Re: [boost] [yap] Re-announcing Yap

31 Mar 2017

      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
    operator() (boost::yap::terminal
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 
struct lazy_vector_expr
{
    using this_type = lazy_vector_expr;

    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 
auto lazy_vector_expr::operator[] (std::size_t n) const
{ return boost::yap::evaluate(boost::yap::transform(*this, take_nth{n})); }

boost::yap::terminal
take_nth::operator() (boost::yap::terminal const & expr)
{
    double x = boost::yap::value(expr)[n];
    return boost::yap::make_terminal(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
    >
{
    lazy_vector () {}

    explicit lazy_vector (std::vector<double> && vec)
    { elements = boost::hana::tuple(std::move(vec)); }

    template 
    lazy_vector & operator+= (lazy_vector_expr 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:
https://tzlaine.github.io/yap
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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