xo-reader2 scaffold (fomo+arena version of xo-reader/) [WIP]

include/xo/reader2/ExpressionParser.hpp

@@ -0,0 +1,80 @@
+/** @file ExpressionParser.hpp
+*
+ *  @author Roland Conybeare, Jan 2026
+ **/
+
+#include "ExprState.hpp"
+#include <xo/alloc2/Allocator.hpp>
+#include <xo/alloc2/arena/IAllocator_DArena.hpp>
+#include <xo/facet/obj.hpp>
+
+namespace xo {
+    namespace scm {
+        /** @class ExpressionParser
+         *  @brief Assemble Schematika expressions from token sequences
+         *
+         *  Parser represents Each partially assembled expression by
+         *  an ExprState object.
+         *  Expreesions form a tree:
+         *  each expression belongs to at most one parent.
+         *
+         **/
+        class ExpressionParser {
+        public:
+            void push_exprstate(obj<AExprState> xstate);
+
+        private:
+            /* TODO:
+             *   ASymbolTable
+             *   DLocalSymtab
+             *   DGlobalSymtab
+             *
+             * Will also need
+             *   DVariable
+             *   DLambda
+             *
+             * For DGlobalSymtab perhaps use DArenaHashMap.
+             * May also want to use DArenaHashMap+DArena to intern strings
+             *
+             * Also:
+             *   TypeUnifier
+             */
+
+            /** Arena for internal parsing stack.
+             *  Must be owned exclusively because destructively
+             *  modified as parser completes parsing of each sub-expression
+             *
+             *  Contents will be a stack of ExprState instances
+             **/
+            DArena parser_alloc_;
+
+#ifdef NOT_YET
+            /** Arena for internal environment stack.
+             *  This represents just nesting for environments.
+             *  Details for each frame survive parsing and are
+             *  stored in @ref expr_alloc_.
+             *  Maybe that means we don't need env_alloc_
+             **/
+            DArena env_alloc_;
+#endif
+
+            /** Allocator for parsed expressions.
+             *  Information available during subsequent execution
+             *  (whether compiling or interpreting) must be stored here.
+             *
+             *  Also use this allocator for error messages arising
+             *  during parsing
+             *
+             *  Memory use patterns for executions are not predictable,
+             *  and require garbage collection, e.g. DX1Collector.
+             *
+             *  May alternatively be able to use DArena in a compile-only
+             *  scenario, where top-level Expressions can be discarded
+             *  once compiled.
+             **/
+            obj<AAllocator> expr_alloc_;
+        };
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/* end ExpressionParser.hpp */