Re: [boost] [outcome] High level summary of review feedback accepted so far

Le 27/05/2017 à 18:28, Niall Douglas via Boost a écrit :

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I would assume an expected could only return a std::variant from its .error(). I can't think what else it could do. As the foremost authority on the nonexistent expected, I can tell you what it does:

// F shall be in E... template<class F> bool has_error() const; template<class F> F error() const;

// if sizeof...(E) == 1 bool has_error() const; E1 error() const;

It returns the equivalent of variant not from error(), but from

unexpected unexpected() const;

which allows you to

expected function() { expected e1 = function1(); if( !e1 ) return e1.unexpected();

expected e2 = function2(); if( !e2 ) return e2.unexpected();

return e1.value() + e2.value(); } That's a second approach.

A third approach could be implementing .index() and std::get<>() for expected.

I believe, expected should implement the SumType operations when we agree on what they are. This doesn't mean that we need .index and std::get, these are specific to variant ;-) I believe that the variant interface is not the one we need to adopt for sum types, in the same way the product type interface shouldn't be the one of tuples. We have other product types (those to which structured binding applies) that I don't see how they can have the tuple interface. Maybe I'm wrong and we are able to do it, but I don't think we should. I suspect that we will have the same issue with future language sum types. Vicente