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

On 6/21/2013 9:35 AM, 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 load_file(...);

vs.

error_code load_file( file_id &, ... );

or

// throws std::exception file_id load_file(...);

**Use 2**: A lightweight alternative to boost::variant when only 2 components are required (either relates to variant in a similar way that pair relates to tuple ).

variant eitherAOrB; if( A * aPtr = boost::get<A>( &eitherAOrB ) ) std::cout << "A: " << *aPtr; else std::cout << "B: " << boost::get<B>( eitherAOrB );

vs.

either eitherAOrB;

if( eitherAOrB.is_left() ) std::cout << "A: " << eitherAOrB.left(); else std::cout << "B: " << eitherAOrB.right();

This misses what is, IMO, the most important use case of Haskell's Either monad: automatic error propagation. This would be more useful if there were a way to call a function such that if any of the function's arguments were an Either-in-error, the function would immediately return the error. I implemented something like this. Check out substitution_failure and try_call starting here: https://github.com/ericniebler/proto-0x/blob/master/boost/proto/v5/utility.h... -- Eric Niebler Boost.org http://www.boost.org