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

Subject: Re: [boost] Interest in an 'either' library?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2013-06-24 08:49:04

On 06/24/13 06:20, Pierre T. wrote:> Hello
[snip]
> I think there is some misunderstanding and confusion between
> Expected/Optional/Either. Indeed, I'm quite sure that Expected and
> Either are completely different. Even if Expected seems to be a subset
> of Either, because you can choose to return a (value,
> exception/error) couple in Either, the behavior will be different. IMHO,
> Expected is more useful because it adds a lot of semantics for this

Could you please highlight what the semantics are which distinguish
Either from Expected?

> particular use-case.
>
> I'd say that if Expected is the brother of Optional, Either is the child
> of Variant (because it's a subset without adding specific semantic).

The gsoc2013 Expected page:

http://google-melange.appspot.com/gsoc/proposal/review/google/gsoc2013/trademark/25002

says:

  expected< T,Error > proposed here is a type that contains a type T or
  a type Error in its storage space.

which sounds a lot like the OP passage:

**Use 1**: Can be used as an alternative to exceptions or the (error
codes+set reference idiom):

and a lot like:

  The Either type is similar to the Maybe type, with one key
  difference: it can carry attached data both for an error and a
  success (“the Right answer”).

from:

  http://book.realworldhaskell.org/read/error-handling.html

Based on a brief look at:

http://www.boost.org/doc/libs/1_53_0/libs/optional/doc/html/boost_optional/detailed_semantics.html

Boost.Optional is like haskell's Maybe<T>(either a T value or
Nothing).  The Nothing in haskell's Maybe is like the none_t
mentioned on the boost_optional/detailed_semantics.html page mentioned
above.

Hence, I don't see any fundamental difference between Either and Expected.
What am I missing?

[snip]

> Finally, the first argument of this Either library was the error/value
> couple facilities, but now we know that Expected is quite better,

Could you explain exactly how Expected is better?

[snip]

-regards,
Larry