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

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

xo-expression/include/xo/expression/Environment.hpp

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+/* file Environment.hpp
+ *
+ * author: Roland Conybeare, Jun 2024
+ */
+
+#pragma once
+
+#include "xo/refcnt/Refcounted.hpp"
+#include "binding_path.hpp"
+
+namespace xo {
+    namespace ast {
+        class Expression;
+
+        class Environment : public ref::Refcount {
+        public:
+            /** true if this is toplevel (global) environment.
+             *  Toplevel environment doesn't have slot numbers.
+             *
+             *  Variables that bind in the global environment have unique
+             *  names, which we rely on instead of slot numbers.
+             **/
+            virtual bool is_global_env() const = 0;
+
+            /** lookup binding path for @p vname in this environment.
+             *
+             *  Reports ingredients needed to address variable at runtime,
+             *  in runtime analog of this environment
+             **/
+            virtual binding_path lookup_binding(const std::string & vname) const = 0;
+
+            /** lookup variable-expression @p vname in this environment.
+             *  returns llvm::Value representing code that produces a value for vname
+             **/
+            virtual ref::brw<Expression> lookup_var(const std::string & vname) const = 0;
+        };
+    } /*namespace ast*/
+} /*namespace xo*/
+
+
+/* end Environment.hpp */