Concepts

Expression

Expression is the central concept in Boost.YAP. It must contain at least an expr_kind and a boost::hana::tuple<> of values.

Here is a summary of the requirements on Expression. In the tables below, E is a type that models Expression; e is an object of type E; Tuple is an instantiation of boost::hana::tuple<>; and t is an object of type Tuple.

Table 47.6. Expression Requirements

Expression	Type	Description	Notes
`E::kind`	`expr_kind`	The kind of expression `E` is.	Must be a compile-time constant.
`e.elements`	`Tuple`	The child expressions of `e`.	The types of the elements must be appropriate to the kind of the expression.
`E e{t}`		Construction/initialization of `e`.	`t` must be stored in `e.elements`.

As stated above, the elements data member must match the kind of the expression:

Table 47.7. Expression Requirements

`E::kind`	`hana::size(e.elements)`	Possible Tuple Element Types	Notes
`expr_kind::expr_ref`	1	Any non-`expr_kind::expr_ref` Expression.
`expr_kind::terminal`	1	Any non-Expression.	A terminal with a `placeholder<>` value will be treated as a placeholder.
Any unary operator	1	Any Expression.
Any binary operator	2	Any Expression.
`expr_kind::if_else`	3	Any Expression.
`expr_kind::call`	Any number >= 1.	Any Expression.

ExpressionTemplate

ExpressionTemplate is any template with two parameters that, when instantiated with an expr_kind and a boost::hana::tuple<>, results in an Expression.

Transform

transform() takes a Transform as its second parameter. A Transform is a Callable that takes expressions and returns values of unconstrained type. There are two sorts of overloads Transform may use: ExpressionTransform and TagTransform.

A Transform may have any number of overloads, including none.

ExpressionTransform

ExpressionTransform takes an Expression as its only parameter. Here are some examples.

This one takes any Expression:

struct xform
{
    template <typename Expr>
    auto operator() (Expr const & expr)
    {
        // ...
    }
};

This one takes any type of Expression that satisfies the constraints imposed by its template parameters:

template <typename Expr1, typename Expr2, typename Expr3>
decltype(auto) xform (
    boost::yap::expression<
        boost::yap::expr_kind::plus,
        boost::hana::tuple<
            boost::yap::expression<
                boost::yap::expr_kind::multiplies,
                boost::hana::tuple<
                    Expr1,
                    Expr2
                >
            >,
            Expr3
        >
    > const & expr
) {
    // ...
}

This one takes only a specific type:

decltype(auto) xform (
    decltype(term<number>{{0.0}} * number{} + number{}) const & expr
) {
    // ...
}

TagTransform

TagTransform takes a tag-type as its first parameter, and the individual elements of an expression as the remaining parameters.

Tags are named such that the tag for an expression with expr_kind expr_kind::foo is named foo_tag. Here are some examples.

This one takes any terminal that contains a user::number (or reference to such a terminal):

struct xform
{
    decltype(auto) operator() (boost::yap::terminal_tag, user::number const & n)
    {
        // ...
    }
};

This one takes any plus expression that contains a pair of user::number terminals (or references to terminals):

decltype(auto) xform (boost::yap::plus_tag, user::number lhs, user::number rhs)
{
    // ...
}

This one takes any negation expression:

struct xform
{
    template <typename Expr>
    decltype(auto) operator() (boost::yap::negate_tag, Expr const & expr)
    {
        // ...
    }
};

This one takes any call expression with two terminals (or references to terminals) containing values convertible to double:

struct xform
{
    decltype(auto) operator() (boost::yap::call_tag, tag_type, double a, double b)
    {
        // ...
    }
}