Re: [boost] Interest in an 'either' library?

Subject: Re: [boost] Interest in an 'either' library?
From: David Sankel (camior_at_[hidden])
Date: 2013-06-23 11:47:57

On Sun, Jun 23, 2013 at 3:52 AM, Paul A. Bristow <pbristow_at_[hidden]>wrote:

>
>
> > -----Original Message-----
> > From: Boost [mailto:boost-bounces_at_[hidden]] On Behalf Of David
> Sankel
> > Sent: Saturday, June 22, 2013 6:47 PM
> > To: boost_at_[hidden]
> > Subject: Re: [boost] Interest in an 'either' library?
> >
> > Hi Larry!
> >
> > On Sat, Jun 22, 2013 at 9:17 AM, Larry Evans <cppljevans_at_[hidden]
> >wrote:
> >
> > > On 06/21/13 11:35, David Sankel wrote:
> > > > **semantics**:
> > > >
> > > > template< typename left_type, typename right_type> struct either {
> > > >   // postcondition, is_left()
> > > >   either( left_type );
> > > >
> > > >   // postcondition, is_right()
> > > >   either( right_type );
> > > >
> > > >   bool is_left() const;
> > > >   bool is_right() const;
> > > >
> > > >   // postcondition, is_left()
> > > >   either & operator=( left_type );
> > > >
> > > >   // postcondition, is_right()
> > > >   either & operator=( right_type );
> > > >
> > > >   // precondition, is_left()
> > > >   left_type left();
> > > >
> > > >   // precondition, is_right()
> > > >   right_type right();
> > > > private:
> > > > };
> > > >
> > > > **uses**:
> > > >
> > > > **Use 1**: Can be used as an alternative to exceptions or the (error
> > > > codes+set reference idiom):
> > > >
> > > > either<error_code, file_id> load_file(...);
> > > >
> > > > vs.
> > > >
> > > > error_code load_file( file_id &, ... );
> > > >
> > > > or
> > > >
> > > > // throws std::exception
> > > > file_id load_file(...);
> > > >
> > > Isn't this like haskell's either:
> > >
> > >
> > > http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/Dat
> > > a-Either.html
> >
> >
> > Yes. The proposed nomenclature (either, left, right) is intentionally
> mimicking that of Haskell.
>
> It's a somewhat cosmetic issue, but I suspect that most of the use cases
> involve an expected and an
> unexpected (rather than both being equally likely), so I'm not sure that
> following Haskell's name is
> really a good idea?

The (either, left, right) wording is somewhat arbitrary. I do see some
serious problems, though, with the (expected,unexpected) wording because it
will mislead readers of code that doesn't use the exception alternative use
case.

For example, compare the following definition,

optional< either< isbn_10, isbn_13 > > parse_isbn_string( std::string );

to the equivalent using the 'expected' wording,

optional< expected< isbn_10, isbn_13 > > parse_isbn_string( std::string );

-- David Sankel