Sample impl of YAP

YAP_impl.hpp

#include <boost/yap/expression.hpp>

#include <algorithm>
#include <cassert>
#include <iostream>
#include <vector>

// TAKEN FROM BOOST.YAP EXAMPLES, BUT MODIFIED BY CODER3101

/*
 * Every YAP expression be it any type must meet this concept.
 *  - It must have a static const member named kind
 *  - It must have a public member named elements
 */
template <boost::yap::expr_kind Kind, typename Tuple>
struct lazy_vector_expr;


/*
 * This is a transform that extracts ith value from terminal node of YAP
 */
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;
};

/*
 * We finally create the lazy_vector_expr class following the 
 * YAP concepts as mentioned above.
 *
 * Few Important things to mention :
 *  - kind is a enum of boost::yap::expr_kind. This value denotes
 *    the operand that has been applied. So if this is expr_kind::plus.
 *    It means that this node in AST represents a plus operation.
 *   
 *  - Tuple is a boost::hana<..> type. YAP uses hana because it provides 
 *    compile time tuple, necessary to build expression templates. This tuple
 *    holds the operands to which the expr_kind::kind operator will be applied.
 *
 *    * If kind = expr_kind::terminal, this means that Tuple elements 
 *    only has one value that is terminal node of AST. Since boost.yap has some
 *    limitations, if expr_kind::terminal it is required that elements must not have
 *    a value that can be converted to any YAP expression concept. It is due to this reason
 *    that we cannot follow CRTP pattern, we want terminal to have some value that cannot be
 *    converted to YAP expression. In this case we use std::vector<>, which is enough to represent 
 *    the complete lazy_vector.
 */

template <boost::yap::expr_kind Kind, typename Tuple>
struct lazy_vector_expr
{
    static const boost::yap::expr_kind kind = Kind;

    Tuple elements;

    // Note that this does not return an expression; it is greedily evaluated.
    auto operator[] (std::size_t n) const;
};

// Some Operator Overloads.
BOOST_YAP_USER_BINARY_OPERATOR(plus, lazy_vector_expr, lazy_vector_expr)
BOOST_YAP_USER_BINARY_OPERATOR(minus, lazy_vector_expr, lazy_vector_expr)

/*
 * This runs the transform and evaluates the terminal expression returned from
 * the transform.
 */
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})); }

/**
 * Defines the transform to extract the ith element from terminal node
 * This then wraps it into a terminal expression type for furthur lazy evaluation
 * if desired.
 */
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));
}

/*
 * This is most Important section. Here we create the lazy_vector class.
 * Because we want that this template class to be trivially convertible to YAP expression terminal.
 * We inherit this class from a terminal type of lazy_vector_expr.
 *
 * Since, lazy_vector now is trivially convertible to an YAP expression, we need to put something else
 * into the terminal node of lazy_vector_expr. In this case we have choosen to put
 * std::vector<double> into the terminal.
 * The thing that we want to put in the terminal must meet following :
 *   - It must have enough information so that expression values can be evaluated greedly.
 *   - It must have enough information so that we can infer the dimension or shape of expression greedly.
 * In this case since it was a vector, std::vector<> values and .size() are enough to 
 * satisfy the needs. In case of matrix or tensor, we may need to put more than one thing
 * say dimension/extent and data. While we can surely put multiple things into terminal,
 * it is usually good to wrap all those things into one single class like matrix/tensor_core.
 * We then put that core into the terminal. Now the Question how do we access the contents of
 * what we have placed in the terminal? See the in-class comments.
 */
struct lazy_vector : public lazy_vector_expr<
        boost::yap::expr_kind::terminal,
        boost::hana::tuple<std::vector<double>>>
{
    lazy_vector () {}

    explicit lazy_vector (std::vector<double> && vec)
    {
	    /*
	     * Since this elements is a hana::tuple of std::vector<double>,
	     * we can assign it a new value at construction. We cannot do it at 
	     * initializer list however because elements does not belong to this class.
	     */
	    elements = boost::hana::tuple<std::vector<double>>(std::move(vec)); 
    }
     
    auto get_data() const {
	    /*
	     * To access the elements, we use hana literals and at 0th index of elements
	     * lies the std::vector we have stored.
	     */
	    using namespace boost::hana::literals;
	    return this->elements[0_c].data();
    }
    auto get_dimension() const {
	    using namespace boost::hana::literals;
	    return this->elements[0_c].size();
    }
};
/**
 * However there is a Issue with the this design.
 * elements is a public member and we inherit it publicly. So anyone can access the std::vector
 * We cannot make elements private because it will violate the YAP concepts.
 *
 * A possible solution as implemented in the tensor is that, we can make all those tensor_core datamembers
 * private and declare tensor and tensor_expression a friend. So, its private memebers are only available in
 * those classes.
 */

/** This is the exact short implementation of tensor_expression of new ublas::tensor.*/