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- [section:concepts Concepts]
- [heading Expression]
- _Expr_ is the central concept in _yap_. It must contain at least an _kind_
- and a _tuple_ of values.
- Here is a summary of the requirements on _Expr_. In the tables below, `E` is
- a type that models _Expr_; `e` is an object of type `E`; `Tuple` is an
- instantiation of _tuple_; and `t` is an object of type `Tuple`.
- [table Expression Requirements
- [[Expression] [Type] [Description] [Notes]]
- [[`E::kind`] [_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 Expression Requirements
- [[`E::kind`] [`hana::size(e.elements)`] [Possible Tuple Element Types] [Notes]]
- [[`expr_kind::expr_ref`] [1] [ Any non-_expr_ref_ _Expr_. ] []]
- [[`expr_kind::terminal`] [1] [ Any non-_Expr_. ] [A terminal with a _placeholder_ value will be treated as a placeholder. ]]
- [[Any unary operator] [1] [ Any _Expr_. ] []]
- [[Any binary operator] [2] [ Any _Expr_. ] []]
- [[`expr_kind::if_else`] [3] [ Any _Expr_. ] []]
- [[`expr_kind::call`] [Any number >= 1.] [ Any _Expr_. ] []]
- ]
- [heading ExpressionTemplate]
- _ExprTmpl_ is any template with two parameters that, when instantiated with an
- _kind_ and a _tuple_, results in an _Expr_.
- [heading Transform]
- _xform_ takes a _XForm_ as its second parameter. A _XForm_ is a _Callable_
- that takes expressions and returns values of unconstrained type. There are
- two sorts of overloads _XForm_ may use: _ExprXForm_ and _TagXForm_.
- A _XForm_ may have any number of overloads, including none.
- [heading ExpressionTransform]
- _ExprXForm_ takes an _Expr_ as its only parameter. Here are some examples.
- This one takes any _Expr_:
- struct xform
- {
- template <typename Expr>
- auto operator() (Expr const & expr)
- {
- // ...
- }
- };
- This one takes any type of _Expr_ 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
- ) {
- // ...
- }
- [heading TagTransform]
- _TagXForm_ 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 _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)
- {
- // ...
- }
- }
- [endsect]
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