May 22 - May 31

From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2020-06-07 19:13:09

On Sun, Jun 7, 2020 at 2:02 AM Joaquin M López Muñoz <
joaquinlopezmunoz_at_[hidden]> wrote:
> I still don't get it. What is the difference between:
>
>      int main()
>      {
>        return boost::leaf::try_handle_some(
>          []()->boost::leaf::result<int>{
>            return 0;
>          },
>          [](boost::leaf::match<int,0>){return 0;},
>          [](boost::leaf::catch_<std::exception>){return 1;},
>          [](const boost::leaf::error_info&){return 2;}
>        ).value();
>      }

One of your handlers uses catch_<>. This means that your try block will
execute inside a try scope, and your handlers will execute in a catch scope.

If the try block throws, LEAF will catch the exception and attempt to find
a handler. It doesn't have to be a handler that uses catch_<>, the first
suitable handler will be called. If your try block returns a failure
without throwing, LEAF will attempt to find a handler also. In this case,
the handler that uses catch_<> will not be called, because no exception was
thrown.

If a suitable handler could not be found, the error is propagated by
rethrowing the exception or returning the result<int> returned from the try
block, as appropriate.

> and
>
>      int main()
>      {
>        return boost::leaf::try_catch(
>          []()->boost::leaf::result<int>{
>            return 0;
>          },
>          [](boost::leaf::match<int,0>){return 0;},
>          [](boost::leaf::catch_<std::exception>){return 1;},
>          [](const boost::leaf::error_info&){return 2;}
>        ).value();
>      }

With try_catch, your try block always executes in a try scope, and your
handlers execute in a catch scope, even if none of them use catch_<>. If an
exception is thrown, try_catch will try to find a suitable handler. It does
not understand the semantics of the result type.