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- // Copyright (C) 2016-2018 T. Zachary Laine
- //
- // Distributed under the Boost Software License, Version 1.0. (See
- // accompanying file LICENSE_1_0.txt or copy at
- // http://www.boost.org/LICENSE_1_0.txt)
- //[ calc3
- #include <boost/yap/expression.hpp>
- #include <boost/hana/maximum.hpp>
- #include <iostream>
- // Look! A transform! This one transforms the expression tree into the arity
- // of the expression, based on its placeholders.
- //[ calc3_get_arity_xform
- struct get_arity
- {
- // Base case 1: Match a placeholder terminal, and return its arity as the
- // result.
- template <long long I>
- boost::hana::llong<I> operator() (boost::yap::expr_tag<boost::yap::expr_kind::terminal>,
- boost::yap::placeholder<I>)
- { return boost::hana::llong_c<I>; }
- // Base case 2: Match any other terminal. Return 0; non-placeholders do
- // not contribute to arity.
- template <typename T>
- auto operator() (boost::yap::expr_tag<boost::yap::expr_kind::terminal>, T &&)
- {
- using namespace boost::hana::literals;
- return 0_c;
- }
- // Recursive case: Match any expression not covered above, and return the
- // maximum of its children's arities.
- template <boost::yap::expr_kind Kind, typename... Arg>
- auto operator() (boost::yap::expr_tag<Kind>, Arg &&... arg)
- {
- return boost::hana::maximum(
- boost::hana::make_tuple(
- boost::yap::transform(
- boost::yap::as_expr(std::forward<Arg>(arg)),
- get_arity{}
- )...
- )
- );
- }
- };
- //]
- int main ()
- {
- using namespace boost::yap::literals;
- // These lambdas wrap our expressions as callables, and allow us to check
- // the arity of each as we call it.
- auto expr_1 = 1_p + 2.0;
- auto expr_1_fn = [expr_1](auto &&... args) {
- auto const arity = boost::yap::transform(expr_1, get_arity{});
- static_assert(arity.value == sizeof...(args), "Called with wrong number of args.");
- return evaluate(expr_1, args...);
- };
- auto expr_2 = 1_p * 2_p;
- auto expr_2_fn = [expr_2](auto &&... args) {
- auto const arity = boost::yap::transform(expr_2, get_arity{});
- static_assert(arity.value == sizeof...(args), "Called with wrong number of args.");
- return evaluate(expr_2, args...);
- };
- auto expr_3 = (1_p - 2_p) / 2_p;
- auto expr_3_fn = [expr_3](auto &&... args) {
- auto const arity = boost::yap::transform(expr_3, get_arity{});
- static_assert(arity.value == sizeof...(args), "Called with wrong number of args.");
- return evaluate(expr_3, args...);
- };
- // Displays "5"
- std::cout << expr_1_fn(3.0) << std::endl;
- // Displays "6"
- std::cout << expr_2_fn(3.0, 2.0) << std::endl;
- // Displays "0.5"
- std::cout << expr_3_fn(3.0, 2.0) << std::endl;
- // Static-asserts with "Called with wrong number of args."
- //std::cout << expr_3_fn(3.0) << std::endl;
- // Static-asserts with "Called with wrong number of args."
- //std::cout << expr_3_fn(3.0, 2.0, 1.0) << std::endl;
- return 0;
- }
- //]
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