Thanks, Edward. I keep mistakenly thinking that "expression template library" is a sufficient explanation on the Boost list, even though I know that this is not the case. Yap allows you to capture a C++ expression as an expression tree that can be subsequently transformed and/or evaluated. For instance, here is some end-user code using lazy vectors. The lazy_vector type and the associated operations are defined using the Yap library. This is mostly a copy-paste from one of the Yap examples, here: https://tzlaine.github.io/yap/doc/html/boost_yap__proposed_/manual/examples/... int main () { // A lazy vector contains a std::vector<double> value lazy_vector v1{std::vector<double>(4, 1.0)}; lazy_vector v2{std::vector<double>(4, 2.0)}; lazy_vector v3{std::vector<double>(4, 3.0)}; // This statement does not create a temporary vector, and // only uses the elements v2[2] and v3[2]. It also generates // the exact same object code as "x2[2] + x3[2]", where x2 // and x3 are non-lazy, plain old std::vector<double>s. double d1 = (v2 + v3)[2]; std::cout << d1 << "\n"; // This statement does an element-wise operation, creating // no temporaries. v1 += v2 - v3; std::cout << '{' << v1[0] << ',' << v1[1] << ',' << v1[2] << ',' << v1[3] << '}' << "\n"; // This expression is disallowed because it does not conform to the // implicit grammar. operator+= is only defined on terminals, not // arbitrary expressions. // (v2 + v3) += v1; return 0; } The Yap code that you must write in order to make this end-user code work is fairly small: // This transform turns a terminal of std::vector<double> into a terminal // containing the nth double in that vector. Think of it as turning our // expression of vectors into an expression of scalars. struct take_nth { boost::yap::terminal<lazy_vector_expr, double> operator() (boost::yap::terminal<lazy_vector_expr, std::vector<double>> const & expr); std::size_t n; }; // A custom expression template that defines lazy + and - operators that // produce expressions, and an eager [] operator that returns the nth element // of the expression. template <boost::yap::expr_kind Kind, typename Tuple> struct lazy_vector_expr { using this_type = lazy_vector_expr<Kind, Tuple>; static const boost::yap::expr_kind kind = Kind; Tuple elements; BOOST_YAP_USER_BINARY_OPERATOR_MEMBER(plus, this_type, ::lazy_vector_expr) BOOST_YAP_USER_BINARY_OPERATOR_MEMBER(minus, this_type, ::lazy_vector_expr) // Note that this does not return an expression; it is greedily evaluated. auto operator[] (std::size_t n) const; }; template <boost::yap::expr_kind Kind, typename Tuple> auto lazy_vector_expr<Kind, Tuple>::operator[] (std::size_t n) const { return boost::yap::evaluate(boost::yap::transform(*this, take_nth{n})); } boost::yap::terminal<lazy_vector_expr, double> take_nth::operator() (boost::yap::terminal<lazy_vector_expr, std::vector<double>> const & expr) { double x = boost::yap::value(expr)[n]; return boost::yap::make_terminal<lazy_vector_expr, double>(std::move(x)); } // In order to define the += operator with the semantics we want, it's // convenient to derive a terminal type from a terminal instantiation of // lazy_vector_expr. note that we could have written a template // specialization here instead -- either one would work. That would of course // have required more typing. struct lazy_vector : lazy_vector_expr< boost::yap::expr_kind::terminal, boost::hana::tuple<std::vector<double>> > { lazy_vector () {} explicit lazy_vector (std::vector<double> && vec) { elements = boost::hana::tuple<std::vector<double>>(std::move(vec)); } template <boost::yap::expr_kind Kind, typename Tuple> lazy_vector & operator+= (lazy_vector_expr<Kind, Tuple> const & rhs) { std::vector<double> & this_vec = boost::yap::value(*this); for (int i = 0, size = (int)this_vec.size(); i < size; ++i) { this_vec[i] += rhs[i]; } return *this; } }; More details and way more examples can be found at: https://tzlaine.github.io/yap Zach On Thu, Mar 30, 2017 at 8:02 PM, Edward Diener via Boost < boost@lists.boost.org> wrote:
On 3/18/2017 6:55 PM, Zach Laine via Boost wrote:
I posted 2-3 months ago about Yap, an expression template library I've written that I intend to propose for Boost.
This is just a reminder that the library exists, and where to find it.
I'm giving a talk about it at C++Now 2017, and some time after that I intend to submit it to the queue. Louis Dionne has offered to serve as review manager when the time comes.
You can find the main repo on GitHub:
https://github.com/tzlaine/yap
And online docs are here:
You might want to mention here what the purpose of the library is and when it might be used by developers, in order to interest others.
Zach
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