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

Subject: Re: [boost] Interest in an 'either' library?
From: David Sankel (camior_at_[hidden])
Date: 2013-06-22 13:47:22

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/Data-Either.html

Yes. The proposed nomenclature (either, left, right) is intentionally
mimicking that of Haskell.

I remember seeing elsewhere that haskell's either was mentioned as
> a replacement for exceptions.  I see if I can find it.
>

The Either Monad is commonly used for this purpose among others (
http://hackage.haskell.org/packages/archive/category-extras/0.53.1/doc/html/Control-Monad-Either.html
).

David Sankel