types/description: add visit dispatcher for descriptions

types/description.hpp

@@ -9,9 +9,14 @@
 #pragma once
 
 #include "../std.hpp"
+#include "../types.hpp"
+#include "../debug.hpp"
 
 #include <cstddef>
 #include <type_traits>
+#include <functional>
+#include <utility>
+
 
 namespace cruft::types {
     //-------------------------------------------------------------------------
@@ -24,7 +29,10 @@ namespace cruft::types {
 
     //-------------------------------------------------------------------------
     struct description {
+        /// The signed, realness, or voidness of the type.
         enum category category;
+
+        /// The number of bytes for an individual instance of this type.
         std::size_t width;
     };
 
@@ -60,6 +68,93 @@ namespace cruft::types {
             .width = sizeof (T)
         };
     }
+
+
+    /// Call a functor with the supplied arguments and a type_tag for the
+    /// described native type.
+    ///
+    /// If the type does not exist then throw and invalid_argument exception.
+    template <typename FunctionT, typename ...Args>
+    decltype(auto)
+    visit (description const &descriminator, FunctionT &&func, Args&&...args)
+    {
+        switch (descriminator.category) {
+        case category ::NONE:
+            return std::invoke (
+                std::forward<FunctionT> (func),
+                std::forward<Args> (args)...,
+                type_tag<void> {}
+            );
+
+        case category::REAL:
+            switch (descriminator.width) {
+                case 4: return std::invoke (
+                    std::forward<FunctionT> (func),
+                    std::forward<Args> (args)...,
+                    type_tag<f32> {}
+                );
+                case 8: return std::invoke (
+                    std::forward<FunctionT> (func),
+                    std::forward<Args> (args)...,
+                    type_tag<f64> {}
+                );
+            default:
+                throw std::invalid_argument ("Unsupported floating point width");
+            }
+
+        case category::SIGNED:
+            switch (descriminator.width) {
+            case 1:  return std::invoke (
+                    std::forward<FunctionT> (func),
+                    std::forward<Args> (args)...,
+                    type_tag<i08> {}
+                );
+            case 2:  return std::invoke (
+                    std::forward<FunctionT> (func),
+                    std::forward<Args> (args)...,
+                    type_tag<i16> {}
+                );
+            case 4:  return std::invoke (
+                    std::forward<FunctionT> (func),
+                    std::forward<Args> (args)...,
+                    type_tag<i32> {}
+                );
+            case 8:  return std::invoke (
+                std::forward<FunctionT> (func),
+                std::forward<Args> (args)...,
+                type_tag<i64> {}
+            );
+            default:
+                throw std::invalid_argument ("Unsupported unsigned width");
+            }
+
+        case category::UNSIGNED:
+            switch (descriminator.width) {
+            case 1:  return std::invoke (
+                    std::forward<FunctionT> (func),
+                    std::forward<Args> (args)...,
+                    type_tag<u08> {}
+                );
+            case 2:  return std::invoke (
+                    std::forward<FunctionT> (func),
+                    std::forward<Args> (args)...,
+                    type_tag<u16> {}
+                );
+            case 4:  return std::invoke (
+                    std::forward<FunctionT> (func),
+                    std::forward<Args> (args)...,
+                    type_tag<u32> {}
+                );
+            case 8:  return std::invoke (
+                    std::forward<FunctionT> (func),
+                    std::forward<Args> (args)...,
+                    type_tag<u64> {}
+                );
+            }
+        }
+
+        unhandled (descriminator.category);
+    }
 }
 
 #include <iosfwd>