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Subject: Re: [boost] [yap] Re-announcing Yap
From: Zach Laine (whatwasthataddress_at_[hidden])
Date: 2017-03-31 15:20:36

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:

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
// 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;


    // Note that this does not return an expression; it is greedily
    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,

// 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
// have required more typing.
struct lazy_vector :
    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:


On Thu, Mar 30, 2017 at 8:02 PM, Edward Diener via Boost <
boost_at_[hidden]> 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:
>> 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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