xo-expression: initial commit (constant + primitive)

include/xo/expression/Expression.hpp

@@ -0,0 +1,48 @@
+/** @file Expression.hpp
+ *
+ *  Author: Roland Conybeare
+ **/
+
+#pragma once
+
+#include "xo/refcnt/Refcounted.hpp"
+#include "exprtype.hpp"
+
+namespace xo {
+    namespace ast {
+        /** @class Expression
+         *  @brief abstract syntax tree for an EGAD program
+         *
+         *  (Expression Graph with Automagic Derivation)
+         *
+         *  Things you can do with an Expression:
+         *  - evaluate it using an interpreter
+         *  - execute it on a VM
+         *  - compile using LLVM
+         *    see xo-jit/
+         **/
+        class Expression : public ref::Refcount {
+        public:
+            explicit Expression(exprtype extype) : extype_{extype} {}
+
+            exprtype extype() const { return extype_; }
+
+            /** write human-readable representation to stream **/
+            virtual void display(std::ostream & os) const = 0;
+            /** human-readable string representation **/
+            virtual std::string display_string() const;
+
+        private:
+            /** expression type (constant | apply | ..) for this expression **/
+            exprtype extype_ = exprtype::invalid;
+        }; /*Expression*/
+
+        inline std::ostream &
+        operator<<(std::ostream & os, const Expression & x) {
+            x.display(os);
+            return os;
+        }
+    } /*namespace ast*/
+} /*namespace xo*/
+
+/** end Expression.hpp **/