Add 'xo-expression/' from commit '5ac3c03a0c'

git-subtree-dir: xo-expression
git-subtree-mainline: d0f5ccc1ce
git-subtree-split: 5ac3c03a0c

xo-expression/include/xo/expression/GeneralizedExpression.hpp

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+/** @file GeneralizedExpression.hpp
+ *
+ *  Author: Roland Conybeare
+ **/
+
+#pragma once
+
+#include "xo/refcnt/Refcounted.hpp"
+#include "xo/reflect/TypeDescr.hpp"
+#include "exprtype.hpp"
+//#include <cstdint>
+
+namespace xo {
+    namespace ast {
+        /** @class GeneralizedExpression
+         *  @brief abstract syntax tree (non-executable) for schematica
+         *
+         *  'Generalized' because it includes both kernel and macro expressions.
+         *  Every macro expression automatically translates to an equivalent kernel expression.
+         *  Kernel expressions are directly executable.
+         **/
+        class GeneralizedExpression : public ref::Refcount {
+        public:
+            using TypeDescr = xo::reflect::TypeDescr;
+
+        public:
+            GeneralizedExpression(exprtype extype, TypeDescr valuetype)
+                : extype_{extype}, valuetype_{valuetype}{}
+
+            exprtype extype() const { return extype_; }
+            TypeDescr valuetype() const { return valuetype_; }
+
+            /** write human-readable representation to stream **/
+            virtual void display(std::ostream & os) const = 0;
+            /** human-readable string representation **/
+            virtual std::string display_string() const;
+
+        protected:
+            /** useful when scaffolding expressions in a parser **/
+            void assign_valuetype(TypeDescr x) { valuetype_ = x; }
+
+        private:
+            /** expression type (constant | apply | ..) for this expression **/
+            exprtype extype_ = exprtype::invalid;
+            /** type information (when available) for values produced by this
+             *  expression.
+             **/
+            TypeDescr valuetype_ = nullptr;
+        };
+
+        inline std::ostream &
+        operator<<(std::ostream & os, const GeneralizedExpression & x) {
+            x.display(os);
+            return os;
+        }
+    } /*namespace ast*/
+} /*namespace xo*/
+
+
+/** end GeneralizedExpression.hpp **/