Reorganize is_true/false and add tests.

src/alia/data_graph.hpp

@@ -871,6 +871,22 @@ struct loop_block : noncopyable
 // takes the context as its first argument. The other form has no trailing
 // underscore and assumes that the context is a variable named 'ctx'.
 
+// is_true(x) evaluates x in a boolean context.
+template<class T>
+std::enable_if_t<!is_readable_signal_type<T>::value, bool>
+is_true(T x)
+{
+    return x ? true : false;
+}
+
+// is_false(x) evaluates x in a boolean context and inverts it.
+template<class T>
+std::enable_if_t<!is_readable_signal_type<T>::value, bool>
+is_false(T x)
+{
+    return x ? false : true;
+}
+
 // if, else_if, else
 
 #define alia_if_(ctx, condition)                                               \

unit_tests/signals/utilities.cpp

@@ -2,6 +2,8 @@
 
 #include <catch.hpp>
 
+#include <alia/signals/basic.hpp>
+
 struct unreadable_regular_signal
     : alia::regular_signal<int, alia::read_only_signal>
 {
@@ -91,3 +93,16 @@ TEST_CASE("lazy_reader", "[signals]")
     REQUIRE(reader.read(generator) == 12);
     REQUIRE(n == 1);
 }
+
+TEST_CASE("is_true/false", "[signals]")
+{
+    using namespace alia;
+
+    REQUIRE(is_true(value(true)));
+    REQUIRE(!is_true(value(false)));
+    REQUIRE(!is_true(empty<bool>()));
+
+    REQUIRE(is_false(value(false)));
+    REQUIRE(!is_false(value(true)));
+    REQUIRE(!is_false(empty<bool>()));
+}