Move user_expression_transform to a standalone test.

test/user_expression_transform.cpp

@@ -0,0 +1,92 @@
+#define BOOST_PROTO17_CONVERSION_OPERATOR_TEMPLATE
+#include "expression.hpp"
+
+#include <gtest/gtest.h>
+
+#include <sstream>
+
+
+template <typename T>
+using term = boost::proto17::terminal<T>;
+
+namespace bp17 = boost::proto17;
+
+
+namespace user_2 {
+
+    struct number
+    {
+        double value;
+    };
+
+    number naxpy (number a, number x, number y)
+    { return number{a.value * x.value + y.value}; }
+
+#if 0
+    auto eval_expression_as (
+        bp17::expression<
+            bp17::expr_kind::plus,
+            bp17::expression<
+                bp17::expr_kind::multiplies,
+                term<number>,
+                term<number>
+            >,
+            term<number>
+        > const & expr,
+        boost::hana::basic_type<number>)
+    {
+        std::cout << "User naxpy!  ";
+        using namespace boost::hana::literals;
+        return naxpy(
+            expr.elements[0_c].elements[0_c].elements[0_c],
+            expr.elements[0_c].elements[1_c].elements[0_c],
+            expr.elements[1_c].elements[0_c]
+        );
+    }
+#else
+    auto eval_expression_as (
+        decltype(term<number>{{0.0}} * number{} + number{}) const & expr,
+        boost::hana::basic_type<number>,
+        boost::hana::tuple<>)
+    {
+        std::cout << "User naxpy!  ";
+        return naxpy(
+            expr.left().left().value(),
+            expr.left().right().value(),
+            expr.right().value()
+        );
+    }
+#endif
+
+}
+
+TEST(user_expression_transform, test_user_expression_transform)
+{
+    term<user_2::number> a{{1.0}};
+    term<user_2::number> x{{42.0}};
+    term<user_2::number> y{{3.0}};
+
+    bp17::expression<
+        bp17::expr_kind::plus,
+        bp17::expression<
+            bp17::expr_kind::multiplies,
+            term<user_2::number>,
+            term<user_2::number>
+        >,
+        term<user_2::number>
+    > expr = a * x + y;
+
+    // TODO: This was an error (user:: vs. user_2::).  Document for users that
+    // they should catch an expression in an auto var to diagnose these sorts
+    // of things.
+#if 0
+    bp17::expression<
+        bp17::expr_kind::multiplies,
+        term<user::number>,
+        term<user::number>
+    > expr = a * x;
+#endif
+
+    user_2::number result = expr;
+    EXPECT_EQ(result.value, 45);
+}