73 lines
2.5 KiB
C++
73 lines
2.5 KiB
C++
// Copyright (C) 2016-2018 T. Zachary Laine
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//
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// Distributed under the Boost Software License, Version 1.0. (See
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// accompanying file LICENSE_1_0.txt or copy at
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// http://www.boost.org/LICENSE_1_0.txt)
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//[ transform_terminals
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#include <boost/yap/yap.hpp>
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struct iota_terminal_transform
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{
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// Base case. Note that we're not treating placeholders specially for this
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// example (they're easy to special-case if necessary).
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template<typename T>
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auto operator()(boost::yap::expr_tag<boost::yap::expr_kind::terminal>, T && t)
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{
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// Like the std::iota() algorithm, we create replacement int terminals
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// with the values index_, index_ + 1, index_ + 2, etc.
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return boost::yap::make_terminal(index_++);
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}
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// Recursive case: Match any call expression.
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template<typename CallableExpr, typename... Arg>
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auto operator()(boost::yap::expr_tag<boost::yap::expr_kind::call>,
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CallableExpr callable, Arg &&... arg)
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{
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// Even though the callable in a call expression is technically a
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// terminal, it doesn't make a lot of sense to replace it with an int,
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// so we'll only transform the argument subexpressions.
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return boost::yap::make_expression<boost::yap::expr_kind::call>(
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boost::yap::as_expr(callable),
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boost::yap::transform(boost::yap::as_expr(arg), *this)...);
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}
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int index_;
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};
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int sum(int a, int b) { return a + b; }
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int main()
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{
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{
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// This simple sum(8, 8) expression requires both overloads of
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// iota_terminal_transform.
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auto expr = boost::yap::make_terminal(sum)(8, 8);
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assert(evaluate(expr) == 16);
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auto iota_expr = boost::yap::transform(expr, iota_terminal_transform{1});
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assert(evaluate(iota_expr) == 3);
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}
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{
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// This expression requires only the terminal case of
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// iota_terminal_transform.
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auto expr = -(boost::yap::make_terminal(8) + 8);
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assert(evaluate(expr) == -16);
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auto iota_expr = boost::yap::transform(expr, iota_terminal_transform{0});
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assert(evaluate(iota_expr) == -1);
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}
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{
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// Like the first expression above, this expression requires both
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// overloads of iota_terminal_transform.
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auto expr = boost::yap::make_terminal(sum)(-(boost::yap::make_terminal(8) + 8), 0);
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assert(evaluate(expr) == -16);
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auto iota_expr = boost::yap::transform(expr, iota_terminal_transform{0});
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assert(evaluate(iota_expr) == -3);
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}
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}
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//]
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